[LLVMdev] Fast-math flags in constant expressions

[Michael Ilseman]

Out of curiosity, how would you envision fast-math flags interacting with constant expressions? Off the top of my head, I can’t think of any flags that would be relevant if the expression can just be constant-folded away at full precision anyways.

> On Nov 28, 2014, at 4:56 AM, Sergey Dmitrouk <sdmitrouk at accesssoftek.com> wrote:
> 
> Doesn't look like a bug, more like a limitation of constant expressions
> as llvm::BinaryConstantExpr doesn't have FastMastFlags.
> 
> -- 
> Sergey
> 
> On Thu, Nov 27, 2014 at 03:39:20PM +0200, Sergey Dmitrouk wrote:
>> Hi,
>> 
>> I'm wondering why lib/AsmParser/LLParser handles fast-math flags in the
>> following IR:
>> 
>>    ...
>>    %val = fmul nnan double 1.0, 1.0
>>    ...
>> 
>> but doesn't allow any flags if "fmul" is inside "phi":
>> 
>>    ...
>>    %val = phi double [ fmul (double 1.0, double 1.0), %cond.true ],
>>                      [ fmul (double 1.0, double 1.0), %cond.false ]
>>    ...
>> 
>> LLParser::ParseValID(...) could call EatFastMathFlagsIfPresent() to
>> handle fast-math flags, but it doesn't.  Is this intended or a bug?
>> 
>> Regards,
>> Sergey
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