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

[Jeroen Dobbelaere via llvm-dev]

Hi Ramshankar,

giving it a try:

> 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

- the first is an 'i8**' (the address of pc),
- the second 'i8***' (aka the address of ppc.)

Those represent two different objects.

Greetings,

Jeroen Dobbelaere


From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of ramshankar r via llvm-dev
Sent: Tuesday, July 7, 2020 18:16
To: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Question about basic-aa's assumptions

Any takers on this?

Best,
Ram

On Mon, Jul 6, 2020 at 12:18 PM ramshankar r <ramshankar543 at gmail.com<mailto:ramshankar543 at gmail.com>> wrote:
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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