https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119223
Bug ID: 119223
Summary: GCC does not optimize with AVX in bitshift with if
condition
Product: gcc
Version: 14.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: kaelfandrew at gmail dot com
Target Milestone: ---
I decided to create 2 C programs that matches newlines in a file (The file is
src/Sema.zig from Zig 0.14) from https://godbolt.org/z/v9hqzPv4b. Both programs
behave the same.
The only difference is at line 56, where the first C code has no if condition.
GCC adds SIMD when no if condition is used as seen in second C program. Clang
optimizes both with SIMD. The difference seems to be at -fdump-tree-optimized.
Gentoo GCC 14.2 was used and both C programs was optimized with -std=gnu23 -O3
-march=icelake-client -D_FILE_OFFSET_BITS=64 -flto.
uname -a is Linux tux 6.6.67-gentoo-gentoo-dist #4 SMP PREEMPT_DYNAMIC Sun Jan
26 03:15:41 EST 2025 x86_64 Intel(R) Core(TM) i5-1035G1 CPU @ 1.00GHz
GenuineIntel GNU/Linux
The results are measured from poop with the following speedups:
./poop './main2' './main1' -d 60000
Benchmark 1 (10000 runs): ./main2
measurement mean ± σ min … max outliers
delta
wall_time 4.58ms ± 972us 2.11ms … 6.88ms 0 ( 0%)
0%
peak_rss 3.10MB ± 64.4KB 2.78MB … 3.20MB 1 ( 0%)
0%
cpu_cycles 4.97M ± 110K 4.47M … 6.18M 1090 (11%)
0%
instructions 12.0M ± 1.19 12.0M … 12.0M 799 ( 8%)
0%
cache_references 31.4K ± 528 30.1K … 32.9K 0 ( 0%)
0%
cache_misses 4.26K ± 808 2.73K … 10.8K 170 ( 2%)
0%
branch_misses 28.1K ± 285 10.4K … 28.2K 153 ( 2%)
0%
Benchmark 2 (10000 runs): ./main1
measurement mean ± σ min … max outliers
delta
wall_time 3.28ms ± 310us 1.54ms … 4.61ms 1807 (18%) -
28.4% ± 0.4%
peak_rss 3.10MB ± 64.0KB 2.78MB … 3.20MB 2 ( 0%)
- 0.0% ± 0.1%
cpu_cycles 2.06M ± 28.2K 2.02M … 2.72M 602 ( 6%) -
58.6% ± 0.0%
instructions 2.37M ± 1.14 2.37M … 2.37M 5 ( 0%) -
80.2% ± 0.0%
cache_references 31.4K ± 378 30.5K … 32.8K 5 ( 0%)
+ 0.3% ± 0.0%
cache_misses 4.25K ± 809 2.71K … 15.6K 246 ( 2%)
- 0.3% ± 0.5%
branch_misses 2.16K ± 35.0 1.44K … 2.32K 110 ( 1%) -
92.3% ± 0.0%
