linux-kernel - [PATCH v2 3/3] lib: zstd: Don't add -O3 to cflags

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Date:   Wed, 17 Nov 2021 12:14:59 -0800
From:   Nick Terrell <nickrterrell@...il.com>
To:     Nick Terrell <terrelln@...com>
Cc:     linux-kernel@...r.kernel.org, linux-parisc@...r.kernel.org,
        Helge Deller <deller@....de>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH v2 3/3] lib: zstd: Don't add -O3 to cflags

From: Nick Terrell <terrelln@...com>

After the update to zstd-1.4.10 passing -O3 is no longer necessary to
get good performance from zstd. Using the default optimization level -O2
is sufficient to get good performance.

I've measured no significant change to compression speed, and a ~1%
decompression speed loss, which is acceptable.

This fixes the reported parisc -Wframe-larger-than=1536 errors [0]. The
gcc-8-hppa-linux-gnu compiler performed very poorly with -O3, generating
stacks that are ~3KB. With -O2 these same functions generate stacks in
the < 100B, completely fixing the problem. Function size deltas are
listed below:

ZSTD_compressBlock_fast_extDict_generic: 3800 -> 68
ZSTD_compressBlock_fast: 2216 -> 40
ZSTD_compressBlock_fast_dictMatchState: 1848 ->  64
ZSTD_compressBlock_doubleFast_extDict_generic: 3744 -> 76
ZSTD_fillDoubleHashTable: 3252 -> 0
ZSTD_compressBlock_doubleFast: 5856 -> 36
ZSTD_compressBlock_doubleFast_dictMatchState: 5380 -> 84
ZSTD_copmressBlock_lazy2: 2420 -> 72

Additionally, this improves the reported code bloat [1]. With gcc-11
bloat-o-meter shows an 80KB code size improvement:

```
> ../scripts/bloat-o-meter vmlinux.old vmlinux
add/remove: 31/8 grow/shrink: 24/155 up/down: 25734/-107924 (-82190)
Total: Before=6418562, After=6336372, chg -1.28%
```

Compared to before the zstd-1.4.10 update we see a total code size
regression of 105KB, down from 374KB at v5.16-rc1:

```
> ../scripts/bloat-o-meter vmlinux.old vmlinux
add/remove: 292/62 grow/shrink: 56/88 up/down: 235009/-127487 (107522)
Total: Before=6228850, After=6336372, chg +1.73%
```

[0] https://lkml.org/lkml/2021/11/15/710
[1] https://lkml.org/lkml/2021/11/14/189

Reported-by: Geert Uytterhoeven <geert@...ux-m68k.org>
Signed-off-by: Nick Terrell <terrelln@...com>
---
 lib/zstd/Makefile | 2 --
 1 file changed, 2 deletions(-)

diff --git a/lib/zstd/Makefile b/lib/zstd/Makefile
index 65218ec5b8f2..fc45339fc3a3 100644
--- a/lib/zstd/Makefile
+++ b/lib/zstd/Makefile
@@ -11,8 +11,6 @@
 obj-$(CONFIG_ZSTD_COMPRESS) += zstd_compress.o
 obj-$(CONFIG_ZSTD_DECOMPRESS) += zstd_decompress.o

-ccflags-y += -O3
-
 zstd_compress-y := \
 		zstd_compress_module.o \
 		common/debug.o \
-- 
2.33.1