Improve add/sub double word splitters (PR rtl-optimization/70467)

Hi!

As the testcase below shows, we generate awful code for double word
additions/subtractions if the last argument is a constant that has the
whole low word 0 (and only nonzero some of the upper bits).
In that case, there is no point doing useless addl $0, ... followed by
adcl $something, ... because the addition won't change anything and the
carry flag will be also clear; we can just add the high word to the high
part.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2016-04-01  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/70467
	* config/i386/i386.md (*add<dwi>3_doubleword, *sub<dwi>3_doubleword):
	If low word of the last operand is 0, just emit addition/subtraction
	for the high word.

	* gcc.target/i386/pr70467-2.c: New test.


	Jakub

On Fri, Apr 1, 2016 at 3:18 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> Hi!
>
> As the testcase below shows, we generate awful code for double word
> additions/subtractions if the last argument is a constant that has the
> whole low word 0 (and only nonzero some of the upper bits).
> In that case, there is no point doing useless addl $0, ... followed by
> adcl $something, ... because the addition won't change anything and the
> carry flag will be also clear; we can just add the high word to the high
> part.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>
> 2016-04-01  Jakub Jelinek  <jakub@redhat.com>
>
>         PR rtl-optimization/70467
>         * config/i386/i386.md (*add<dwi>3_doubleword, *sub<dwi>3_doubleword):
>         If low word of the last operand is 0, just emit addition/subtraction
>         for the high word.
>
>         * gcc.target/i386/pr70467-2.c: New test.

OK with a small testcase adjustment.

Thanks,
Uros.

> --- gcc/config/i386/i386.md.jj  2016-03-29 19:31:23.000000000 +0200
> +++ gcc/config/i386/i386.md     2016-03-31 17:33:36.848167239 +0200
> @@ -5449,7 +5449,14 @@ (define_insn_and_split "*add<dwi>3_doubl
>                        (match_dup 4))
>                      (match_dup 5)))
>               (clobber (reg:CC FLAGS_REG))])]
> -  "split_double_mode (<DWI>mode, &operands[0], 3, &operands[0], &operands[3]);")
> +{
> +  split_double_mode (<DWI>mode, &operands[0], 3, &operands[0], &operands[3]);
> +  if (operands[2] == const0_rtx)
> +    {
> +      ix86_expand_binary_operator (PLUS, <MODE>mode, &operands[3]);
> +      DONE;
> +    }
> +})
>
>  (define_insn "*add<mode>_1"
>    [(set (match_operand:SWI48 0 "nonimmediate_operand" "=r,rm,r,r")
> @@ -6379,7 +6386,14 @@ (define_insn_and_split "*sub<dwi>3_doubl
>                        (ltu:DWIH (reg:CC FLAGS_REG) (const_int 0)))
>                      (match_dup 5)))
>               (clobber (reg:CC FLAGS_REG))])]
> -  "split_double_mode (<DWI>mode, &operands[0], 3, &operands[0], &operands[3]);")
> +{
> +  split_double_mode (<DWI>mode, &operands[0], 3, &operands[0], &operands[3]);
> +  if (operands[2] == const0_rtx)
> +    {
> +      ix86_expand_binary_operator (MINUS, <MODE>mode, &operands[3]);
> +      DONE;
> +    }
> +})
>
>  (define_insn "*sub<mode>_1"
>    [(set (match_operand:SWI 0 "nonimmediate_operand" "=<r>m,<r>")
> --- gcc/testsuite/gcc.target/i386/pr70467-2.c.jj        2016-04-01 12:29:15.611785157 +0200
> +++ gcc/testsuite/gcc.target/i386/pr70467-2.c   2016-04-01 12:31:19.980092446 +0200
> @@ -0,0 +1,20 @@
> +/* PR rtl-optimization/70467 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2" } */
> +
> +unsigned long long
> +foo (unsigned long long x)
> +{
> +  return x + 0x12345600000000ULL;
> +}
> +
> +unsigned long long
> +bar (unsigned long long x)
> +{
> +  return x - 0x12345600000000ULL;
> +}
> +
> +/* { dg-final { scan-assembler-not "addl\[ \t\]*.0," { target ia32 } } } */
> +/* { dg-final { scan-assembler-not "subl\[ \t\]*.0," { target ia32 } } } */
> +/* { dg-final { scan-assembler-not "adcl\[^\n\r\]*%" { target ia32 } } } */
> +/* { dg-final { scan-assembler-not "sbbl\[^\n\r\]*%" { target ia32 } } } */

Please compile the test only for ia32 target. The purpose of the test
is to scan assembly on ia32, and obviously, there is no point to
compile it on x86_64.