https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69442
--- Comment #2 from Zdenek Sojka <zsojka at seznam dot cz> ---
(In reply to ktkachov from comment #1)
> Does this occur for other optimisations levels?
It happens only with -Og.
Difference -O1 -> -Og:
@@ -22,6 +22,8 @@
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
push {r4, r5}
+ mvn r0, r0, lsr #16
+ mvn r0, r0, lsl #16
subs r2, r2, #255
sbc r3, r3, #0
mvn r4, #24
@@ -29,15 +31,12 @@
movt r5, 65535
cmp r3, r5
cmpeq r2, r4
- movcs r1, #1
- movcc r1, #0
- mvn r4, #24
- umull r4, r5, r1, r4
- sub r5, r5, r1, lsl #16
+ movcs r4, #1
+ movcc r4, #0
+ mvn r1, #24
+ umull r4, r5, r4, r1
subs r2, r2, r4
sbc r3, r3, r5
- mvn r0, r0, lsr #16
- mvn r0, r0, lsl #16
adds r4, r2, r0
adc r5, r3, #0
mov r0, r4
The code is the same, except;
- sub r5, r5, r1, lsl #16
which is missing in the -Og code.
> The modulo by a power of 2 expansion was changed for arm for GCC 6 but that
> was for signed values, whereas the testcase uses unsigned variables and the
> modulo operation is not by a power of 2...
Maybe the problem is in the older GCC branches as well, but it is not exposed
on this particular testcase.
