[LLVMdev] Inlining sqrt library function in X86

[Nadav Rotem]

Does fast-math imply no-math-errno ?

Thanks,
Nadav

On May 17, 2013, at 15:36, Chris Lattner <clattner at apple.com> wrote:

> 
> On May 17, 2013, at 3:33 PM, "Gurd, Preston" <preston.gurd at intel.com> wrote:
> 
>> Using the following example program
>>  
>> #include <math.h>
>>  
>> double f(double d){
>>   return sqrt(d);
>> }
>>  
>> and compiling it with “clang –O3 …”, I was trying to determine what it would take to get the X86 code generator to replace the call to sqrt with a sqrtsd instruction inline.
>>  
>> It turns out that it could do exactly that, were it not for the fact that in the function visitUnaryFloatCall() at line 5514 in SelectionDAGBuilder.cpp, the result of
>>  
>> !I.onlyReadsMemory()
>>  
>> Is true, so the code is unable to replace the function call with an ISD::FSQRT SDNode. If I remove the above test, then the compiler will emit a sqrtsd instruction.
>>  
>> I am hoping that someone might be able to comment on what onlyReadsMemory is supposed to do be doing in general and why it is returning false in this case.
> 
> The compiler is being conservative about errno (which sqrt in libm can set, but "sqrtss" doesn't).  Try building with -fno-math-errno.
> 
> -Chris
> 
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