Patchwork [ARM,ifcvt] Improve use of conditional execution in thumb mode.

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Submitter Andrew Stubbs
Date Feb. 14, 2012, 4:53 p.m.
Message ID <4F3A91A0.3040209@codesourcery.com>
Download mbox | patch
Permalink /patch/141141/
State New
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Comments

Andrew Stubbs - Feb. 14, 2012, 4:53 p.m.
Hi all,

I've discovered that GCC does not use ARM conditional execution for 
16-bit Thumb opcodes in many cases. It's fine for individual 
instructions, but if-conversion of basic blocks with more than one 
instruction fails.

E.g.

int
foo (int a, int b)
{
   if (a != b)
     {
       a = a << b;
       a = a >> 1;
     }

   return a + b;
}

The current compiler gives:

foo:
         cmp     r0, r1
         beq     .L2
         lsls    r0, r0, r1
         asrs    r0, r0, #1
.L2:
         adds    r0, r0, r1
         bx      lr

With my patch I get this:

foo:
         cmp     r0, r1
         itt     ne
         lslne   r0, r0, r1
         asrne   r0, r0, #1
         adds    r0, r0, r1
         bx      lr

The problem comes from the fact that the compiler prefers "lsls" over 
"lsl" because the former is a 16-bit encoding, and the latter a 32-bit 
encoding. There's actually a peephole optimization defined to make this 
happen wherever the CC register is not live.

This is fine in unconditional code, but the CC register clobber means 
that it's only possible to convert it to conditional code if it is the 
last instruction in the IT block, so if-conversion fails on the above 
example.

My patch introduces a new target hook "IFCVT_OVERRIDE_MODIFIED_TEST" 
that allows the CC clobber to be ignored on such instructions, and uses 
IFCVT_MODIFY_INSN to convert from "lsls" to "lsl<c>" where possible.

I've also introduced a new instruction attribute "it_cc" to indicate 
which instruction patterns are affected.

OK for trunk, once stage 1 reopens?

Andrew
Richard Earnshaw - Feb. 14, 2012, 5:30 p.m.
On 14/02/12 16:53, Andrew Stubbs wrote:
> Hi all,
> 
> I've discovered that GCC does not use ARM conditional execution for 
> 16-bit Thumb opcodes in many cases. It's fine for individual 
> instructions, but if-conversion of basic blocks with more than one 
> instruction fails.
> 
> E.g.
> 
> int
> foo (int a, int b)
> {
>    if (a != b)
>      {
>        a = a << b;
>        a = a >> 1;
>      }
> 
>    return a + b;
> }
> 
> The current compiler gives:
> 
> foo:
>          cmp     r0, r1
>          beq     .L2
>          lsls    r0, r0, r1
>          asrs    r0, r0, #1
> .L2:
>          adds    r0, r0, r1
>          bx      lr
> 
> With my patch I get this:
> 
> foo:
>          cmp     r0, r1
>          itt     ne
>          lslne   r0, r0, r1
>          asrne   r0, r0, #1
>          adds    r0, r0, r1
>          bx      lr
> 
> The problem comes from the fact that the compiler prefers "lsls" over 
> "lsl" because the former is a 16-bit encoding, and the latter a 32-bit 
> encoding. There's actually a peephole optimization defined to make this 
> happen wherever the CC register is not live.
> 
> This is fine in unconditional code, but the CC register clobber means 
> that it's only possible to convert it to conditional code if it is the 
> last instruction in the IT block, so if-conversion fails on the above 
> example.
> 
> My patch introduces a new target hook "IFCVT_OVERRIDE_MODIFIED_TEST" 
> that allows the CC clobber to be ignored on such instructions, and uses 
> IFCVT_MODIFY_INSN to convert from "lsls" to "lsl<c>" where possible.
> 
> I've also introduced a new instruction attribute "it_cc" to indicate 
> which instruction patterns are affected.
> 
> OK for trunk, once stage 1 reopens?
> 
> Andrew
> 

Bernds checked in a patch last year (or maybe even before that) to make
the selection of flags clobbered insns run very late (certainly after
condexec had run), so I'm not sure why you're not seeing this.

R.

