https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82989
Bug ID: 82989
Summary: Inexplicable use of NEON for 64-bit math
Product: gcc
Version: 7.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: matthijsvanduin at gmail dot com
Target Milestone: ---
The following function:
void foo( uint64_t *a ) {
*a += *a >> 32;
}
compiled with arm-linux-gnueabihf-gcc -mcpu=cortex-a8 -mfpu=neon -O2
produces the following code:
push {r4, r5}
ldrd r4, [r0]
vmov d16, r4, r5
vshr.u64 d16, d16, #32
vmov r2, r3, d16
adds r2, r2, r4
adcs r3, r3, r5
strd r2, [r0]
pop {r4, r5}
bx lr
Since -mneon-for-64bits is not enabled (I double-checked using -Q just to be
sure), the use of neon instructions here is highly unexpected.
(Moreover, shifting right by 32 bits should of course not involve any actual
arithmetic whatsoever. Ideally this function would compile to
ldrd r2, [r0]
adds r2, r2, r3
adcs r3, r3, #0
strd r2, [r0]
bx lr
)
