[llvm-dev] Basic AA fail. Bug or feature?

Jon Chesterfield via llvm-dev llvm-dev at lists.llvm.org
Tue Jun 20 16:33:51 PDT 2017

basicaa with pointers to distinct globals.

static const int x = 4;
static const int y = 5;
static const double z = 6.0;

int types_match(void)
  return (char*)&x == (char*)&y;

int types_differ(void)
  return (char*)&x == (char*)&z;

I'd like both tests to resolve to 'false' at the IR level, but the mixed
type one doesn't at O3:

target triple = "x86_64-unknown-linux-gnu"
@x = internal constant i32 4, align 4
@z = internal constant double 6.000000e+00, align 8

define i32 @types_match() {
  ret i32 0

define i32 @types_differ() {
  ret i32 zext (i1 icmp eq (double* bitcast (i32* @x to double*), double*
@z) to i32)

Discovered in code which uses a static variable's address as a failure
indicator. Imagine a function which returns 0 for out of memory, &some_var
for bad arguments, pointer to something useful otherwise. The static
variable happens to have a different type because flexible array members
are involved with length 0 for the failure indicator and length !=0 in

I consider this a failure in the alias analysis - would a patch be welcome,
or is this intended behaviour that I've missed the point of?


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