[LLVMdev] Min and max

[Chris Lattner]

On May 27, 2008, at 2:49 AM, Nicolas Capens wrote:

> Hi all,
>
> I’m trying to implement a floating-point ‘min’ and ‘max’ operation  
> using select. For ‘min’ I get the expected x86 assembly minss  
> instruction, but for ‘max’ I get a branch instead of maxss.
>
> The corresponding C syntax code looks like this:
>
> float z = (x > y) ? x : y;
>
> Any clues?

Your code is not safe for NaNs.  This is the correct way to write  
maxss in C:

float max(float x, float y) {
   return !(x < y) ? x : y;
}

If you don't care about NaNs, you can pass -ffast-math to llvm-gcc, or  
set "UnsafeFPMath=true" from <llvm/Target/TargetOptions.h>

> Could someone maybe explain to me the basics of LLVM’s target  
> specific optimizations and code generation? I’d love to analyze  
> things like this myself but I don’t know where to start.

This one specifically boils down to the semantics of maxss and LLVM IR  
instructions.  For example, this code:

float not_max(float x, float y) {
   return (x > y) ? x : y;
}

float really_max(float x, float y) {
   return !(x < y) ? x : y;
}

compiles into this LLVM IR (llvm-gcc t.c -S -o - -O -emit-llvm):

define float @not_max(float %x, float %y) nounwind  {
entry:
	%tmp3 = fcmp ogt float %x, %y		; <i1> [#uses=1]
	%iftmp.0.0 = select i1 %tmp3, float %x, float %y		; <float> [#uses=1]
	ret float %iftmp.0.0
}

define float @really_max(float %x, float %y) nounwind  {
entry:
	%tmp3 = fcmp uge float %x, %y		; <i1> [#uses=1]
	%iftmp.1.0 = select i1 %tmp3, float %x, float %y		; <float> [#uses=1]
	ret float %iftmp.1.0
}

If you're interested in target-specific x86 optimizations to be done,  
take a look at lib/Target/X86/README*.txt

-Chris
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