https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64304
Bug ID: 64304
Summary: AArch64 miscompilation with -mgeneral-regs-only
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: me at williamgrant dot id.au
The aarch64 target, from 4.8.0 to today's master (tested up to 4b686186), drops
an "and x0, x0, #0x7" in some integer-only code when built with
-mgeneral-regs-only. Ignoring register assignment, the only change in the
resultant code is the missing and. It works fine without -mgeneral-regs-only
and on several other targets.
This test case is quite fragile. The original code didn't need -fno-inline to
reproduce it, and the problem disappeared in an earlier test with
-fno-tree-vrp. -fdump-rtl-all reveals that the adddi vanishes between ud_dce
and combine, but I can't see any significant differences in the input RTL to
explain that.
$ gcc -mgeneral-regs-only -O2 -fno-inline -o gen-test gen-test.c
$ ./gen-test && echo success || echo fail
fail
$ gcc -O2 -fno-inline -o gen-test gen-test.c
$ ./gen-test && echo success || echo fail
success
----
unsigned char byte = 0;
void set_bit(unsigned int bit, unsigned char value) {
unsigned char mask = (unsigned char)(1 << (bit & 7));
if (!value) {
byte &= (unsigned char)~mask;
} else {
byte |= mask;
}
}
int main(int argc, char **argv) {
set_bit(8, 1);
set_bit(9, 1);
return byte != 3;
}
----
