[LLVMdev] Missing optimization in constant propagation?

[Eli Friedman]

On Fri, Jul 15, 2011 at 12:21 AM, Martin Apel <martin.apel at simpack.de> wrote:
> Hi all,
>
> I stumbled across a peculiarity regarding constant propagation that I don't understand. I'm not sure, if I oversee anything or if it's a missing feature.
>
> I have created the following simple test function in C:
>
> int times_zero(int a)
> {
>  return (a * 0);
> }
>
> Compiling this with GCC using dragonegg generates the following code:
>
> %int = type i32
>
> define i32 @times_zero(i32 %a) nounwind {
> entry:
>  %a_addr = alloca i32, align 4
>  %memtmp = alloca i32
>  %"alloca point" = bitcast i32 0 to i32
>  store i32 %a, i32* %a_addr
>  %"ssa point" = bitcast i32 0 to i32
>  br label %"2"
>
> "2":                                              ; preds = %entry
>  store i32 0, i32* %memtmp, align 1
>  br label %return
>
> return:                                           ; preds = %"2"
>  %retval = load i32* %memtmp
>  ret i32 %retval
> }
>
> Running this through "opt -O2" generates
>
> define i32 @times_zero(i32 %a) nounwind readnone {
> entry:
>  ret i32 0
> }
>
> So far everything works as expected. LLVM recognizes the multiplication by zero and replaces the multiplication by its result zero.
> Doing the same for doubles however does not do the same optimization.
>
> double times_zero(double a)
> {
>  return (a * 0.0);
> }
>
>
> Instead the following code is generated after optimization:
> define double @times_zero(double %a) nounwind readnone {
> entry:
>  %0 = fmul double %a, 0.000000e+00
>  ret double %0
> }
>
> Is there a reason, why this optimization opportunity is not taken? For the record, I am using LLVM 2.9

Because that isn't how FP arithmetic works; 1.0*0.0 is 0.0, but
(-1.0)*0.0 is -0.0, and (0.0/0.0)*0.0 is NaN.

-Eli