[LLVMdev] LLVM constant propagation optimization question

[Seb]

Hi Duncan,

What do you mean by "a data layout string in your module" ?
Best Regards
Seb

2011/10/18 Duncan Sands <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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>
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