> 
> ifcvt_modify_insn.patch
> 
> 
> 2012-02-14  Andrew Stubbs  <ams@codesourcery.com>
> 
> 	gcc/
> 	* config/arm/arm-protos.h (arm_ifcvt_modify_insn): New prototype.
> 	(arm_ifcvt_override_modified_test): New prototype.
> 	* config/arm/arm.c (thumb_insn_suitable_for_ifcvt): New function.
> 	(arm_ifcvt_override_modified_test): New function.
> 	(arm_ifcvt_modify_insn): New function.
> 	* config/arm/arm.h (IFCVT_OVERRIDE_MODIFIED_TEST): New macro.
> 	(IFCVT_MODIFY_INSN): New macro.
> 	* config/arm/thumb2.md (it_cc): New attribute.
> 	(thumb2_alusi3_short): Set it_cc attribute.
> 	(thumb2_shiftsi3_short, thumb2_mov<mode>_shortim): Likewise.
> 	(thumb2_addsi_short, thumb2_subsi_short): Likewise.
> 	(thumb2_mulsi_short, thumb2_one_cmplsi2_short): Likewise.
> 	(thumb2_negsi2_short): Likewise.
> 	* doc/tm.texi: Regenerate.
> 	* doc/tm.texi.in (IFCVT_OVERRIDE_MODIFIED_TEST): Document.
> 	* ifcvt.c (cond_exec_process_insns): Add IFCVT_OVERRIDE_MODIFIED_TEST.
> 
> 	gcc/testsuite/
> 	* gcc.target/arm/thumb-ifcvt.c: New test case.
> 
> ---
>  gcc/config/arm/arm-protos.h                |    5 ++
>  gcc/config/arm/arm.c                       |   67 ++++++++++++++++++++++++++++
>  gcc/config/arm/arm.h                       |    5 ++
>  gcc/config/arm/thumb2.md                   |   11 +++++
>  gcc/doc/tm.texi                            |   13 +++++
>  gcc/doc/tm.texi.in                         |   13 +++++
>  gcc/ifcvt.c                                |   14 +++++-
>  gcc/testsuite/gcc.target/arm/thumb-ifcvt.c |   19 ++++++++
>  8 files changed, 146 insertions(+), 1 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.target/arm/thumb-ifcvt.c
> 
> diff --git a/gcc/config/arm/arm-protos.h b/gcc/config/arm/arm-protos.h
> index 296550a..67396f0 100644
> --- a/gcc/config/arm/arm-protos.h
> +++ b/gcc/config/arm/arm-protos.h
> @@ -244,4 +244,9 @@ extern const struct tune_params *current_tune;
>  extern int vfp3_const_double_for_fract_bits (rtx);
>  #endif /* RTX_CODE */
>  
> +#ifdef BB_HEAD
> +extern int arm_ifcvt_override_modified_test (rtx, rtx);
> +extern rtx arm_ifcvt_modify_insn (ce_if_block_t *, rtx, rtx);
> +#endif
> +
>  #endif /* ! GCC_ARM_PROTOS_H */
> diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
> index 0bded8d..803e1c9 100644
> --- a/gcc/config/arm/arm.c
> +++ b/gcc/config/arm/arm.c
> @@ -25139,5 +25139,72 @@ vfp3_const_double_for_fract_bits (rtx operand)
>    return 0;
>  }
>  
> +/* Find the portion of INSN that is suitable for if-conversion.
> +
> +   Some Thumb mode 16-bit instructions have two variants: one that is
> +   unconditional but clobbers the condition flags, and one that is
> +   conditional (in an IT block) and does not clobber anything.
> +
> +   There are 32-bit variants that are unconditional but don't clobber
> +   anything, so the peephole2 pass adds the clobber in order to use the
> +   smaller instruction encoding.  Unfortunately this defeats the
> +   if-conversion pass since CC must not be modified in an IT block.
> +
> +   The peephole can be reversed, and the instruction converted to
> +   conditional execution without affecting the size optimization.
> +
> +   This function detects these instructions and returns the sub-expression
> +   that contains the operation, without the clobber.  */
> +
> +static rtx
> +thumb_insn_suitable_for_ifcvt (rtx pattern, rtx insn)
> +{
> +  if (GET_CODE (pattern) == PARALLEL
> +      && XVECLEN (pattern, 0) == 2)
> +    {
> +      rtx op = XVECEXP (pattern, 0, 0);
> +      rtx clobber = XVECEXP (pattern, 0, 1);
> +
> +      if (GET_CODE (clobber) == CLOBBER
> +	  && GET_CODE (XEXP (clobber, 0)) == REG
> +	  && REGNO (XEXP (clobber, 0)) == CC_REGNUM
> +	  && get_attr_it_cc (insn) == IT_CC_NOCLOBBER)
> +	return op;
> +    }
> +
> +  return NULL_RTX;
> +}
> +
> +/* Prevent if-conversion thinking that certain Thumb1 instructions
> +   clobber CC.  These instruction in fact do not when in an IT block.  */
> +int
> +arm_ifcvt_override_modified_test (rtx test, rtx insn)
> +{
> +  if (thumb_insn_suitable_for_ifcvt (PATTERN (insn), insn) != NULL_RTX)
> +    return 0; /* Force *not* modified.  */
> +
> +  return -1;
> +}
> +
> +/* Modify insns to enable conditional execution.
> +
> +   This function removes CC clobers that impede if-conversion.  See the
> +   comments on thumb_insn_suitable_for_ifcvt.  */
> +rtx
> +arm_ifcvt_modify_insn (ce_if_block_t *ce, rtx pattern, rtx insn)
> +{
> +  rtx op, cond;
> +
> +  gcc_assert (GET_CODE (pattern) == COND_EXEC);
> +
> +  cond = XEXP (pattern, 0);
> +  op = thumb_insn_suitable_for_ifcvt (XEXP (pattern, 1), insn);
> +
> +  if (op != NULL_RTX)
> +    return gen_rtx_COND_EXEC (VOIDmode, cond, op);
> +
> +  return pattern;
> +}
> +
>  #include "gt-arm.h"
>  
> diff --git a/gcc/config/arm/arm.h b/gcc/config/arm/arm.h
> index 5a78125..0b7f332 100644
> --- a/gcc/config/arm/arm.h
> +++ b/gcc/config/arm/arm.h
> @@ -2220,4 +2220,9 @@ extern const char *host_detect_local_cpu (int argc, const char **argv);
>  
>  #define DRIVER_SELF_SPECS MCPU_MTUNE_NATIVE_SPECS
>  
> +#define IFCVT_OVERRIDE_MODIFIED_TEST(TEST,INSN) \
> +  arm_ifcvt_override_modified_test ((TEST), (INSN))
> +#define IFCVT_MODIFY_INSN(CE,PATTERN,INSN) \
> +  (PATTERN) = arm_ifcvt_modify_insn ((CE), (PATTERN), (INSN))
> +
>  #endif /* ! GCC_ARM_H */
> diff --git a/gcc/config/arm/thumb2.md b/gcc/config/arm/thumb2.md
> index 05585da..454c42f 100644
> --- a/gcc/config/arm/thumb2.md
> +++ b/gcc/config/arm/thumb2.md
> @@ -24,6 +24,9 @@
>  ;; changes made in armv5t as "thumb2".  These are considered part
>  ;; the 16-bit Thumb-1 instruction set.
>  
> +;; Does an insn clobber CC if it appears in an IT block.  */
> +(define_attr "it_cc" "undef,clobber,noclobber" (const_string "undef"))
> +
>  (define_insn "*thumb2_incscc"
>    [(set (match_operand:SI 0 "s_register_operand" "=r,r")
>          (plus:SI (match_operator:SI 2 "arm_comparison_operator"
> @@ -674,6 +677,7 @@
>     && GET_CODE(operands[3]) != MINUS"
>    "%I3%!\\t%0, %1, %2"
>    [(set_attr "predicable" "yes")
> +   (set_attr "it_cc" "noclobber")
>     (set_attr "length" "2")]
>  )
>  
> @@ -708,6 +712,7 @@
>         || REG_P(operands[2]))"
>    "* return arm_output_shift(operands, 2);"
>    [(set_attr "predicable" "yes")
> +   (set_attr "it_cc" "noclobber")
>     (set_attr "shift" "1")
>     (set_attr "length" "2")
>     (set (attr "type") (if_then_else (match_operand 2 "const_int_operand" "")
> @@ -736,6 +741,7 @@
>    "TARGET_THUMB2 && reload_completed"
>    "mov%!\t%0, %1"
>    [(set_attr "predicable" "yes")
> +   (set_attr "it_cc" "noclobber")
>     (set_attr "length" "2")]
>  )
>  
> @@ -778,6 +784,7 @@
>        return \"add%!\\t%0, %1, %2\";
>    "
>    [(set_attr "predicable" "yes")
> +   (set_attr "it_cc" "noclobber")
>     (set_attr "length" "2")]
>  )
>  
> @@ -789,6 +796,7 @@
>    "TARGET_THUMB2 && reload_completed"
>    "sub%!\\t%0, %1, %2"
>    [(set_attr "predicable" "yes")
> +   (set_attr "it_cc" "noclobber")
>     (set_attr "length" "2")]
>  )
>  
> @@ -905,6 +913,7 @@
