https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93010
Bug ID: 93010 Summary: Wrong optimization: provenance affects comparison of saved bits of addresses of dead auto variables 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: --- It's known that the value of a pointer to an object becomes indeterminate after the object is dead (C11, 6.2.4p2). Whether its representation becomes indeterminate is up for debate but let's bypass the issue by saving the representation while the object is still alive. For example, we can cast it to an integer. And we'll get an ordinary integer, with a stable value etc., not affected by changes in the life of the original object. Right? This seems to be broken for the equality operators when the operands are constructed from addresses of automatic variables and at least one of these variables is dead at the time of comparison. ---------------------------------------------------------------------- #include <stdio.h> int main() { unsigned long u, v; { int x[5]; u = (unsigned long)x; } { int y[5]; v = (unsigned long)y; } printf("u = %#lx\n", u); printf("v = %#lx\n", v); printf("diff = %#lx\n", u - v); printf("eq = %d\n", u == v); } ---------------------------------------------------------------------- $ gcc -std=c11 -pedantic -Wall -Wextra -O3 test.c && ./a.out u = 0x7ffeb6326180 v = 0x7ffeb6326180 diff = 0 eq = 0 ---------------------------------------------------------------------- gcc x86-64 version: gcc (GCC) 10.0.0 20191219 (experimental) If "diff == 0" then "eq" should be 1.