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[linux-kernel] Don't add -O3 to CFLAGS

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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:

| Compiler | Before (Bytes) | After (Bytes) | Delta (Bytes) |
|----------|----------------|---------------|---------------|
| gcc-11   |         952754 |        738954 |       -213800 |
| clang-12 |         976290 |        938826 |        -37464 |
This commit is contained in:
Nick Terrell
2021-11-16 14:25:18 -08:00
parent ddae153947
commit e2d01863bc
1 changed files with 0 additions and 1 deletions
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contrib/linux-kernel/linux.mk
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@ -11,7 +11,6 @@
obj-$(CONFIG_ZSTD_COMPRESS) += zstd_compress.o obj-$(CONFIG_ZSTD_COMPRESS) += zstd_compress.o
obj-$(CONFIG_ZSTD_DECOMPRESS) += zstd_decompress.o obj-$(CONFIG_ZSTD_DECOMPRESS) += zstd_decompress.o
ccflags-y += -O3
ccflags-y += -Wno-error=deprecated-declarations ccflags-y += -Wno-error=deprecated-declarations
zstd_compress-y := \ zstd_compress-y := \