>    "TARGET_THUMB2 && optimize_size && reload_completed"
>    "mul%!\\t%0, %2, %0"
>    [(set_attr "predicable" "yes")
> +   (set_attr "it_cc" "noclobber")
>     (set_attr "length" "2")
>     (set_attr "insn" "muls")])
>  
> @@ -999,6 +1008,7 @@
>    "TARGET_THUMB2 && reload_completed"
>    "mvn%!\t%0, %1"
>    [(set_attr "predicable" "yes")
> +   (set_attr "it_cc" "noclobber")
>     (set_attr "length" "2")]
>  )
>  
> @@ -1022,6 +1032,7 @@
>    "TARGET_THUMB2 && reload_completed"
>    "neg%!\t%0, %1"
>    [(set_attr "predicable" "yes")
> +   (set_attr "it_cc" "noclobber")
>     (set_attr "length" "2")]
>  )
>  
> diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
> index 6d41cee..7d09ddb 100644
> --- a/gcc/doc/tm.texi
> +++ b/gcc/doc/tm.texi
> @@ -10879,6 +10879,19 @@ if-statements into conditions combined by @code{and} and @code{or} operations.
>  being processed and about to be turned into a condition.
>  @end defmac
>  
> +@defmac IFCVT_OVERRIDE_MODIFIED_TEST (@var{test}, @var{insn})
> +Used if the target has instructions that modify the condition
> +register when executed unconditionally, but do not do so when executed
> +conditionally, or vice-versa.  @var{TEST} contains the condition test, and
> +@var{INSN} contains the original insn to be conditionalized.  The macro
> +should return minus-one if there is to be no override, zero to disregard
> +an apparent modified condition register, or any other value to indicate
> +that the instruction does modify the condition register, even if it does
> +not appear to.  Additionally, if conditionalizing the instruction does
> +change its behaviour, @code{IFCVT_MODIFY_INSN} should make the appropriate
> +adjustments to match.
> +@end defmac
> +
>  @defmac IFCVT_MODIFY_INSN (@var{ce_info}, @var{pattern}, @var{insn})
>  A C expression to modify the @var{PATTERN} of an @var{INSN} that is to
>  be converted to conditional execution format.  @var{ce_info} points to
> diff --git a/gcc/doc/tm.texi.in b/gcc/doc/tm.texi.in
> index 396f244..52d9e96 100644
> --- a/gcc/doc/tm.texi.in
> +++ b/gcc/doc/tm.texi.in
> @@ -10759,6 +10759,19 @@ if-statements into conditions combined by @code{and} and @code{or} operations.
>  being processed and about to be turned into a condition.
>  @end defmac
>  
> +@defmac IFCVT_OVERRIDE_MODIFIED_TEST (@var{test}, @var{insn})
> +Used if the target has instructions that modify the condition
> +register when executed unconditionally, but do not do so when executed
> +conditionally, or vice-versa.  @var{TEST} contains the condition test, and
> +@var{INSN} contains the original insn to be conditionalized.  The macro
> +should return minus-one if there is to be no override, zero to disregard
> +an apparent modified condition register, or any other value to indicate
> +that the instruction does modify the condition register, even if it does
> +not appear to.  Additionally, if conditionalizing the instruction does
> +change its behaviour, @code{IFCVT_MODIFY_INSN} should make the appropriate
> +adjustments to match.
> +@end defmac
> +
>  @defmac IFCVT_MODIFY_INSN (@var{ce_info}, @var{pattern}, @var{insn})
>  A C expression to modify the @var{PATTERN} of an @var{INSN} that is to
>  be converted to conditional execution format.  @var{ce_info} points to
> diff --git a/gcc/ifcvt.c b/gcc/ifcvt.c
> index ce60ce2..ff620da 100644
> --- a/gcc/ifcvt.c
> +++ b/gcc/ifcvt.c
> @@ -310,6 +310,7 @@ cond_exec_process_insns (ce_if_block_t *ce_info ATTRIBUTE_UNUSED,
>    rtx insn;
>    rtx xtest;
>    rtx pattern;
> +  int override_modified = -1;
>  
>    if (!start || !end)
>      return FALSE;
> @@ -338,7 +339,18 @@ cond_exec_process_insns (ce_if_block_t *ce_info ATTRIBUTE_UNUSED,
>        if (must_be_last)
>  	return FALSE;
>  
> -      if (modified_in_p (test, insn))
> +      /* Some instructions modify CC flags when executed unconditionally
> +	 but not when executed conditionally.
> +         Return values:
> +	    -1 = no override
> +	    0 = force false
> +	    other = force true.  */
> +#ifdef IFCVT_OVERRIDE_MODIFIED_TEST
> +      override_modified = IFCVT_OVERRIDE_MODIFIED_TEST (test, insn);
> +#endif
> +
> +      if ((override_modified == -1 && modified_in_p (test, insn))
> +	  || override_modified)
>  	{
>  	  if (!mod_ok)
>  	    return FALSE;
> diff --git a/gcc/testsuite/gcc.target/arm/thumb-ifcvt.c b/gcc/testsuite/gcc.target/arm/thumb-ifcvt.c
> new file mode 100644
> index 0000000..b03bbce
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/arm/thumb-ifcvt.c
> @@ -0,0 +1,19 @@
> +/* Check that Thumb 16-bit shifts can be if-converted.  */
> +/* { dg-do compile } */
> +/* { dg-require-effective-target arm_thumb2_ok } */
> +/* { dg-options "-O2" } */
> +
> +int
> +foo (int a, int b)
> +{
> +  if (a != b)
> +    {
> +      a = a << b;
> +      a = a >> 1;
> +    }
> +
> +  return a + b;
> +}
> +
> +/* { dg-final { scan-assembler "lslne" } } */
> +/* { dg-final { scan-assembler "asrne" } } */
Richard Earnshaw - Feb. 14, 2012, 5:35 p.m.
On 14/02/12 17:30, Richard Earnshaw wrote:
> On 14/02/12 16:53, Andrew Stubbs wrote:
>> Hi all,
>>
>> I've discovered that GCC does not use ARM conditional execution for
>> 16-bit Thumb opcodes in many cases. It's fine for individual
>> instructions, but if-conversion of basic blocks with more than one
>> instruction fails.
>>
>> E.g.
>>
>> int
>> foo (int a, int b)
>> {
>>    if (a != b)
>>      {
>>        a = a << b;
>>        a = a >> 1;
>>      }
>>
>>    return a + b;
>> }
>>
>> The current compiler gives:
>>
>> foo:
>>          cmp     r0, r1
>>          beq     .L2
>>          lsls    r0, r0, r1
>>          asrs    r0, r0, #1
>> .L2:
>>          adds    r0, r0, r1
>>          bx      lr
>>
>> With my patch I get this:
>>
>> foo:
>>          cmp     r0, r1
>>          itt     ne
>>          lslne   r0, r0, r1
>>          asrne   r0, r0, #1
>>          adds    r0, r0, r1
>>          bx      lr
>>
>> The problem comes from the fact that the compiler prefers "lsls" over
>> "lsl" because the former is a 16-bit encoding, and the latter a 32-bit
>> encoding. There's actually a peephole optimization defined to make this
>> happen wherever the CC register is not live.
>>
>> This is fine in unconditional code, but the CC register clobber means
>> that it's only possible to convert it to conditional code if it is the
>> last instruction in the IT block, so if-conversion fails on the above
>> example.
>>
>> My patch introduces a new target hook "IFCVT_OVERRIDE_MODIFIED_TEST"
>> that allows the CC clobber to be ignored on such instructions, and uses
>> IFCVT_MODIFY_INSN to convert from "lsls" to "lsl<c>" where possible.
>>
>> I've also introduced a new instruction attribute "it_cc" to indicate
>> which instruction patterns are affected.
>>
>> OK for trunk, once stage 1 reopens?
>>
>> Andrew
>>
> 
> Bernds checked in a patch last year (or maybe even before that) to make
> the selection of flags clobbered insns run very late (certainly after
> condexec had run), so I'm not sure why you're not seeing this.
> 

