[linux-kernel] Don't add -O3 to CFLAGS #2866
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Cyan4973
approved these changes
Nov 16, 2021
It is no longer necessary to get good performance, there is only a small speed difference between -O2 and -O3, so just stick to the default of -O2. I've measured neutral compression speed and a ~3% decompression speed loss in userspace with clang & gcc. I've also measured neutral compression speed and a ~1% decompression speed loss in the kernel benchmarks. This also fixes the stack space usage on parisc. The compiler was buggy for -O3 and used ~3KB of stack space for several functions. With -O2 the problem is completely resolved, and stack space is back to a few hundred bytes. Additionally, we get a large code size win on gcc: | Compiler | Before (Bytes) | After (Bytes) | Delta (Bytes) | |----------|----------------|---------------|---------------| | gcc-11 | 952754 | 738954 | -213800 | | clang-12 | 976290 | 938826 | -37464 |
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It is no longer necessary to get good performance, there is only a small
speed difference between -O2 and -O3, so just stick to the default of
-O2. I've measured neutral compression speed and a ~3% decompression
speed loss in userspace with clang & gcc. I've also measured neutral
compression speed and a ~1% decompression speed loss in the kernel
benchmarks.
This also fixes the stack space usage on parisc. The compiler was buggy
for -O3 and used ~3KB of stack space for several functions. With -O2 the
problem is completely resolved, and stack space is back to a few hundred
bytes.
Additionally, we get a large code size win on gcc: