[llvm-dev] Finding Store Instructions that possibly affect load instruction or func call instruction

[sushant gokhale via llvm-dev]

Hi,

I want to track StoreInst that affect loadInst/CallInst.

e.g %1 = alloc i32

      store 10,%1

      foo(%1)      -------> %1 should take the value 10 (defined by store ins)

I tried to use MemSSA for this but for situations I can't find these correct dependencies possibly due to insufficient Alias information

e.g

Consider the C code:

__attribute__((noinline)) void foo(int i,int j)
{
    printf("%d %d",i,j);
}
int main()
{
    int j;
    int k;
    scanf("%d%d",&j,&k);
    j+=10;
    k-=3;

    //func call to force the stores
    foo(j,k);

    //func call to force the loads
    printf("%d %d",j,k);
}

MemSSA generated:

define dso_local i32 @main() #2 {
  %1 = alloca i32, align 4
  %2 = alloca i32, align 4
  %3 = bitcast i32* %1 to i8*
; 1 = MemoryDef(liveOnEntry)
  call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %3) #5
  %4 = bitcast i32* %2 to i8*
; 2 = MemoryDef(1)
  call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %4) #5
  %5 = getelementptr inbounds [5 x i8], [5 x i8]* @.str.1, i64 0, i64 0
; 3 = MemoryDef(2)
  %6 = call i32 (i8*, ...) @__isoc99_scanf(i8* %5, i32* nonnull %1, i32* nonnull %2)
; MemoryUse(3) MayAlias
  %7 = load i32, i32* %1, align 4, !tbaa !5
  %8 = add nsw i32 %7, 10
; 4 = MemoryDef(3)
  store i32 %8, i32* %1, align 4, !tbaa !5
; MemoryUse(3) MayAlias
  %9 = load i32, i32* %2, align 4, !tbaa !5
  %10 = add nsw i32 %9, -3
; 5 = MemoryDef(4)
  store i32 %10, i32* %2, align 4, !tbaa !5
; 6 = MemoryDef(5)
  call void @foo(i32 %8, i32 %10)
; MemoryUse(6) MayAlias
  %11 = load i32, i32* %1, align 4, !tbaa !5
; MemoryUse(6) MayAlias
  %12 = load i32, i32* %2, align 4, !tbaa !5
  %13 = getelementptr inbounds [6 x i8], [6 x i8]* @.str, i64 0, i64 0
; 7 = MemoryDef(6)
  %14 = call i32 (i8*, ...) @printf(i8* nonnull dereferenceable(1) %13, i32 %11, i32 %12)

Two  issues for me:

1. I am not able to track from where the values for %11 and %12 are coming. There are coming from 2 store instructions before the foo call. But since both loads   have defining ins as 6 = MemoryDef(5), I couldn't get these stores.

The only way I could find them is traverse back using def chain created in MemSSA

2. For function foo(), I can't trace from where parameter values are coming from because MemSSA only gives 1 link up  i.e it tracks only 5 = MemoryDef(4)and not the other store.  That is to say, its tracking only 1 parameter.

Is there any solution to this?

Regards

Sushant

[Florian Hahn via llvm-dev]

Hi,

> On Mar 8, 2021, at 03:46, sushant gokhale via llvm-dev <llvm...@lists.llvm.org> wrote:
>
> Hi,
>
> I want to track StoreInst that affect loadInst/CallInst.
>
> e.g %1 = alloc i32
> store 10,%1
> foo(%1) -------> %1 should take the value 10 (defined by store ins)
>
> I tried to use MemSSA for this but for situations I can't find these correct dependencies possibly due to insufficient Alias information

MemorySSA should be helpful here. One thing to note is that MemorySSA is not completely optimized at construction, but it guarantees that all potentially clobbering instructions are in the memory def-use chain.

It is up the the so-called walkers to skip over non-aliasing accesses. Please take a look at https://llvm.org/doxygen/classllvm_1_1MemorySSAWalker.html . They provide getClobberingMemoryAccess, which skip non-aliasing instructions, depending on the walker implementation. You can use MemorySSA::getWalker to get the default walker. If that doesn’t do what you want, you probably need to define your own walker. At last LLVM Developers meeting, there was a talk covering MemorySSA: https://www.youtube.com/watch?v=1e5y6WDbXCQ

Cheers,
Florian
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