Do not drop fast-math flags in SimplifyBinOp

[Michael Zolotukhin]

> On Jan 29, 2015, at 4:21 PM, Chandler Carruth <chandlerc at google.com> wrote:
> 
> Do we use the context instruction to extract fast math flags elsewhere?
> 
> I wouldn't expect that to be reliably at all. Nothing *requires* the context instruction to be the one governing the semantics.
> 
> I think we need to pass the fast math flags directly into SimplifyBinOp for this to work.

I tried to avoid adding a new argument, but you are right: using the context instruction isn’t appropriate for this.

Updated patch is attached, is it better?



Michael

> 
> On Thu, Jan 29, 2015 at 4:12 PM, Michael Zolotukhin <mzolotukhin at apple.com <mailto:mzolotukhin at apple.com>> wrote:
> Hi,
> 
> Currently InlineCost pass fails to use fast-math flags when analyzing profitability of inlining. Here is a small example to demonstrate that:
> 
> float foo(float *a, float b) {
>   float r;
>   if (a[1] * b)
>     r = a[1]+a[2]/a[3]-a[4]*a[5]+a[6]-a[7]+a[8]*a[9]+
>             a[11]+a[12]/a[13]-a[14]*a[15]+a[16]-a[17]+a[18]*a[19]+
>             a[21]+a[22]/a[23]-a[24]*a[25]+a[26]-a[27]+a[28]*a[29]+
>             a[31]+a[32]/a[33]-a[34]*a[35]+a[36]-a[37]+a[38]*a[39]+
>             a[41]+a[42]/a[43]-a[44]*a[45]+a[46]-a[47]+a[48]*a[49]+
>             a[51]+a[52]/a[53]-a[54]*a[55]+a[56]-a[57]+a[58]*a[59]+
>             a[61]+a[62]/a[63]-a[64]*a[65]+a[66]-a[67]+a[68]*a[69]+
>             a[0]*a[0]+b;
>   else
>     r = 1;
>   return r;
> }
> float boo(float *a) {
>   return foo(a, 0.0);
> }
> 
> Here inliner can’t figure out that a[1]*b can be folded to 0 if the call is inlined, and decides not to inline.
> 
> That happens because inliner drops fast-math flags in SimplifyBinOp. The attached patch fixes it by looking into the instruction math flags. Does it look good?
> 
> Without patch:
> > clang inline.c -O3 -S -Rpass=inline -ffast-math
> // Empty output
> 
> With patch:
> > clang inline.c -O3 -S -Rpass=inline -ffast-math
> inline.c:17:10: remark: foo inlined into boo [-Rpass=inline]
>   return foo(a, 0.0);
>          ^
> 
> 
> 
> 
> Thanks,
> Michael
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