On 2026/2/26 10:32, Yifan Zhao wrote:
Hi Lee,
A quick search shows that x86 kernel implementation also use `div` instruction
under Linux v6.19 and GCC 15.2.1, add GCC correctly generate shift instruction
Could we try more GCC versions and find the impacts?
Does it a recent GCC regression?
in my arm64 machine with GCC 14.2.0.
Could you also consider evaluate this optimization in kernel?
Yes, but I wonder if `div` is already used for these years on x86,
could you check this?
Thanks,
Gao Xiang
Thanks,
Yifan
On 2/26/2026 1:30 AM, Ashley Lee wrote:
perf on fsck.erofs in gcc reports that z_erofs_load_compact_lcluster
was spending 20% of its time doing the div instruction. While the
function itself is ~40% of user runtime. In the source code, it seems
that dividing by vcnt doesn't optimize to a shift despite the two
possible states being powers of 2.
Changing the division into a ilog2() function call encourages the
compiler to recognize it as a power of 2. Thus performing a shift.
Running a benchmark on lzma compressed freebsd code on x86, shows
there is a ~4% increase in performance in gcc. While clang shows
virtually no regression in performance. The tradeoff is slightly
obfuscated source code.
The following command was run locally on x86.
$ hyperfine -w 5 -m 30 "./fsck.erofs ./bsd.erofs.lzma"
patch on gcc 15.2.1
Time (mean ± σ): 354.8 ms ± 6.0 ms \
[User: 227.8 ms, System: 126.1 ms]
Range (min … max): 345.8 ms … 366.2 ms 30 runs
dev on gcc 15.2.1
Time (mean ± σ): 370.7 ms ± 6.7 ms \
[User: 246.5 ms, System: 123.4 ms]
Range (min … max): 362.7 ms … 390.7 ms 30 runs
patch on clang 21.1.8
Time (mean ± σ): 371.9 ms ± 2.4 ms \
[User: 247.2 ms, System: 123.9 ms]
Range (min … max): 369.1 ms … 380.0 ms 30 runs
dev on clang 21.1.8
Time (mean ± σ): 371.0 ms ± 1.9 ms \
[User: 245.5 ms, System: 124.5 ms]
Range (min … max): 368.4 ms … 377.7 ms 30 runs
Signed-off-by: Ashley Lee <[email protected]>
---
v2: changed vdiv to ilog2 call
lib/zmap.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/lib/zmap.c b/lib/zmap.c
index baec278..3ac7fe9 100644
--- a/lib/zmap.c
+++ b/lib/zmap.c
@@ -160,7 +160,7 @@ static int z_erofs_load_compact_lcluster(struct
z_erofs_maprecorder *m,
m->nextpackoff = round_down(pos, vcnt << amortizedshift) +
(vcnt << amortizedshift);
lobits = max(lclusterbits, ilog2(Z_EROFS_LI_D0_CBLKCNT) + 1U);
- encodebits = ((vcnt << amortizedshift) - sizeof(__le32)) * 8 / vcnt;
+ encodebits = (((vcnt << amortizedshift) - sizeof(__le32)) * 8) >>
ilog2(vcnt);
bytes = pos & ((vcnt << amortizedshift) - 1);
in -= bytes;
i = bytes >> amortizedshift;
