[Bug c/87683] New: Inline asm input/output operand does not prevent compiler optimization

[sebastian.hu...@embedded-brains.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87683

            Bug ID: 87683
           Summary: Inline asm input/output operand does not prevent
                    compiler optimization
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sebastian.hu...@embedded-brains.de
  Target Milestone: ---

I use this macro since 2016 to prevent certain compiler optimizations:

#define OBFUSCATE_VARIABLE(var) __asm__("" : "+r" (var))

This was suggested by:

https://gcc.gnu.org/ml/gcc/2016-09/msg00115.html

It seems that it doesn't prevent some compiler optimizations in combination
with the non-null attribute. Consider the following test case:

#include <stddef.h>

#define OBFUSCATE_VARIABLE(var) __asm__("" : "+r" (var))

int posix_memalign(void **, size_t, size_t) __attribute__((__nonnull__(1)))
    __attribute__((__alloc_align__(2))) __attribute__((__alloc_size__(3)))
    __attribute__((__warn_unused_result__));

void a(void);

void b(void);

void c(void)
{
        int s;
        void **p;

        p = 0;
        OBFUSCATE_VARIABLE(p);
        s = posix_memalign(p, 16, 16);

        if (s != 22) {
                a();
        } else {
                b();
        }
}

GCC 7, 8, 9 unconditionally calls a() with -O2 without any warnings:

gcc -O2 -S -Wall -Wextra -pedantic test.c -o -
        .file   "test.c"
        .text
        .p2align 4,,15
        .globl  c
        .type   c, @function
c:
.LFB0:
        .cfi_startproc
        subq    $8, %rsp
        .cfi_def_cfa_offset 16
        movl    $16, %edx
        movl    $16, %esi
        xorl    %edi, %edi
        call    posix_memalign
        addq    $8, %rsp
        .cfi_def_cfa_offset 8
        jmp     a
        .cfi_endproc
.LFE0:
        .size   c, .-c
        .ident  "GCC: (SUSE Linux) 7.3.1 20180920 [gcc-7-branch revision
264438]"
        .section        .note.GNU-stack,"",@progbits

If I remove the inline asm, then I get a warning:

test.c: In function ‘c’:
test.c:19:4: warning: argument 1 null where non-null expected [-Wnonnull]
  s = posix_memalign(p, 16, 16);
  ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~