[LLVMdev] LLVM constant propagation optimization question

[Duncan Sands]

Hi Seb,

> I'm writting following LLVM assembly:
>
> ; ModuleID = 'structaccess.ll'
>

not having a data layout string in your module disables many optimizations.

Ciao, Duncan.

> %struct._anon0 = type <{ i32, i32, i32 }>
>
> @s = common global %struct._anon0 zeroinitializer
>
> define arm_aapcscc void @foo() nounwind {
> L.entry:
>    store i32 5, i32* getelementptr inbounds (%struct._anon0* @s, i32 0, i32 0)
>    store i32 10, i32* getelementptr inbounds (%struct._anon0* @s, i32 0, i32 1)
>    %0 = load i32* getelementptr inbounds (%struct._anon0* @s, i32 0, i32 0)
>    %1 = load i32* getelementptr inbounds (%struct._anon0* @s, i32 0, i32 1)
>    %2 = add i32 %0, %1
>    store i32 %2, i32* getelementptr inbounds (%struct._anon0* @s, i32 0, i32 2)
>    ret void
> }
>
> Using 'opt' utility as follows :
>
> opt -O2 structaccess.ll -S -o structaccess-opt.ll
>
> I've got following code for structaccess-opt.ll file:
>
> ; ModuleID = 'structaccess.ll'
>
> %struct._anon0 = type <{ i32, i32, i32 }>
>
> @s = common global %struct._anon0 zeroinitializer
>
> define arm_aapcscc void @foo() nounwind {
> L.entry:
>    store i32 5, i32* getelementptr inbounds (%struct._anon0* @s, i32 0, i32 0)
>    store i32 10, i32* getelementptr inbounds (%struct._anon0* @s, i32 0, i32 1)
>    %0 = load i32* getelementptr inbounds (%struct._anon0* @s, i32 0, i32 0)
>    %1 = add i32 %0, 10
>    store i32 %1, i32* getelementptr inbounds (%struct._anon0* @s, i32 0, i32 2)
>    ret void
> }
>
> I would have expected constant 5 to be propagated by 'opt' to its use and thus
> LLVM assembly after opt to be :
>
> ; ModuleID = 'structaccess.ll'
>
> %struct._anon0 = type <{ i32, i32, i32 }>
>
> @s = common global %struct._anon0 zeroinitializer
>
> define arm_aapcscc void @foo() nounwind {
> L.entry:
>    store i32 5, i32* getelementptr inbounds (%struct._anon0* @s, i32 0, i32 0)
>    store i32 10, i32* getelementptr inbounds (%struct._anon0* @s, i32 0, i32 1)
>    store i32 15, i32* getelementptr inbounds (%struct._anon0* @s, i32 0, i32 2)
>    ret void
> }
>
> Can someone explain me why this is not the case ?
>
> Note that C equivalent would be something like:
>
> struct {
>      int x, y, z;
> } s;
>
> void foo()
> {
>     s.x = 5 ;
>     s.y = 10 ;
>     s.z = s.x + s.y ;
> }
>
>
>
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