https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110423
Bug ID: 110423
Summary: Redundant constants not gettign eliminated on RISCV.
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: vineetg at gcc dot gnu.org
Target Milestone: ---
Redundant constants, across basic blocks, don't seem to be eliminated robustly
by gcc. I'd reported this last year [1] and this is just recapturing that info
as a bugzilla PR.
[1] https://gcc.gnu.org/pipermail/gcc/2022-October/239645.html
When analyzing coremark build for RISC-V, noticed redundant constants
not being eliminated. While this is a recurrent issue with RV, this
specific instance is not unique to RV as I can trigger similar output on
aarch64 with -fno-if-conversion, hence something which could be
addressed in common passes.
-O3 -march=rv64gc_zba_zbb
crcu8:
xor a3,a0,a1
andi a3,a3,1
srli a4,a0,1
srli a5,a1,1
beq a3,zero,.L2
li a3,-24576 # 0xFFFF_A000
addi a3,a3,1 # 0xFFFF_A001
xor a5,a5,a3
zext.h a5,a5
.L2:
xor a4,a4,a5
andi a4,a4,1
srli a3,a0,2
srli a5,a5,1
beq a4,zero,.L3
li a4,-24576 # 0xFFFF_A000
addi a4,a4,1 # 0xFFFF_A001
xor a5,a5,a4
zext.h a5,a5
.L3:
xor a3,a3,a5
andi a3,a3,1
srli a4,a0,3
srli a5,a5,1
beq a3,zero,.L4
li a3,-24576 # 0xFFFF_A000
addi a3,a3,1 # 0xFFFF_A001
[...]
.L8
andi a3,a5,1
srli a0,a5,1
beq a3,a4,.L9
li a5,-24576 # 0xFFFF_A000
addi a5,a5,1 # 0xFFFF_A001
xor a0,a0,a5
slli a0,a0,48
srli a0,a0,48
.L9:
ret
cse can't handle this: as explained by Jeff in [2] EBB can have jumps out but
not jumps in, which misses the cfg paths needed to be traversed to find the
equivalents.
[2] https://gcc.gnu.org/pipermail/gcc/2022-October/239646.html
Note that since gcc 13.1, this specific test generates different code since the
match.pd change 6508d5e5a1a ("match.pd: rewrite select to branchless
expression") now removes the branches and the arithmatic needing the large
const.
But this test is very convenient, so I'm continuing to use it and just revert
the match.pd change in my local gcc build.
