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

[Jakob Stoklund Olesen]

On Jan 2, 2013, at 10:08 PM, Craig Topper <craig.topper at gmail.com> wrote:

> Does this apply to other targets too? The ARM divide instructions aren't marked as having side effects for instance.

It's complicated ;-)

AFAIK, on ARMv7-A division by zero doesn't trap, but simply returns 0 instead. On ARMv7-R, division by zero can be configured to raise an undefined instruction exception, but I don't know if any targets use that.

Floating point division by zero can also be configured to trap, but normally just sets a bit in the FPSCR.

/jakob



> On Wed, Jan 2, 2013 at 12:58 PM, Jakob Stoklund Olesen <stoklund at 2pi.dk> wrote:
> 
> On Jan 2, 2013, at 11:44 AM, Craig Topper <craig.topper at gmail.com> wrote:
> 
>> OK. Should we more explicitly set "hasSideEffects = 1" then so that its not subtly happening due to lack of a pattern?
> 
> Yes, I think that is best. Ideally, all targets should work with guessInstructionProperties = 0. I would like to make that the default.
> 
> You can add the test case to test/CodeGen/X86/early-ifcvt.ll 
> 
> Thanks,
> /jakob
> 
> 
>> 
>> On Wed, Jan 2, 2013 at 11:11 AM, Jakob Stoklund Olesen <stoklund at 2pi.dk> wrote:
>> 
>> 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
>> 
>> 
>> 
>> 
>> -- 
>> ~Craig
> 
> 
> 
> 
> -- 
> ~Craig

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20130103/24fc3f34/attachment.html>