[llvm-dev] MayAlias in MemorySSA

[sushant gokhale via llvm-dev]

Thanks for the information.

Regards
Sushant

On Wed, Mar 10, 2021 at 1:57 AM Alina Sbirlea <alina.sbirlea at gmail.com>
wrote:

> Hi,
>
> 1. Alias information is only informative. It is displayed for accesses
> that have been optimized and it is stored as part of the memory access that
> displays it.
> 2. MayAlias for that instruction means the load instruction reads from a
> location that may overlap with locations written to by `call void @foo`.
> In your example foo only reads to I suspect that if you add `readnone` to
> its attribute list, `MemoryUse(6) MayAlias` will become `MemoryUse(4)
> MustAlias`.
>
> Best,
> Alina
>
> On Tue, Mar 9, 2021 at 10:16 AM sushant gokhale via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> For the following 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);}
>>
>>
>> The MemSSA generated is:
>> 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)
>>
>> Now, considering the block:
>> MemoryUse(6) MayAlias
>>   %11 = load i32, i32* %1, align 4, !tbaa !5
>>
>> I have 2 questions:
>> 1.why is Alias information shown alongwith MemSSA graph?
>> 2. what does MayAlias mean here? Does it mean that this load may depend
>> upon operand being referred/modified  in 6 = MemoryDef(5) ?  Or is it
>> something else?
>>
>> Regards
>> Sushant
>> _______________________________________________
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>> llvm-dev at lists.llvm.org
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>>
>
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