On 7/10/25 8:44 AM, Dusan Stojkovic wrote:
This peephole pattern combines the following instructions:
bswap8:
rev8 a5,a0
-> li a4,-65536
-> srai a5,a5,32
-> and a5,a5,a4
-> roriw a5,a5,16
and a0,a0,a4
or a0,a0,a5
sext.w a0,a0
ret
And emits this assembly:
bswap8:
rev8 a5,a0
-> li a4,-65536
-> srai a5,a5,48
and a0,a0,a4
or a0,a0,a5
sext.w a0,a0
ret
Since the load instruction is required for the rest of the test function
in the PR, the pattern conserves the load.
2025-07-10 Dusan Stojkovic <dusan.stojko...@rt-rk.com>
PR target/120920
gcc/ChangeLog:
* config/riscv/peephole.md: New pattern.
gcc/testsuite/ChangeLog:
* gcc.target/riscv/zbb_bswap8.c: New test.
So I'm not sure this transformation is correct.
Let's consider the case where a5 has the value 0xffff000000000000 at the
"li" instruction.
a5 = 0xff00000000000000
li a4, -65536 // a4 = 0xffffffffffff0000
srai a5,a5,32 // a5 = 0xffffffffff000000
and a5,a5,a4 // a5 = 0xffffffffff000000
roriw a5,a5,16 // a5 = 0x000000000000ff00
So with your change:
a5 = 0xff00000000000000
li a4, -65536 // a4 = 0xffffffffffff0000
srai a5,a5,48 // a5 = 0xfffffffffffff000
Am I missing something?
Jeff
