[LLVMdev] Missing optimization in constant propagation?

[Martin Apel]

On 15/07/11 09:36, Eli Friedman wrote:
> 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
> .
>
Hi Eli,

thanks for the enlightenment! I did not think of this, but you are completely right. Floating point arithmetic is kind of strange sometimes.

Martin