https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125405
Bug ID: 125405
Summary: Missed optimization: mask/sub sign-extension idiom not
canonicalized to sign extension
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: [email protected]
Target Milestone: ---
Build: -O2
The following Hacker's Delight idioms compute sign extension from
the low byte / low halfword:
(x & 0x7f) - (x & 0x80)
(x & 0x7fff) - (x & 0x8000)
However, GCC does not currently canonicalize these idioms to a signed
narrow conversion / sign-extension operation.
Testcase:
typedef unsigned long xlen_t;
xlen_t sext8_hd(xlen_t x)
{
return (x & 0x7fUL) - (x & 0x80UL);
}
xlen_t sext16_hd(xlen_t x)
{
return (x & 0x7fffUL) - (x & 0x8000UL);
}
Command line:
gcc -O2 -march=rv64gc -mabi=lp64d -S test.c
Current output for sext8_hd:
sext8_hd:
andi a5,a0,127
andi a0,a0,128
sub a0,a5,a0
ret
Current output for sext16_hd:
sext16_hd:
slli a5,a0,49
li a4,32768
srli a5,a5,49
and a0,a0,a4
sub a0,a5,a0
ret
Expected output:
sext8_hd:
slliw a0,a0,24
sraiw a0,a0,24
ret
sext16_hd:
slliw a0,a0,16
sraiw a0,a0,16
ret
Example link: https://godbolt.org/z/3h9z3vf95
