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Seb

2011-Oct-18 13:09 UTC

[LLVMdev] LLVM constant propagation optimization question

Hi all,

I'm writting following LLVM assembly:

; 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 = 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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Duncan Sands

2011-Oct-18 13:42 UTC

head link

[LLVMdev] LLVM constant propagation optimization question

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 ;
> }
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Seb

2011-Oct-18 13:52 UTC

head link

[LLVMdev] LLVM constant propagation optimization question

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 ;
> > }
> >
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
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