Hm, you mentioned some peepholes.  Try removing them....

R.

> R.
> 
>>
>> ifcvt_modify_insn.patch
>>
>>
>> 2012-02-14  Andrew Stubbs  <ams@codesourcery.com>
>>
>>       gcc/
>>       * config/arm/arm-protos.h (arm_ifcvt_modify_insn): New prototype.
>>       (arm_ifcvt_override_modified_test): New prototype.
>>       * config/arm/arm.c (thumb_insn_suitable_for_ifcvt): New function.
>>       (arm_ifcvt_override_modified_test): New function.
>>       (arm_ifcvt_modify_insn): New function.
>>       * config/arm/arm.h (IFCVT_OVERRIDE_MODIFIED_TEST): New macro.
>>       (IFCVT_MODIFY_INSN): New macro.
>>       * config/arm/thumb2.md (it_cc): New attribute.
>>       (thumb2_alusi3_short): Set it_cc attribute.
>>       (thumb2_shiftsi3_short, thumb2_mov<mode>_shortim): Likewise.
>>       (thumb2_addsi_short, thumb2_subsi_short): Likewise.
>>       (thumb2_mulsi_short, thumb2_one_cmplsi2_short): Likewise.
>>       (thumb2_negsi2_short): Likewise.
>>       * doc/tm.texi: Regenerate.
>>       * doc/tm.texi.in (IFCVT_OVERRIDE_MODIFIED_TEST): Document.
>>       * ifcvt.c (cond_exec_process_insns): Add IFCVT_OVERRIDE_MODIFIED_TEST.
>>
>>       gcc/testsuite/
>>       * gcc.target/arm/thumb-ifcvt.c: New test case.
>>
>> ---
>>  gcc/config/arm/arm-protos.h                |    5 ++
>>  gcc/config/arm/arm.c                       |   67 ++++++++++++++++++++++++++++
>>  gcc/config/arm/arm.h                       |    5 ++
>>  gcc/config/arm/thumb2.md                   |   11 +++++
>>  gcc/doc/tm.texi                            |   13 +++++
>>  gcc/doc/tm.texi.in                         |   13 +++++
>>  gcc/ifcvt.c                                |   14 +++++-
>>  gcc/testsuite/gcc.target/arm/thumb-ifcvt.c |   19 ++++++++
>>  8 files changed, 146 insertions(+), 1 deletions(-)
>>  create mode 100644 gcc/testsuite/gcc.target/arm/thumb-ifcvt.c
>>
>> diff --git a/gcc/config/arm/arm-protos.h b/gcc/config/arm/arm-protos.h
>> index 296550a..67396f0 100644
>> --- a/gcc/config/arm/arm-protos.h
>> +++ b/gcc/config/arm/arm-protos.h
>> @@ -244,4 +244,9 @@ extern const struct tune_params *current_tune;
>>  extern int vfp3_const_double_for_fract_bits (rtx);
>>  #endif /* RTX_CODE */
>>
>> +#ifdef BB_HEAD
>> +extern int arm_ifcvt_override_modified_test (rtx, rtx);
>> +extern rtx arm_ifcvt_modify_insn (ce_if_block_t *, rtx, rtx);
>> +#endif
>> +
>>  #endif /* ! GCC_ARM_PROTOS_H */
>> diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
>> index 0bded8d..803e1c9 100644
>> --- a/gcc/config/arm/arm.c
>> +++ b/gcc/config/arm/arm.c
>> @@ -25139,5 +25139,72 @@ vfp3_const_double_for_fract_bits (rtx operand)
>>    return 0;
>>  }
>>
>> +/* Find the portion of INSN that is suitable for if-conversion.
>> +
>> +   Some Thumb mode 16-bit instructions have two variants: one that is
>> +   unconditional but clobbers the condition flags, and one that is
>> +   conditional (in an IT block) and does not clobber anything.
>> +
>> +   There are 32-bit variants that are unconditional but don't clobber
>> +   anything, so the peephole2 pass adds the clobber in order to use the
>> +   smaller instruction encoding.  Unfortunately this defeats the
>> +   if-conversion pass since CC must not be modified in an IT block.
>> +
>> +   The peephole can be reversed, and the instruction converted to
>> +   conditional execution without affecting the size optimization.
>> +
>> +   This function detects these instructions and returns the sub-expression
>> +   that contains the operation, without the clobber.  */
>> +
>> +static rtx
>> +thumb_insn_suitable_for_ifcvt (rtx pattern, rtx insn)
>> +{
>> +  if (GET_CODE (pattern) == PARALLEL
>> +      && XVECLEN (pattern, 0) == 2)
>> +    {
>> +      rtx op = XVECEXP (pattern, 0, 0);
>> +      rtx clobber = XVECEXP (pattern, 0, 1);
>> +
>> +      if (GET_CODE (clobber) == CLOBBER
>> +       && GET_CODE (XEXP (clobber, 0)) == REG
>> +       && REGNO (XEXP (clobber, 0)) == CC_REGNUM
>> +       && get_attr_it_cc (insn) == IT_CC_NOCLOBBER)
>> +     return op;
>> +    }
>> +
>> +  return NULL_RTX;
>> +}
>> +
>> +/* Prevent if-conversion thinking that certain Thumb1 instructions
>> +   clobber CC.  These instruction in fact do not when in an IT block.  */
>> +int
>> +arm_ifcvt_override_modified_test (rtx test, rtx insn)
>> +{
>> +  if (thumb_insn_suitable_for_ifcvt (PATTERN (insn), insn) != NULL_RTX)
>> +    return 0; /* Force *not* modified.  */
>> +
>> +  return -1;
>> +}
>> +
>> +/* Modify insns to enable conditional execution.
>> +
>> +   This function removes CC clobers that impede if-conversion.  See the
>> +   comments on thumb_insn_suitable_for_ifcvt.  */
>> +rtx
>> +arm_ifcvt_modify_insn (ce_if_block_t *ce, rtx pattern, rtx insn)
>> +{
>> +  rtx op, cond;
>> +
>> +  gcc_assert (GET_CODE (pattern) == COND_EXEC);
>> +
>> +  cond = XEXP (pattern, 0);
>> +  op = thumb_insn_suitable_for_ifcvt (XEXP (pattern, 1), insn);
>> +
>> +  if (op != NULL_RTX)
>> +    return gen_rtx_COND_EXEC (VOIDmode, cond, op);
>> +
>> +  return pattern;
>> +}
>> +
>>  #include "gt-arm.h"
>>
>> diff --git a/gcc/config/arm/arm.h b/gcc/config/arm/arm.h
>> index 5a78125..0b7f332 100644
>> --- a/gcc/config/arm/arm.h
>> +++ b/gcc/config/arm/arm.h
>> @@ -2220,4 +2220,9 @@ extern const char *host_detect_local_cpu (int argc, const char **argv);
>>
>>  #define DRIVER_SELF_SPECS MCPU_MTUNE_NATIVE_SPECS
>>
>> +#define IFCVT_OVERRIDE_MODIFIED_TEST(TEST,INSN) \
>> +  arm_ifcvt_override_modified_test ((TEST), (INSN))
>> +#define IFCVT_MODIFY_INSN(CE,PATTERN,INSN) \
>> +  (PATTERN) = arm_ifcvt_modify_insn ((CE), (PATTERN), (INSN))
>> +
>>  #endif /* ! GCC_ARM_H */
>> diff --git a/gcc/config/arm/thumb2.md b/gcc/config/arm/thumb2.md
>> index 05585da..454c42f 100644
>> --- a/gcc/config/arm/thumb2.md
>> +++ b/gcc/config/arm/thumb2.md
>> @@ -24,6 +24,9 @@
>>  ;; changes made in armv5t as "thumb2".  These are considered part
>>  ;; the 16-bit Thumb-1 instruction set.
>>
>> +;; Does an insn clobber CC if it appears in an IT block.  */
>> +(define_attr "it_cc" "undef,clobber,noclobber" (const_string "undef"))
>> +
>>  (define_insn "*thumb2_incscc"
>>    [(set (match_operand:SI 0 "s_register_operand" "=r,r")
>>          (plus:SI (match_operator:SI 2 "arm_comparison_operator"
>> @@ -674,6 +677,7 @@
>>     && GET_CODE(operands[3]) != MINUS"
>>    "%I3%!\\t%0, %1, %2"
>>    [(set_attr "predicable" "yes")
>> +   (set_attr "it_cc" "noclobber")
>>     (set_attr "length" "2")]
