https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93301
--- Comment #6 from Alexander Cherepanov <ch3root at openwall dot com> ---
(In reply to Alexander Cherepanov from comment #2)
> But my guess is that the C++ rules will not help. The problem is the
> internal inconsistency so everything will blow up independently of any
> external rules.
Well, the following example should illustrate what I mean. The uninitialized
variable `y` is only used in dead code:
----------------------------------------------------------------------
#include <stdio.h>
__attribute__((noipa)) // imagine it in a separate TU
static int opaque(int i) { return i; }
int main()
{
short x = opaque(1);
short y;
opaque(x - 1);
while (opaque(1)) {
printf("x = %d; x - 1 = %d\n", x, opaque(1) ? x - 1 : 5);
if (opaque(1))
break;
if (x - 1 == y)
opaque(y);
}
}
----------------------------------------------------------------------
$ gcc -std=c11 test.c && ./a.out
x = 1; x - 1 = 0
$ gcc -std=c11 -O3 test.c && ./a.out
x = 1; x - 1 = 5
----------------------------------------------------------------------
gcc x86-64 version: gcc (GCC) 10.0.1 20200126 (experimental)
----------------------------------------------------------------------
There is some defence in tree-ssa-loop-unswitch.c against loop unswitching on
undefined values but, as you wrote in comment 1, it's probably not that easy.
