llvm dev - May 2013 - [LLVMdev] Inlining sqrt library function in X86

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.

Thanks!

Preston

--
Preston Gurd <preston.gurd at intel.com>
  Intel Waterloo

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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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If you want sqrtss to be used without fast-math you can generate sqrtss and a
call to sqrtf if NaN is returned. This is how gcc appears to handle it.

Paul

From: <Gurd>, Preston <preston.gurd at intel.com<mailto:preston.gurd
at intel.com>>
Date: Friday, 17 May, 2013 6:33 PM
To: "LLVMdev (LLVMdev at cs.uiuc.edu<mailto:LLVMdev at
cs.uiuc.edu>)" <LLVMdev at cs.uiuc.edu<mailto:LLVMdev at
cs.uiuc.edu>>
Subject: [LLVMdev] Inlining sqrt library function in X86

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.

Thanks!

Preston

--
Preston Gurd <preston.gurd at intel.com<mailto:preston.gurd at
intel.com>>
  Intel Waterloo

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
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
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