>>  )
>>
>> @@ -708,6 +712,7 @@
>>         || REG_P(operands[2]))"
>>    "* return arm_output_shift(operands, 2);"
>>    [(set_attr "predicable" "yes")
>> +   (set_attr "it_cc" "noclobber")
>>     (set_attr "shift" "1")
>>     (set_attr "length" "2")
>>     (set (attr "type") (if_then_else (match_operand 2 "const_int_operand" "")
>> @@ -736,6 +741,7 @@
>>    "TARGET_THUMB2 && reload_completed"
>>    "mov%!\t%0, %1"
>>    [(set_attr "predicable" "yes")
>> +   (set_attr "it_cc" "noclobber")
>>     (set_attr "length" "2")]
>>  )
>>
>> @@ -778,6 +784,7 @@
>>        return \"add%!\\t%0, %1, %2\";
>>    "
>>    [(set_attr "predicable" "yes")
>> +   (set_attr "it_cc" "noclobber")
>>     (set_attr "length" "2")]
>>  )
>>
>> @@ -789,6 +796,7 @@
>>    "TARGET_THUMB2 && reload_completed"
>>    "sub%!\\t%0, %1, %2"
>>    [(set_attr "predicable" "yes")
>> +   (set_attr "it_cc" "noclobber")
>>     (set_attr "length" "2")]
>>  )
>>
>> @@ -905,6 +913,7 @@
>>    "TARGET_THUMB2 && optimize_size && reload_completed"
>>    "mul%!\\t%0, %2, %0"
>>    [(set_attr "predicable" "yes")
>> +   (set_attr "it_cc" "noclobber")
>>     (set_attr "length" "2")
>>     (set_attr "insn" "muls")])
>>
>> @@ -999,6 +1008,7 @@
>>    "TARGET_THUMB2 && reload_completed"
>>    "mvn%!\t%0, %1"
>>    [(set_attr "predicable" "yes")
>> +   (set_attr "it_cc" "noclobber")
>>     (set_attr "length" "2")]
>>  )
>>
>> @@ -1022,6 +1032,7 @@
>>    "TARGET_THUMB2 && reload_completed"
>>    "neg%!\t%0, %1"
>>    [(set_attr "predicable" "yes")
>> +   (set_attr "it_cc" "noclobber")
>>     (set_attr "length" "2")]
>>  )
>>
>> diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
>> index 6d41cee..7d09ddb 100644
>> --- a/gcc/doc/tm.texi
>> +++ b/gcc/doc/tm.texi
>> @@ -10879,6 +10879,19 @@ if-statements into conditions combined by @code{and} and @code{or} operations.
>>  being processed and about to be turned into a condition.
>>  @end defmac
>>
>> +@defmac IFCVT_OVERRIDE_MODIFIED_TEST (@var{test}, @var{insn})
>> +Used if the target has instructions that modify the condition
>> +register when executed unconditionally, but do not do so when executed
>> +conditionally, or vice-versa.  @var{TEST} contains the condition test, and
>> +@var{INSN} contains the original insn to be conditionalized.  The macro
>> +should return minus-one if there is to be no override, zero to disregard
>> +an apparent modified condition register, or any other value to indicate
>> +that the instruction does modify the condition register, even if it does
>> +not appear to.  Additionally, if conditionalizing the instruction does
>> +change its behaviour, @code{IFCVT_MODIFY_INSN} should make the appropriate
>> +adjustments to match.
>> +@end defmac
>> +
>>  @defmac IFCVT_MODIFY_INSN (@var{ce_info}, @var{pattern}, @var{insn})
>>  A C expression to modify the @var{PATTERN} of an @var{INSN} that is to
>>  be converted to conditional execution format.  @var{ce_info} points to
>> diff --git a/gcc/doc/tm.texi.in b/gcc/doc/tm.texi.in
>> index 396f244..52d9e96 100644
>> --- a/gcc/doc/tm.texi.in
>> +++ b/gcc/doc/tm.texi.in
>> @@ -10759,6 +10759,19 @@ if-statements into conditions combined by @code{and} and @code{or} operations.
>>  being processed and about to be turned into a condition.
>>  @end defmac
>>
>> +@defmac IFCVT_OVERRIDE_MODIFIED_TEST (@var{test}, @var{insn})
>> +Used if the target has instructions that modify the condition
>> +register when executed unconditionally, but do not do so when executed
>> +conditionally, or vice-versa.  @var{TEST} contains the condition test, and
>> +@var{INSN} contains the original insn to be conditionalized.  The macro
>> +should return minus-one if there is to be no override, zero to disregard
>> +an apparent modified condition register, or any other value to indicate
>> +that the instruction does modify the condition register, even if it does
>> +not appear to.  Additionally, if conditionalizing the instruction does
>> +change its behaviour, @code{IFCVT_MODIFY_INSN} should make the appropriate
>> +adjustments to match.
>> +@end defmac
>> +
>>  @defmac IFCVT_MODIFY_INSN (@var{ce_info}, @var{pattern}, @var{insn})
>>  A C expression to modify the @var{PATTERN} of an @var{INSN} that is to
>>  be converted to conditional execution format.  @var{ce_info} points to
>> diff --git a/gcc/ifcvt.c b/gcc/ifcvt.c
>> index ce60ce2..ff620da 100644
>> --- a/gcc/ifcvt.c
>> +++ b/gcc/ifcvt.c
>> @@ -310,6 +310,7 @@ cond_exec_process_insns (ce_if_block_t *ce_info ATTRIBUTE_UNUSED,
>>    rtx insn;
>>    rtx xtest;
>>    rtx pattern;
>> +  int override_modified = -1;
>>
>>    if (!start || !end)
>>      return FALSE;
>> @@ -338,7 +339,18 @@ cond_exec_process_insns (ce_if_block_t *ce_info ATTRIBUTE_UNUSED,
>>        if (must_be_last)
>>       return FALSE;
>>
>> -      if (modified_in_p (test, insn))
>> +      /* Some instructions modify CC flags when executed unconditionally
>> +      but not when executed conditionally.
>> +         Return values:
>> +         -1 = no override
>> +         0 = force false
>> +         other = force true.  */
>> +#ifdef IFCVT_OVERRIDE_MODIFIED_TEST
>> +      override_modified = IFCVT_OVERRIDE_MODIFIED_TEST (test, insn);
>> +#endif
>> +
>> +      if ((override_modified == -1 && modified_in_p (test, insn))
>> +       || override_modified)
>>       {
>>         if (!mod_ok)
>>           return FALSE;
>> diff --git a/gcc/testsuite/gcc.target/arm/thumb-ifcvt.c b/gcc/testsuite/gcc.target/arm/thumb-ifcvt.c
>> new file mode 100644
>> index 0000000..b03bbce
>> --- /dev/null
>> +++ b/gcc/testsuite/gcc.target/arm/thumb-ifcvt.c
>> @@ -0,0 +1,19 @@
>> +/* Check that Thumb 16-bit shifts can be if-converted.  */
>> +/* { dg-do compile } */
>> +/* { dg-require-effective-target arm_thumb2_ok } */
>> +/* { dg-options "-O2" } */
>> +
>> +int
>> +foo (int a, int b)
>> +{
>> +  if (a != b)
>> +    {
>> +      a = a << b;
>> +      a = a >> 1;
>> +    }
>> +
>> +  return a + b;
>> +}
>> +
>> +/* { dg-final { scan-assembler "lslne" } } */
>> +/* { dg-final { scan-assembler "asrne" } } */
> 
> 
>
Andrew Stubbs - Feb. 14, 2012, 6 p.m.
On Tue 14 Feb 2012 17:35:36 GMT, Richard Earnshaw wrote:
>> Bernds checked in a patch last year (or maybe even before that) to make
>> the selection of flags clobbered insns run very late (certainly after
>> condexec had run), so I'm not sure why you're not seeing this.
>
> Hm, you mentioned some peepholes.  Try removing them....

