llvm dev - Jul 2020 - Question about basic-aa's assumptions

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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Any takers on this?

Best,
Ram

On Mon, Jul 6, 2020 at 12:18 PM ramshankar r <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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On 7/6/20 11:18 AM, ramshankar r via llvm-dev 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
Isn't this talking about `&pc[0]` and `&ppc`? If so, NoAlias seems 
reasonable to me.

I mean, you print the value of `ppc` and `pc` while the above talks 
about the addresses of these.

(I'm a bit tired so take this as a potential answer only.)


~ Johannes


> 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
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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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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