llvm dev - Jul 2008 - [LLVMdev] Problem while using mem2reg Optimization

Hi,

I am using various LLVM optimization passes and I noticed a strange
behaviour in Mem2Reg Optimization. These pass is used for SSA construction
and basically removed alloca of the corresponding pointers are just used in
load/stores.

I tried the following .ll file

*define i32 @test(i32 %y,i32 %z) {
entry:
        %X = alloca i32           ; type of %X is i32*.
        %X.0 = add i32 %y ,%z
        store i32 %X.0, i32* %X   ; Update X
        br label %cond_next*
**
*cond_next:
        %tmp1 = alloca i32
        %tmp2 = sub i32  %y, 3
        store i32 %tmp2,i32* %tmp1
        ret i32 0
}*
**
I tried the Mem2Reg pass on above file and I got following output:

*; ModuleID = 'test4.bc'*
*define i32 @test(i32 %y, i32 %z) {
entry:
        %X.0 = add i32 %y, %z           ; <i32> [#uses=0]
        br label %cond_next*
**
*cond_next:              ; preds = %entry
        %tmp1 = alloca i32              ; <i32*> [#uses=1]
        %tmp2 = sub i32 %y, 3           ; <i32> [#uses=1]
        store i32 %tmp2, i32* %tmp1
        ret i32 0
}*

So, it removed the alloca from first basic block and not from others. If
I remove the alloca  from cond_next basic block and instead put that also in
first Basic Block, the mem2Reg optimizes this and I get following output

*; ModuleID = 'test4.bc'*
*define i32 @test(i32 %y, i32 %z) {
entry:
        %X.0 = add i32 %y, %z           ; <i32> [#uses=0]
        br label %cond_next*
*cond_next:              ; preds = %entry
        %tmp2 = sub i32 %y, 3           ; <i32> [#uses=0]
        ret i32 0
}*
**
**
So, is there any restriction that all the alloca should be put in first
basic block?

Thanks


Kapil

*

*
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On Wed, Jul 2, 2008 at 10:07 AM, kapil anand <kapilanand2 at gmail.com>
wrote:
> Hi,
>
> I am using various LLVM optimization passes and I noticed a strange
> behaviour in Mem2Reg Optimization. These pass is used for SSA construction
> and basically removed alloca of the corresponding pointers are just used in
> load/stores.
> So, is there any restriction that all the alloca should be put in first
> basic block?
Yes, there is exactly such a restriction;
http://llvm.org/docs/tutorial/LangImpl7.html has a good description of
using mem2reg for SSA construction.

-Eli

> Yes, there is exactly such a restriction;
> http://llvm.org/docs/tutorial/LangImpl7.html has a good description of
> using mem2reg for SSA construction.
From that page:
"mem2reg only looks for alloca instructions in the entry block of the
function. Being in the entry block guarantees that the alloca is only executed
once, which makes analysis simpler."

I was thinking you could just move any alloca to the entry block for free, but
I hadn't considered looping.

Gr.

Matthijs
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