Hmmm, it seems you're right. The machine reorg pass now takes care of 
this. Well ... that was easy!

Here's a patch to remove the obsolete peepholes. I've confirmed it works 
with the testcase.

OK?

Andrew

Patch

2012-02-14  Andrew Stubbs  <ams@codesourcery.com>

	gcc/
	* config/arm/arm-protos.h (arm_ifcvt_modify_insn): New prototype.
	(arm_ifcvt_override_modified_test): New prototype.
	* config/arm/arm.c (thumb_insn_suitable_for_ifcvt): New function.
	(arm_ifcvt_override_modified_test): New function.
	(arm_ifcvt_modify_insn): New function.
	* config/arm/arm.h (IFCVT_OVERRIDE_MODIFIED_TEST): New macro.
	(IFCVT_MODIFY_INSN): New macro.
	* config/arm/thumb2.md (it_cc): New attribute.
	(thumb2_alusi3_short): Set it_cc attribute.
	(thumb2_shiftsi3_short, thumb2_mov<mode>_shortim): Likewise.
	(thumb2_addsi_short, thumb2_subsi_short): Likewise.
	(thumb2_mulsi_short, thumb2_one_cmplsi2_short): Likewise.
	(thumb2_negsi2_short): Likewise.
	* doc/tm.texi: Regenerate.
	* doc/tm.texi.in (IFCVT_OVERRIDE_MODIFIED_TEST): Document.
	* ifcvt.c (cond_exec_process_insns): Add IFCVT_OVERRIDE_MODIFIED_TEST.

	gcc/testsuite/
	* gcc.target/arm/thumb-ifcvt.c: New test case.

