https://gcc.gnu.org/g:80491b0493ac1e2b0cdbdfc3eab8c5c5a390d77c
commit r15-6322-g80491b0493ac1e2b0cdbdfc3eab8c5c5a390d77c
Author: Xi Ruoyao <xry...@xry111.site>
Date: Thu Dec 5 14:19:02 2024 +0800
LoongArch: Combine xor and crc instructions
For a textbook-style CRC implementation:
uint32_t crc = 0xffffffffu;
for (size_t k = 0; k < len; k++)
{
crc ^= data[k];
for (int i = 0; i < 8 * sizeof (T); i++)
if (crc & 1)
crc = (crc >> 1) ^ poly;
else
crc >>= 1;
}
return crc;
The generic code reports:
Data and CRC are xor-ed before for loop. Initializing data with 0.
resulting in:
ld.bu $t1, $a0, 0
xor $t0, $t0, $t1
crc.w.b.w $t0, $zero, $t0
But it's just better to use
ld.bu $t1, $a0, 0
crc.w.b.w $t0, $t1, $t0
instead. Implement this optimization now.
gcc/ChangeLog:
* config/loongarch/loongarch.md (*crc_combine): New
define_insn_and_split.
Diff:
---
gcc/config/loongarch/loongarch.md | 25 +++++++++++++++++++++++++
1 file changed, 25 insertions(+)
diff --git a/gcc/config/loongarch/loongarch.md
b/gcc/config/loongarch/loongarch.md
index 806b0ec0be9e..7a110ca9de6e 100644
--- a/gcc/config/loongarch/loongarch.md
+++ b/gcc/config/loongarch/loongarch.md
@@ -4477,6 +4477,31 @@
DONE;
})
+(define_insn_and_split "*crc_combine"
+ [(set (match_operand:SI 0 "register_operand" "=r,r")
+ (unspec:SI
+ [(reg:SUBDI 0)
+ (subreg:SI
+ (xor:DI
+ (match_operand:DI 1 "register_operand" "r,r")
+ ; Our LOAD_EXTEND_OP makes this same as sign_extend
+ ; if SUBDI is SI, or zero_extend if SUBDI is QI or HI.
+ ; For the former the high bits in rk are ignored by
+ ; crc.w.w.w anyway, for the latter the zero extension is
+ ; necessary for the correctness of this transformation.
+ (subreg:DI
+ (match_operand:SUBDI 2 "memory_operand" "m,k") 0)) 0)]
+ CRC))]
+ "TARGET_64BIT && loongarch_pre_reload_split ()"
+ "#"
+ "&& true"
+ [(set (match_dup 3) (match_dup 2))
+ (set (match_dup 0)
+ (unspec:SI [(match_dup 3) (subreg:SI (match_dup 1) 0)] CRC))]
+ {
+ operands[3] = gen_reg_rtx (<MODE>mode);
+ })
+
;; With normal or medium code models, if the only use of a pc-relative
;; address is for loading or storing a value, then relying on linker
;; relaxation is not better than emitting the machine instruction directly.
