[llvm-dev] Question about basic-aa's assumptions

[ramshankar r via llvm-dev]

Hi,

I have a test-case for which I seem to be getting incorrect information
from basic-aa alias-analysis. Here it is:
#include <stdio.h>
#define ARR_SIZE 4
char c[ARR_SIZE];
char *pc[ARR_SIZE];
char **ppc = pc;

int main() {

  pc[0] = &c[0];
  pc[1] = &c[1];
  pc[2] = &c[2];
  pc[3] = &c[3];

  printf(" ppc: %p\n"
      " pc : %p\n"
      " c  : %p\n"
      " *pc: %p\n",
      ppc, pc, c, *pc);

  return 0;
}

The way I compiled it is as follows:
clang -m32 -g pointer.c -emit-llvm -S -c
opt pointer.ll -aa-pipeline=basic-aa -passes=aa-eval
-print-all-alias-modref-info -disable-output

The result from the AA-Evaluator prints this:
  NoAlias: i8** getelementptr inbounds ([4 x i8*], [4 x i8*]* @pc, i32 0,
i32 0), i8*** @ppc

If we run the program, the result is this:
 ppc: 0x565ee028
 pc : 0x565ee028
 c  : 0x565ee024
 *pc: 0x565ee024

Basically, I would have liked if basic-aa said ppc and pc are may-aliased
to start with for this kind of usage. This is how the globals look like.
The second one: "ppc" has "pc" on the right hand side.

@pc = common dso_local global [4 x i8*] zeroinitializer, align 4, !dbg !0
@ppc = dso_local global i8** getelementptr inbounds ([4 x i8*], [4 x i8*]*
@pc, i32 0, i32 0), align 4, !dbg !6

Best regards,
Ram
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