https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93301
Bug ID: 93301
Summary: Wrong optimization: instability of uninitialized
variables leads to nonsense
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ch3root at openwall dot com
Target Milestone: ---
Instability is inconsistency, which leads to logical contradictions, which
leads to total chaos. Similar to bug 61502, comment 42, but with uninitialized
variables:
----------------------------------------------------------------------
#include <stdio.h>
__attribute__((noipa)) // imagine it in a separate TU
static void *opaque(void *p) { return p; }
int main()
{
int c = 1;
opaque(&c);
unsigned char x = 0;
opaque(&x);
unsigned char y; // no trap representation possible
(void)&y; // disarm C11, 6.3.2.1p2
unsigned char z;
int b;
if (x == y) {
b = 1;
z = x;
} else {
b = 0;
z = y;
}
opaque(&b);
if (b)
printf("b = %d c = %d x = %d e = %d\n", b, c, x, c ? z : 5);
}
----------------------------------------------------------------------
$ gcc -std=c11 -O3 test.c && ./a.out
b = 1 c = 1 x = 0 e = 5
----------------------------------------------------------------------
gcc x86-64 version: gcc (GCC) 10.0.1 20200117 (experimental)
----------------------------------------------------------------------
Given that the printf has fired, `b` is `1`, hence `z` is the same as `x` and
`e = 0` should be printed.
According to my reading of C11 this program doesn't invoke UB. (And I thought
that most proposals about "wobbly" values wouldn't change this but I'm not sure
anymore:-)
Even if this particular example is deemed undefined by gcc, I guess
inconsistencies could blow everything up even without any help from a
programmer.
clang bug -- https://bugs.llvm.org/show_bug.cgi?id=44512.
