https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113980
Bug ID: 113980
Summary: risc-v: unnecessary sign-extend after lw, and more
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: Simon.Richter at hogyros dot de
Target Milestone: ---
On RISC-V with -O3,
#include <stdint.h>
extern uint32_t volatile d;
extern uint8_t volatile o;
void f()
{
uint32_t t;
while((t = d) & 0x80000000)
;
o = t;
}
generates
f:
lui a3,%hi(d)
.L2:
lw a5,%lo(d)(a3)
sext.w a4,a5
blt a5,zero,.L2
andi a4,a4,0xff
lui a5,%hi(o)
sb a4,%lo(o)(a5)
ret
The result of the lw instruction is already sign-extended, so the sext.w
instruction is unnecessary in any case. It is extra unnecessary when the value
is subsequently truncated with an and instruction, which itself is unnecessary
for a sb instruction.
To keep this bug report focused, I'd limit the problem description to the
sign-extend -- if the others warrant opening extra bug reports, I can do that
as well.
