[llvm-dev] Why LLVM cannot optimize this?

[Hongbin Zheng via llvm-dev]

if you replace "zero *= a;" by "zero += a;",  you get:

; Function Attrs: norecurse nounwind readnone uwtable
define i32 @foo(i32 %a) #0 {
entry:
  %0 = mul i32 %a, 10000
  ret i32 %0
}

I think the problem is ScalarEvolution only have "SCEVAddRecExpr" for zero
+= a, but no corresponding expression to represent and optimize zero *= a;



On Wed, Mar 2, 2016 at 3:24 PM, zet via llvm-dev <llvm-dev at lists.llvm.org>
wrote:

> test.c :
> -----------------------------------------------------------------------
> int foo(int a)
> {
>   int zero = 0;
>   for (int i = 0; i < 10000; i++)
>     zero *= a;
>   return zero;
> }
> -------------------------------------------------------------------------
>
> run clang : clang -O2 -S test.c -o test.s
>
> My clang version is 3.7.1.
> We will get a horrible assembly output.
>
> Why constant propagation and other optimization skills can not find out
> that variable zero is initialized 0, and the only statement in for loop
> (i.e. zero *= a) always get a 0?
>
> I can read the clang/llvm source, Please tell me more details.
>
> THX
>
>
> --
> 业精于勤,荒于嬉..
>
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