[Bug c/103887] New: -fsanitize=shift affects constness of an expression

2022-01-01 Thread mike--- via Gcc-bugs 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103887

Bug ID: 103887
   Summary: -fsanitize=shift affects constness of an expression
   Product: gcc
   Version: 11.2.1
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c
  Assignee: unassigned at gcc dot gnu.org
  Reporter: m...@mk-sys.cz
  Target Milestone: ---

This simplified testcase

---
#include 

#define ASSERT_ON_COMPILE_SELECTOR_SIZE(e)\
_Static_assert(((__builtin_constant_p(e) && ((e) >> 16) == 0)) || \
   (sizeof(e) <= 2), "problem")

int f(uint16_t tr)
{
ASSERT_ON_COMPILE_SELECTOR_SIZE(tr);
return 0;
}
---

builds cleanly with "-O2" but fails with "-O2 -fsanitize=shift":

---
mike@lion:/tmp/gcc> gcc-11 -Wall -Wextra -O2 -c foo.c
mike@lion:/tmp/gcc> gcc-11 -Wall -Wextra -O2 -fsanitize=shift -c foo.c
foo.c: In function ‘f’:
foo.c:4:72: error: expression in static assertion is not constant
4 | _Static_assert(((__builtin_constant_p(e) && ((e) >> 16) == 0))
|| \
  |   
^~~~
5 |(sizeof(e) <= 2), "problem")
  | 
foo.c:9:9: note: in expansion of macro ‘ASSERT_ON_COMPILE_SELECTOR_SIZE’
9 | ASSERT_ON_COMPILE_SELECTOR_SIZE(tr);
  | ^~~
---

I was able to reproduce the same with various gcc version from gcc7 to gcc12.

[Bug c/103887] -fsanitize=shift affects constness of an expression

2022-01-01 Thread mike--- via Gcc-bugs 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103887

--- Comment #2 from Michal Kubecek  ---
(In reply to Andrew Pinski from comment #1)
> The problem is rather __builtin_constant_p not resolving to a constant. This
> is a dup of bug 79482.

There is a difference: your modified testcase fails to compile regardless of
-fsanitize=shift while mine fails only with this option and succeeds without
it. If __builtin_constant_p(tr) not being handled as constant were the problem,
the result should not depend on presence of "-fsanitize=shift".

[Bug c/103887] -fsanitize=shift affects constness of an expression

2022-01-01 Thread mike--- via Gcc-bugs 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103887

--- Comment #4 from Michal Kubecek  ---
It's probably even more complicated as

#define ASSERT_ON_COMPILE_SELECTOR_SIZE(expr)   \
_Static_assert( \
__builtin_choose_expr(__builtin_constant_p(expr), \
  ((expr) >> 16) == 0, \
  sizeof(expr) <= 2), \
"problem")

works as expected (tested with "uint16_t tr", "uint32_t tr", 6 and 7)
even if the documentation says first argument of __builtin_choose_expr() has
to be a constant expression.

As this is nicer than the original code, I guess I'll use it and hope it is
not just a happy coincidence that it works.