On 10/23/26 4:39 AM, Michiel Derhaeg wrote:
Michiel Derhaeg <[email protected]> writes:
From: Loeka Rogge <[email protected]>
This removes the guards that disables the clmulr pattern added by
patch r17-4206-ge91f79891fc657: Use clmulr for reversed crc32 on rv32
from being used on rv64.
This changes the calculation of the reflected quotient (on the host) to
fit inside 64 bits. gf2n_poly_long_div_quotient calculates
x^128 / (x^64 + lower order terms) = x^64 + lower order terms.
Meaning the result is missing an implicit 1 in front of the msb.
After reversing, this missing 1 should be after the lsb. This is
re-added with a shift and an or. This causes the msb to fall outside
the result stored in t0.
The value in t0 is only used by the emitted clmul or vclmul
instructions. Clmul only emits the bottom 64 bits, representing x^63 and
lower order terms. This is unaffected by the missing bit that
represents x^64, because it would always fall outside the result.
Regtested on rv32gc, rv32gc_zbc, rv64gc, and rv64gc_zbc.
gcc/ChangeLog:
* config/riscv/bitmanip.md: Remove 64 bit guard.
* config/riscv/riscv.cc (riscv_optab_supported_p): Likewise.
64 bit guard.
(expand_reversed_crc_using_clmul): Adapt quotient calculation
for 65 bits
gcc/testsuite/ChangeLog:
* gcc.target/riscv/crc-builtin-zbc64.c: test whether clmulr is
applied for crc64
I noticed too late that we forgot to add a changelog for the new test.
No worries. I routinely fix ChangeLog entries since pre-commit hooks
catch this kind of stuff at push time.
jeff
