https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108987
Bug ID: 108987
Summary: [13 Regression] RISC-V: shiftadd cost model bug
needlessly preferring syth_multiply
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: vineetg at rivosinc dot com
Target Milestone: ---
gcc trunk is preferring synthetic multiply using shift+add even when they are
costlier than multiply.
unsigned long long f5(unsigned long long i)
{
return i * 0x0202020202020202ULL;
}
riscv64-unknown-linux-gnu-gcc -c -O2 -march=rv64gc_zba
f5:
slli a5,a0,8
add a0,a5,a0
slli a5,a0,16
add a0,a0,a5
slli a5,a0,32
add a0,a0,a5
slli a0,a0,1
ret
With gcc 12.2 this used to be
f5:
lui a5,%hi(.LC0)
ld a5,%lo(.LC0)(a5)
mul a0,a0,a5
ret
This is a regression introduced by commit f90cb39235c4 ("RISC-V: costs: support
shift-and-add in strength-reduction"). It introduced the cost for
shift[1-3]+add (to favor SH*ADD) but due to a coding bug ended up doing this
for all shift values, affecting synth multiply among others.
This showed up as dynamic icount regression in SPEC 531.deepsjeng.