---
 gcc/config/arm/arm-protos.h                |    5 ++
 gcc/config/arm/arm.c                       |   67 ++++++++++++++++++++++++++++
 gcc/config/arm/arm.h                       |    5 ++
 gcc/config/arm/thumb2.md                   |   11 +++++
 gcc/doc/tm.texi                            |   13 +++++
 gcc/doc/tm.texi.in                         |   13 +++++
 gcc/ifcvt.c                                |   14 +++++-
 gcc/testsuite/gcc.target/arm/thumb-ifcvt.c |   19 ++++++++
 8 files changed, 146 insertions(+), 1 deletions(-)
 create mode 100644 gcc/testsuite/gcc.target/arm/thumb-ifcvt.c

diff --git a/gcc/config/arm/arm-protos.h b/gcc/config/arm/arm-protos.h
index 296550a..67396f0 100644
--- a/gcc/config/arm/arm-protos.h
+++ b/gcc/config/arm/arm-protos.h
@@ -244,4 +244,9 @@  extern const struct tune_params *current_tune;
 extern int vfp3_const_double_for_fract_bits (rtx);
 #endif /* RTX_CODE */
 
+#ifdef BB_HEAD
+extern int arm_ifcvt_override_modified_test (rtx, rtx);
+extern rtx arm_ifcvt_modify_insn (ce_if_block_t *, rtx, rtx);
+#endif
+
 #endif /* ! GCC_ARM_PROTOS_H */
diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index 0bded8d..803e1c9 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -25139,5 +25139,72 @@  vfp3_const_double_for_fract_bits (rtx operand)
   return 0;
 }
 
+/* Find the portion of INSN that is suitable for if-conversion.
+
+   Some Thumb mode 16-bit instructions have two variants: one that is
+   unconditional but clobbers the condition flags, and one that is
+   conditional (in an IT block) and does not clobber anything.
+
+   There are 32-bit variants that are unconditional but don't clobber
+   anything, so the peephole2 pass adds the clobber in order to use the
+   smaller instruction encoding.  Unfortunately this defeats the
+   if-conversion pass since CC must not be modified in an IT block.
+
+   The peephole can be reversed, and the instruction converted to
+   conditional execution without affecting the size optimization.
+
+   This function detects these instructions and returns the sub-expression
+   that contains the operation, without the clobber.  */
+
+static rtx
+thumb_insn_suitable_for_ifcvt (rtx pattern, rtx insn)
+{
+  if (GET_CODE (pattern) == PARALLEL
+      && XVECLEN (pattern, 0) == 2)
+    {
+      rtx op = XVECEXP (pattern, 0, 0);
+      rtx clobber = XVECEXP (pattern, 0, 1);
+
+      if (GET_CODE (clobber) == CLOBBER
+	  && GET_CODE (XEXP (clobber, 0)) == REG
+	  && REGNO (XEXP (clobber, 0)) == CC_REGNUM
+	  && get_attr_it_cc (insn) == IT_CC_NOCLOBBER)
+	return op;
+    }
+
+  return NULL_RTX;
+}
+
+/* Prevent if-conversion thinking that certain Thumb1 instructions
+   clobber CC.  These instruction in fact do not when in an IT block.  */
+int
+arm_ifcvt_override_modified_test (rtx test, rtx insn)
+{
+  if (thumb_insn_suitable_for_ifcvt (PATTERN (insn), insn) != NULL_RTX)
+    return 0; /* Force *not* modified.  */
+
+  return -1;
+}
+
+/* Modify insns to enable conditional execution.
+
+   This function removes CC clobers that impede if-conversion.  See the
+   comments on thumb_insn_suitable_for_ifcvt.  */
+rtx
+arm_ifcvt_modify_insn (ce_if_block_t *ce, rtx pattern, rtx insn)
+{
+  rtx op, cond;
+
+  gcc_assert (GET_CODE (pattern) == COND_EXEC);
+
+  cond = XEXP (pattern, 0);
+  op = thumb_insn_suitable_for_ifcvt (XEXP (pattern, 1), insn);
+
+  if (op != NULL_RTX)
+    return gen_rtx_COND_EXEC (VOIDmode, cond, op);
+
+  return pattern;
+}
+
 #include "gt-arm.h"
 
diff --git a/gcc/config/arm/arm.h b/gcc/config/arm/arm.h
index 5a78125..0b7f332 100644
--- a/gcc/config/arm/arm.h
+++ b/gcc/config/arm/arm.h
@@ -2220,4 +2220,9 @@  extern const char *host_detect_local_cpu (int argc, const char **argv);
 
 #define DRIVER_SELF_SPECS MCPU_MTUNE_NATIVE_SPECS
 
+#define IFCVT_OVERRIDE_MODIFIED_TEST(TEST,INSN) \
+  arm_ifcvt_override_modified_test ((TEST), (INSN))
+#define IFCVT_MODIFY_INSN(CE,PATTERN,INSN) \
+  (PATTERN) = arm_ifcvt_modify_insn ((CE), (PATTERN), (INSN))
+
 #endif /* ! GCC_ARM_H */
diff --git a/gcc/config/arm/thumb2.md b/gcc/config/arm/thumb2.md
index 05585da..454c42f 100644
--- a/gcc/config/arm/thumb2.md
+++ b/gcc/config/arm/thumb2.md
@@ -24,6 +24,9 @@ 
 ;; changes made in armv5t as "thumb2".  These are considered part
 ;; the 16-bit Thumb-1 instruction set.
 
