On Thu, Aug 24, 2023 at 8:06 PM libreknight via Gcc-bugs
<gcc-bugs@gcc.gnu.org> wrote:
>
> Greetings.
>
> I have come across erroneous behavior whilst comparing optimizations
> performed by different compilers. Said behavior persists through
> different versions of GCC and flags. The output from GCC is incorrect
> and diverges from all compilers.
>
> In order to reproduce aforementioned behavior, compile with any flags of
> your liking the following code:
>
> #include <stdio.h>
> #include <stdlib.h>
>
> typedef struct
> {
> unsigned char value:1;
> } boolean;
>
> int
> main (void)
> {
> boolean var;
>
> var.value = 0;
>
> printf ("%d %d\n", --var.value, --var.value);
GCC does produce a warning:
<source>: In function 'main':
<source>:16:35: warning: operation on 'var.value' may be undefined
[-Wsequence-point]
16 | printf ("%d %d\n", --var.value, --var.value);
| ^~~~~~~~~~~
Anyways the order of evaluation is not specified in the C standard
when it comes to function arguments; left to right or right to left is
both valid.
Thanks,
Andrew Pinski
>
> return EXIT_SUCCESS;
> }
>
> The outcome disparates from the expected by producing the opposite
> result.
>
> GCC:
> 0 1
>
> clang, tcc, icc, icx, msvc, compcert:
> 1 0
>
> Inasmuch as the potential consequences of faulty logic allows for
> undefined behavior, security vulnerabilities emerges. Nevertheless,
> acknowledging the atypicality of causal application, I evaluate the
> significance of low-medium priority.
>
> Thanks in advance.
