https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=259789
Bug ID: 259789
Summary: clang 9 and later performance regression using
pointers to traverse arrays
Product: Base System
Version: Unspecified
Hardware: amd64
OS: Any
Status: New
Severity: Affects Some People
Priority: ---
Component: bin
Assignee: [email protected]
Reporter: [email protected]
This may be an upstream issue, but I'm starting here in hopes that someone
would know for sure. I have not yet tried to reproduce this on other platforms.
1. The clang man page states that -O is equivalent to -O2, which is clearly not
the case for clang11 per the benchmark data below.
2. I wrote a very simple benchmark suite using selection sort to compare the
performance of various compilers and interpreters. There are subscript and
pointer versions for C. I just noticed that the pointer implementation takes
more than twice as long as the subscript implementations using clang90 or
later. On amd64 with a modern compiler, I would expect very little difference,
maybe a slight advantage to the pointer implementation, which is what I see
with clang80 and gcc10.
To reproduce:
git clone https://github.com/outpaddling/Lang-speed
cd Lang-speed
./clang-check
clang version 8.0.1 (tags/RELEASE_801/final)
Target: x86_64-portbld-freebsd13.0
Thread model: posix
InstalledDir: /usr/local/llvm80/bin
-O
Subscripts: 2.27 real 2.27 user 0.00 sys
Pointers: 2.05 real 2.05 user 0.00 sys
-O2
Subscripts: 2.10 real 2.10 user 0.00 sys
Pointers: 2.02 real 2.02 user 0.00 sys
clang version 9.0.1
Target: x86_64-portbld-freebsd13.0
Thread model: posix
InstalledDir: /usr/local/llvm90/bin
-O
Subscripts: 2.09 real 2.09 user 0.00 sys
Pointers: 4.67 real 4.67 user 0.00 sys
-O2
Subscripts: 2.05 real 2.05 user 0.00 sys
Pointers: 4.62 real 4.62 user 0.00 sys
FreeBSD clang version 11.0.1 ([email protected]:llvm/llvm-project.git
llvmorg-11.0.1-0-g43ff75f2c3fe)
Target: x86_64-unknown-freebsd13.0
Thread model: posix
InstalledDir: /usr/bin
-O
Subscripts: 4.62 real 4.62 user 0.00 sys
Pointers: 4.67 real 4.67 user 0.00 sys
-O2
Subscripts: 2.04 real 2.04 user 0.00 sys
Pointers: 4.61 real 4.61 user 0.00 sys
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