+;; Does an insn clobber CC if it appears in an IT block.  */
+(define_attr "it_cc" "undef,clobber,noclobber" (const_string "undef"))
+
 (define_insn "*thumb2_incscc"
   [(set (match_operand:SI 0 "s_register_operand" "=r,r")
         (plus:SI (match_operator:SI 2 "arm_comparison_operator"
@@ -674,6 +677,7 @@ 
    && GET_CODE(operands[3]) != MINUS"
   "%I3%!\\t%0, %1, %2"
   [(set_attr "predicable" "yes")
+   (set_attr "it_cc" "noclobber")
    (set_attr "length" "2")]
 )
 
@@ -708,6 +712,7 @@ 
        || REG_P(operands[2]))"
   "* return arm_output_shift(operands, 2);"
   [(set_attr "predicable" "yes")
+   (set_attr "it_cc" "noclobber")
    (set_attr "shift" "1")
    (set_attr "length" "2")
    (set (attr "type") (if_then_else (match_operand 2 "const_int_operand" "")
@@ -736,6 +741,7 @@ 
   "TARGET_THUMB2 && reload_completed"
   "mov%!\t%0, %1"
   [(set_attr "predicable" "yes")
+   (set_attr "it_cc" "noclobber")
    (set_attr "length" "2")]
 )
 
@@ -778,6 +784,7 @@ 
       return \"add%!\\t%0, %1, %2\";
   "
   [(set_attr "predicable" "yes")
+   (set_attr "it_cc" "noclobber")
    (set_attr "length" "2")]
 )
 
@@ -789,6 +796,7 @@ 
   "TARGET_THUMB2 && reload_completed"
   "sub%!\\t%0, %1, %2"
   [(set_attr "predicable" "yes")
+   (set_attr "it_cc" "noclobber")
    (set_attr "length" "2")]
 )
 
@@ -905,6 +913,7 @@ 
   "TARGET_THUMB2 && optimize_size && reload_completed"
   "mul%!\\t%0, %2, %0"
   [(set_attr "predicable" "yes")
+   (set_attr "it_cc" "noclobber")
    (set_attr "length" "2")
    (set_attr "insn" "muls")])
 
@@ -999,6 +1008,7 @@ 
   "TARGET_THUMB2 && reload_completed"
   "mvn%!\t%0, %1"
   [(set_attr "predicable" "yes")
+   (set_attr "it_cc" "noclobber")
    (set_attr "length" "2")]
 )
 
@@ -1022,6 +1032,7 @@ 
   "TARGET_THUMB2 && reload_completed"
   "neg%!\t%0, %1"
   [(set_attr "predicable" "yes")
+   (set_attr "it_cc" "noclobber")
    (set_attr "length" "2")]
 )
 
diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
index 6d41cee..7d09ddb 100644
--- a/gcc/doc/tm.texi
+++ b/gcc/doc/tm.texi
@@ -10879,6 +10879,19 @@  if-statements into conditions combined by @code{and} and @code{or} operations.
 being processed and about to be turned into a condition.
 @end defmac
 
+@defmac IFCVT_OVERRIDE_MODIFIED_TEST (@var{test}, @var{insn})
+Used if the target has instructions that modify the condition
+register when executed unconditionally, but do not do so when executed
+conditionally, or vice-versa.  @var{TEST} contains the condition test, and
+@var{INSN} contains the original insn to be conditionalized.  The macro
+should return minus-one if there is to be no override, zero to disregard
+an apparent modified condition register, or any other value to indicate
+that the instruction does modify the condition register, even if it does
+not appear to.  Additionally, if conditionalizing the instruction does
+change its behaviour, @code{IFCVT_MODIFY_INSN} should make the appropriate
+adjustments to match.
+@end defmac
+
 @defmac IFCVT_MODIFY_INSN (@var{ce_info}, @var{pattern}, @var{insn})
 A C expression to modify the @var{PATTERN} of an @var{INSN} that is to
 be converted to conditional execution format.  @var{ce_info} points to
diff --git a/gcc/doc/tm.texi.in b/gcc/doc/tm.texi.in
index 396f244..52d9e96 100644
--- a/gcc/doc/tm.texi.in
+++ b/gcc/doc/tm.texi.in
@@ -10759,6 +10759,19 @@  if-statements into conditions combined by @code{and} and @code{or} operations.
 being processed and about to be turned into a condition.
 @end defmac
 
+@defmac IFCVT_OVERRIDE_MODIFIED_TEST (@var{test}, @var{insn})
+Used if the target has instructions that modify the condition
+register when executed unconditionally, but do not do so when executed
+conditionally, or vice-versa.  @var{TEST} contains the condition test, and
+@var{INSN} contains the original insn to be conditionalized.  The macro
+should return minus-one if there is to be no override, zero to disregard
+an apparent modified condition register, or any other value to indicate
+that the instruction does modify the condition register, even if it does
+not appear to.  Additionally, if conditionalizing the instruction does
+change its behaviour, @code{IFCVT_MODIFY_INSN} should make the appropriate
+adjustments to match.
+@end defmac
+
 @defmac IFCVT_MODIFY_INSN (@var{ce_info}, @var{pattern}, @var{insn})
 A C expression to modify the @var{PATTERN} of an @var{INSN} that is to
 be converted to conditional execution format.  @var{ce_info} points to
diff --git a/gcc/ifcvt.c b/gcc/ifcvt.c
index ce60ce2..ff620da 100644
--- a/gcc/ifcvt.c
+++ b/gcc/ifcvt.c
@@ -310,6 +310,7 @@  cond_exec_process_insns (ce_if_block_t *ce_info ATTRIBUTE_UNUSED,
   rtx insn;
   rtx xtest;
   rtx pattern;
+  int override_modified = -1;
 
   if (!start || !end)
     return FALSE;
@@ -338,7 +339,18 @@  cond_exec_process_insns (ce_if_block_t *ce_info ATTRIBUTE_UNUSED,
       if (must_be_last)
 	return FALSE;
 
-      if (modified_in_p (test, insn))
+      /* Some instructions modify CC flags when executed unconditionally
+	 but not when executed conditionally.
+         Return values:
+	    -1 = no override
+	    0 = force false
+	    other = force true.  */
+#ifdef IFCVT_OVERRIDE_MODIFIED_TEST
+      override_modified = IFCVT_OVERRIDE_MODIFIED_TEST (test, insn);
+#endif
+
+      if ((override_modified == -1 && modified_in_p (test, insn))
+	  || override_modified)
 	{
 	  if (!mod_ok)
 	    return FALSE;
diff --git a/gcc/testsuite/gcc.target/arm/thumb-ifcvt.c b/gcc/testsuite/gcc.target/arm/thumb-ifcvt.c
new file mode 100644
index 0000000..b03bbce
--- /dev/null
+++ b/gcc/testsuite/gcc.target/arm/thumb-ifcvt.c
@@ -0,0 +1,19 @@ 
+/* Check that Thumb 16-bit shifts can be if-converted.  */
+/* { dg-do compile } */
+/* { dg-require-effective-target arm_thumb2_ok } */
+/* { dg-options "-O2" } */
+
+int
+foo (int a, int b)
+{
+  if (a != b)
+    {
+      a = a << b;
+      a = a >> 1;
+    }
+
+  return a + b;
+}
+
+/* { dg-final { scan-assembler "lslne" } } */
+/* { dg-final { scan-assembler "asrne" } } */