[llvm-commits] [llvm] r171136 - /llvm/trunk/lib/Target/X86/X86InstrArithmetic.td

[Jakob Stoklund Olesen]

On Jan 2, 2013, at 11:04 AM, Jakob Stoklund Olesen <stoklund at 2pi.dk> wrote:

> 
> On Dec 26, 2012, at 7:01 PM, Craig Topper <craig.topper at gmail.com> wrote:
> 
>> Author: ctopper
>> Date: Wed Dec 26 21:01:18 2012
>> New Revision: 171136
>> 
>> URL: http://llvm.org/viewvc/llvm-project?rev=171136&view=rev
>> Log:
>> Mark the divide instructions as hasSideEffects=0.
> 
> Hi Craig,
> 
> I don't think this is safe because div instructions can trap when the divisor is 0, and that means it is not safe to move them between basic blocks. See also llvm::isSafeToSpeculativelyExecute().

For example:

$ cat unsafe-div.c 
int f(int a, int b) {
  return b ? a/b : a;
}

$ Release+Asserts/bin/clang -O -o- -S unsafe-div.c -mllvm -x86-early-ifcvt -mllvm -stress-early-ifcvt
	.section	__TEXT,__text,regular,pure_instructions
	.globl	_f
	.align	4, 0x90
_f:                                     ## @f
	.cfi_startproc
## BB#0:                                ## %entry
	pushq	%rbp
Ltmp2:
	.cfi_def_cfa_offset 16
Ltmp3:
	.cfi_offset %rbp, -16
	movq	%rsp, %rbp
Ltmp4:
	.cfi_def_cfa_register %rbp
	movl	%edi, %eax
	cltd
	idivl	%esi
	testl	%esi, %esi
	cmovel	%edi, %eax
	popq	%rbp
	ret
	.cfi_endproc


.subsections_via_symbols