llvm dev - Mar 2010 - [LLVMdev] constant folding for standard math functions

Hi!

I'd like to replace all calls to standard math functions (e.g. sin(0.5)) by
their result.
What strategy do you recommend?
Should I write a pass that does only this or should I copy and
modify the SCCP pass?

A problem with an extra pass could be that that I need to alternate
my pass and SCCP several times since the results of the math functions
could be folded again.

-Jochen

On Mar 1, 2010, at 9:44 AM, Jochen Wilhelmy wrote:
> Hi!
> 
> I'd like to replace all calls to standard math functions (e.g.
sin(0.5)) by
> their result.
> What strategy do you recommend?
> Should I write a pass that does only this or should I copy and
> modify the SCCP pass?
> 
> A problem with an extra pass could be that that I need to alternate
> my pass and SCCP several times since the results of the math functions
> could be folded again.
sin/cos etc should already be handled by lib/Analysis/ConstantFolding.cpp.

-Chris

Hi!

> sin/cos etc should already be handled by lib/Analysis/ConstantFolding.cpp.
>    
Thanks for the hint and it works!
Now I have a new Problem:

I have this function:

float foo(float a, float b)
{
     float x = a * b * 0.0f;
     return cos(0.5) * sin(0.5) * x;
};

after compiling it with clang (cpp mode) and renaming _ZSt3sinf to sin
and _ZSt3cosf to cos I get the following:

define float @_Z3fooff(float %a, float %b) nounwind {
entry:
   %mul = fmul float %a, %b                        ; <float> [#uses=1]
   %mul2 = fmul float %mul, 0.000000e+000          ; <float> [#uses=1]
   %mul6 = fmul float 0x3FDAED54A0000000, %mul2    ; <float> [#uses=1]
   ret float %mul6
}

the sin and cos calls are folded, but not the mul by zero.
May be this is missing in llvm::ConstantFoldInstOperands in 
ConsantFolding.cpp?

I would expect the following optimizations, but didn't find them in the 
code:
x  + 0 = x
x * 0 = 0
x * 1 = 1
x * -1 = -x

-Jochen