[LLVMdev] LLVM constant propagation optimization question

[Duncan Sands]

Hi Seb,

> What do you mean by "a data layout string in your module" ?

something like this:

target datalayout = 
"e-p:64:64:64-S128-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-f128:128:128-n8:16:32:64"
target triple = "x86_64--linux-gnu"

Ciao, duncan.

> Best Regards
> Seb
>
> 2011/10/18 Duncan Sands <baldrick at free.fr <mailto:baldrick at free.fr>>
>
>     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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