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

sushant gokhale via llvm-dev llvm-dev at lists.llvm.org
Sun Mar 7 19:46:56 PST 2021

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?

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