[llvm-bugs] [Bug 26118] New: TBAA failed to alias union in a very obvious code

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26118

            Bug ID: 26118
           Summary: TBAA failed to alias union in a very obvious code
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: yinma at codeaurora.org
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

###---t.c----###
typedef union
{
    unsigned int b32[2];
    long long int b64;
} Vector;

int bar();

unsigned int h264p_get_bits(Vector *c)
{
    unsigned int v = c->b64;
    c->b32[0]= bar();
    return v;
}

clang t.c -S -O1 -mllvm -print-after-all
*** IR Dump After Module Verifier ***
; Function Attrs: nounwind
define i32 @h264p_get_bits(%union.Vector* %c) #0 {
entry:
  %c.addr = alloca %union.Vector*, align 4
  %v = alloca i32, align 4
  store %union.Vector* %c, %union.Vector** %c.addr, align 4, !tbaa !3
  %0 = bitcast i32* %v to i8*
  call void @llvm.lifetime.start(i64 4, i8* %0) #3
  %1 = load %union.Vector*, %union.Vector** %c.addr, align 4, !tbaa !3
  %b64 = bitcast %union.Vector* %1 to i64*
  %2 = load i64, i64* %b64, align 8, !tbaa !7 <<<----- !7
  %conv = trunc i64 %2 to i32
  store i32 %conv, i32* %v, align 4, !tbaa !9
  %call = call i32 bitcast (i32 (...)* @bar to i32 ()*)()
  %3 = load %union.Vector*, %union.Vector** %c.addr, align 4, !tbaa !3
  %b32 = bitcast %union.Vector* %3 to [2 x i32]*
  %arrayidx = getelementptr inbounds [2 x i32], [2 x i32]* %b32, i32 0, i32 0
  store i32 %call, i32* %arrayidx, align 8, !tbaa !9 <<<----- !9, not !7
  %4 = load i32, i32* %v, align 4, !tbaa !9
  %5 = bitcast i32* %v to i8*
  call void @llvm.lifetime.end(i64 4, i8* %5) #3
  ret i32 %4
}

Although I understand
Strictly speaking, reading a member of a union different from the one written
to is undefined in ANSI/ISO C99 except in the special case of type-punning to a
char*, similar to the example below: Casting to char*. However, it is an
extremely common idiom and is well-supported by all major compilers. As a
practical matter, reading and writing to any member of a union, in any order,
is acceptable practice.

And for type based AA, it should not strict aligned. However, for union, this
is very general usage, tbaa should output they are aliased, right.

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