[llvm-dev] Question about Alias Analysis with restrict keyword

[jingu@codeplay.com via llvm-dev]

Hello All,

I have met a case with restrict keyword and I have a question about it. 
Let's look at a simple example.

char buf[4];

void test(char *restrict a, char *restrict b, int count) {
   for (unsigned i = 0; i < count; i++) {
     *a = *b;
     a++;
     b++;
     buf[i] = i;
   }
}

I think there are no aliasing among pointers such as 'a', 'b' and 'buf' 
because of the restrict keyword.

The IR snippet with '-O1' is as following:

@buf = common local_unnamed_addr global [4 x i8] zeroinitializer, align 1

; Function Attrs: norecurse nounwind uwtable
define void @test(i8* noalias nocapture, i8* noalias nocapture readonly, 
i32) local_unnamed_addr #0 {
   %4 = icmp eq i32 %2, 0
   br i1 %4, label %7, label %5

; <label>:5:                                      ; preds = %3
   %6 = zext i32 %2 to i64
   br label %8

; <label>:7:                                      ; preds = %8, %3
   ret void

; <label>:8:                                      ; preds = %8, %5
   %9 = phi i64 [ 0, %5 ], [ %17, %8 ]
   %10 = phi i8* [ %0, %5 ], [ %13, %8 ]
   %11 = phi i8* [ %1, %5 ], [ %14, %8 ]
   %12 = load i8, i8* %11, align 1, !tbaa !2
   store i8 %12, i8* %10, align 1, !tbaa !2
   %13 = getelementptr inbounds i8, i8* %10, i64 1
   %14 = getelementptr inbounds i8, i8* %11, i64 1
   %15 = trunc i64 %9 to i8
   %16 = getelementptr inbounds [4 x i8], [4 x i8]* @buf, i64 0, i64 %9
   store i8 %15, i8* %16, align 1, !tbaa !2
   %17 = add nuw nsw i64 %9, 1
   %18 = icmp eq i64 %17, %6
   br i1 %18, label %7, label %8
}

As you can see, there are noalias attribute with arguments and we can 
expect the alias analysis will consider them.

The Alias Set is as below.

Alias sets for function 'test':
Alias Set Tracker: 2 alias sets for 3 pointer values.
   AliasSet[0x4a1e3a0, 3] may alias, Mod/Ref   Pointers: (i8* %11, 1), 
(i8* %10, 1), (i8* %16, 1)
   AliasSet[0x4a1e440, 1] must alias, Mod        forwarding to 0x4a1e3a0

Alias analysis returns 'May- alias' for above code. When I look at the 
alias analysis code, below code causes may-alias.

On BasicAAResult::aliasGEP()

1297     // Check to see if these two pointers are related by the 
getelementptr
1298     // instruction.  If one pointer is a GEP with a non-zero index 
of the other
1299     // pointer, we know they cannot alias.
1300
1301     // If both accesses are unknown size, we can't do anything 
useful here.
1302     if (V1Size == MemoryLocation::UnknownSize &&
1303         V2Size == MemoryLocation::UnknownSize)
1304       return MayAlias;
1305
1306     AliasResult R = aliasCheck(UnderlyingV1, 
MemoryLocation::UnknownSize,
1307                                AAMDNodes(), V2, 
MemoryLocation::UnknownSize,
1308                                V2AAInfo, nullptr, UnderlyingV2);

On line 1306, we can see the V1Size and V2Size are set up with 
"MemoryLocation::UnknownSize" and it causes may-alias. On line 1302 and 
1033, there are checking code for "MemoryLocation::UnknownSize"... I am 
not sure whether it is bug or not... Can someone let me know why it 
passes "MemoryLocation::UnknownSize" please? If I missed something, 
please let me know.

Thanks,

JinGu Kang