| Message ID | 3316944.sc4MgGFZdG@polaris |
|---|---|
| State | New |
| Headers | show |
On Fri, Apr 25, 2014 at 12:47:32PM +0200, Eric Botcazou wrote: > /* PR target/60941 */ > /* Reported by Martin Husemann <martin@netbsd.org> */ > > extern void abort (void); > > static void __attribute__((noinline)) > set (unsigned long *l) > { > *l = 31; > } > > int main (void) > { > unsigned long l; > int i; > > set (&l); > i = (int) l; > l = (unsigned long)(2U << i); > if (l != 0) > abort (); > return 0; > } I'm afraid the testcase will fail on int16 targets, so perhaps you should guard the body of main other than return 0; with #if __INT_MAX__ >= 2147483647ULL or #if __SIZEOF_INT__ >= 4 or similar (or require int32plus target, but gcc.c-torture/execute/ is not a good place for that and *.x files are terribly ugly, so better would be to move the test to gcc.dg/torture/ in that case). Jakub
> I'm afraid the testcase will fail on int16 targets, so perhaps > you should guard the body of main other than return 0; with > #if __INT_MAX__ >= 2147483647ULL > or > #if __SIZEOF_INT__ >= 4 > or similar (or require int32plus target, but gcc.c-torture/execute/ > is not a good place for that and *.x files are terribly ugly, so > better would be to move the test to gcc.dg/torture/ in that case). Not clear to me, (2U << i) should be zero if the shift count is masked.
On Fri, Apr 25, 2014 at 01:24:13PM +0200, Eric Botcazou wrote: > > I'm afraid the testcase will fail on int16 targets, so perhaps > > you should guard the body of main other than return 0; with > > #if __INT_MAX__ >= 2147483647ULL > > or > > #if __SIZEOF_INT__ >= 4 > > or similar (or require int32plus target, but gcc.c-torture/execute/ > > is not a good place for that and *.x files are terribly ugly, so > > better would be to move the test to gcc.dg/torture/ in that case). > > Not clear to me, (2U << i) should be zero if the shift count is masked. 2U << 31 is undefined behavior on those targets. Jakub
> > Not clear to me, (2U << i) should be zero if the shift count is masked. > > 2U << 31 is undefined behavior on those targets. Precisely not, or else we are not talking about the same notion of masking.
Eric Botcazou writes: > > > Not clear to me, (2U << i) should be zero if the shift count is masked. > > > > 2U << 31 is undefined behavior on those targets. > > Precisely not, or else we are not talking about the same notion of masking. I believe Jakub is referring to the following in the C standard: "Bitwise shift operators ... Semantics ... If the value of the right operand ... is greater than or equal to the width of the promoted left operand, the behavior is undefined." So on 16-bit int systems you can't portably shift 2U by more than 15. (I'm not worried about the HW blowing up, but GCC itself has a long history of exploiting no-undefinedness assumptions.) /Mikael
On Sat, Apr 26, 2014 at 03:30:25PM +0200, Eric Botcazou wrote: > > > Not clear to me, (2U << i) should be zero if the shift count is masked. > > > > 2U << 31 is undefined behavior on those targets. > > Precisely not, or else we are not talking about the same notion of masking. Eh, C99, 6.5.7/3: "If the value of the right operand is negative or is greater than or equal to the width of the promoted left operand, the behavior is undefined." So, if you have int16 target, where unsigned int is 16-bit and UINT_MAX 65535, then shift count must be >= 0 and < 16, therefore, 2U << 31 is undefined behavior. Jakub
> So, if you have int16 target, where unsigned int is 16-bit and UINT_MAX > 65535, then shift count must be >= 0 and < 16, therefore, 2U << 31 is > undefined behavior. Well, if the shift count is masked by the target, if will be masked according to the width of the register and the result will nevertheless be zero.
> I believe Jakub is referring to the following in the C standard: > > "Bitwise shift operators > ... > Semantics > ... If the value of the right operand ... is greater than or equal to the > width of the promoted left operand, the behavior is undefined." > > So on 16-bit int systems you can't portably shift 2U by more than 15. Sure, but we're talking about targets for which the shift count is masked, not about the C standard.
Index: config/sparc/sparc.md =================================================================== --- config/sparc/sparc.md (revision 209778) +++ config/sparc/sparc.md (working copy) @@ -5795,19 +5795,6 @@ (define_insn "ashlsi3" } [(set_attr "type" "shift")]) -(define_insn "*ashlsi3_extend" - [(set (match_operand:DI 0 "register_operand" "=r") - (zero_extend:DI - (ashift:SI (match_operand:SI 1 "register_operand" "r") - (match_operand:SI 2 "arith_operand" "rI"))))] - "TARGET_ARCH64" -{ - if (GET_CODE (operands[2]) == CONST_INT) - operands[2] = GEN_INT (INTVAL (operands[2]) & 0x1f); - return "sll\t%1, %2, %0"; -} - [(set_attr "type" "shift")]) - (define_expand "ashldi3" [(set (match_operand:DI 0 "register_operand" "=r") (ashift:DI (match_operand:DI 1 "register_operand" "r")