https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84562
Bug ID: 84562
Summary: -faggressive-loop-optimizations makes decisions based
on weak data structures
Product: gcc
Version: 8.0.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: jnordholz at sect dot tu-berlin.de
Target Milestone: ---
Created attachment 43501
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43501&action=edit
minimal example for both cases (int and array-of-char*)
(Minimal test case attached as source tar.gz, as the problem is inherently
multi-compilation-unit and doesn't involve any headers. Snippets below for
illustration.)
Compiling the following code with -c -O2...
__attribute__((weak)) const int y = 0;
void foo(void) {
for (i = 0; i < y; i++) {
write(2, "X", 1);
}
}
yields an empty foo() even though 'y' is weak (which I consider a bug, as
linking with another unit with a proper symbol 'y' with nonzero value will Do
The Wrong Thing and not even issue a warning - -fsanitize=undefined cannot help
either, as the whole loop is optimized away).
In a similar case, having
__attribute__((weak)) const char *arr[] = { NULL };
[...]
for (i = 0; arr[i]; i++) {
in one unit and overriding 'arr' in another with
const char *arr[] = { "abc", "def", NULL };
causes related problems: this time the loop doesn't disappear completely, so
-fsanitize=undefined complains about illegal array accesses - but I don't
believe this is justified, unless the different implicit array size is illegal
by the C standard; I couldn't find anything on that.
Tested with Debian gcc7 (7.3.0) and gcc8 (8.0.1) on x86_64, thus assigning to
the latest version.
