On 5/15/24 12:49 AM, Christoph Müllner wrote:
GCC has a generic cmpmemsi expansion via the by-pieces framework,
which shows some room for target-specific optimizations.
E.g. for comparing two aligned memory blocks of 15 bytes
we get the following sequence:
my_mem_cmp_aligned_15:
li a4,0
j .L2
.L8:
bgeu a4,a7,.L7
.L2:
add a2,a0,a4
add a3,a1,a4
lbu a5,0(a2)
lbu a6,0(a3)
addi a4,a4,1
li a7,15 // missed hoisting
subw a5,a5,a6
andi a5,a5,0xff // useless
beq a5,zero,.L8
lbu a0,0(a2) // loading again!
lbu a5,0(a3) // loading again!
subw a0,a0,a5
ret
.L7:
li a0,0
ret
Diff first byte: 15 insns
Diff second byte: 25 insns
No diff: 25 insns
Possible improvements:
* unroll the loop and use load-with-displacement to avoid offset increments
* load and compare multiple (aligned) bytes at once
* Use the bitmanip/strcmp result calculation (reverse words and
synthesize (a2 >= a3) ? 1 : -1 in a branchless sequence)
When applying these improvements we get the following sequence:
my_mem_cmp_aligned_15:
ld a5,0(a0)
ld a4,0(a1)
bne a5,a4,.L2
ld a5,8(a0)
ld a4,8(a1)
slli a5,a5,8
slli a4,a4,8
bne a5,a4,.L2
li a0,0
.L3:
sext.w a0,a0
ret
.L2:
rev8 a5,a5
rev8 a4,a4
sltu a5,a5,a4
neg a5,a5
ori a0,a5,1
j .L3
Diff first byte: 11 insns
Diff second byte: 16 insns
No diff: 11 insns
This patch implements this improvements.
The tests consist of a execution test (similar to
gcc/testsuite/gcc.dg/torture/inline-mem-cmp-1.c) and a few tests
that test the expansion conditions (known length and alignment).
Similar to the cpymemsi expansion this patch does not introduce any
gating for the cmpmemsi expansion (on top of requiring the known length,
alignment and Zbb).
Bootstrapped and SPEC CPU 2017 tested.
gcc/ChangeLog:
* config/riscv/riscv-protos.h (riscv_expand_block_compare): New
prototype.
* config/riscv/riscv-string.cc (GEN_EMIT_HELPER2): New helper
for zero_extendhi.
(do_load_from_addr): Add support for HI and SI/64 modes.
(do_load): Add helper for zero-extended loads.
(emit_memcmp_scalar_load_and_compare): New helper to emit memcmp.
(emit_memcmp_scalar_result_calculation): Likewise.
(riscv_expand_block_compare_scalar): Likewise.
(riscv_expand_block_compare): New RISC-V expander for memory compare.
* config/riscv/riscv.md (cmpmemsi): New cmpmem expansion.
gcc/testsuite/ChangeLog:
* gcc.target/riscv/cmpmemsi-1.c: New test.
* gcc.target/riscv/cmpmemsi-2.c: New test.
* gcc.target/riscv/cmpmemsi-3.c: New test.
* gcc.target/riscv/cmpmemsi.c: New test.
[ ... ]
I fixed some of the nits from the linter (whitespace stuff) and pushed
both patches of this series.
Jeff
