[llvm-dev] One more Question about handling pointer with restrict keyword on Alias Analysis

[jingu@codeplay.com via llvm-dev]

Hi All,

I have a question about handling pointer on Alias Analysis. Let's look 
at simple code snippet.

struct element {
   char fieldA;
   char fieldB;
};

char c;

void test(int idx, struct element *restrict *restrict buf1,
           struct element *restrict *restrict buf2) {
   struct element *a = buf1[idx];
   struct element *b = buf2[idx];

   a->fieldA = b->fieldB;

   c++;

   a->fieldA = b->fieldB | 1;
}

The IR snippet is as following:

%struct.element = type { i8, i8 }

@c = common local_unnamed_addr global i8 0, align 1

define void @test(i32, %struct.element** noalias nocapture readonly, 
%struct.element** noalias nocapture readonly) local_unnamed_addr #0 {
   %4 = sext i32 %0 to i64
   %5 = getelementptr inbounds %struct.element*, %struct.element** %1, 
i64 %4
   %6 = load %struct.element*, %struct.element** %5, align 8, !tbaa !2
   %7 = getelementptr inbounds %struct.element*, %struct.element** %2, 
i64 %4
   %8 = load %struct.element*, %struct.element** %7, align 8, !tbaa !2
   %9 = getelementptr inbounds %struct.element, %struct.element* %8, i64 
0, i32 1
   %10 = load i8, i8* %9, align 1, !tbaa !6
   %11 = getelementptr inbounds %struct.element, %struct.element* %6, 
i64 0, i32 0
   store i8 %10, i8* %11, align 1, !tbaa !8
   %12 = load i8, i8* @c, align 1, !tbaa !9
   %13 = add i8 %12, 1
   store i8 %13, i8* @c, align 1, !tbaa !9
   %14 = or i8 %10, 1
   store i8 %14, i8* %11, align 1, !tbaa !8
   ret void
}

The result of AliasSet is as below.

Alias sets for function 'test':
Alias Set Tracker: 4 alias sets for 5 pointer values.
   AliasSet[0x64ac600, 1] must alias, Ref       Pointers: 
(%struct.element** %5, 8)
   AliasSet[0x64aa990, 1] must alias, Ref       Pointers: 
(%struct.element** %7, 8)
   AliasSet[0x64ab320, 1] must alias, Ref       Pointers: (i8* %9, 1)
   AliasSet[0x64ac800, 2] may alias, Mod/Ref   Pointers: (i8* %11, 1), 
(i8* @c, 1)

As you can see, there is 'May-Alias' between %11 and @c. I think we 
could say 'No-Alias' between them because of the restrict keyword. It 
seems the basicAA does not go through 'load' instruction with its 
recursive way and GetUnderlyingObject. I am not sure why it does not 
always go though it. Is there something illegal to do it? Could someone 
let me know it please? If I missed something, please let me know.

Thanks,

JinGu Kang