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authorJeff King <peff@peff.net>2020-09-24 19:21:11 (GMT)
committerJunio C Hamano <gitster@pobox.com>2020-09-24 19:30:09 (GMT)
commitc578e29ba0791041ad7fabf1166dd6f7e7f26d1f (patch)
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parent47ae905ffb98cc4d4fd90083da6bc8dab55d9ecc (diff)
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bswap.h: drop unaligned loads
Our put_be32() routine and its variants (get_be32(), put_be64(), etc) has two implementations: on some platforms we cast memory in place and use nothl()/htonl(), which can cause unaligned memory access. And on others, we pick out the individual bytes using bitshifts. This introduces extra complexity, and sometimes causes compilers to generate warnings about type-punning. And it's not clear there's any performance advantage. This split goes back to 660231aa97 (block-sha1: support for architectures with memory alignment restrictions, 2009-08-12). The unaligned versions were part of the original block-sha1 code in d7c208a92e (Add new optimized C 'block-sha1' routines, 2009-08-05), which says it is: Based on the mozilla SHA1 routine, but doing the input data accesses a word at a time and with 'htonl()' instead of loading bytes and shifting. Back then, Linus provided timings versus the mozilla code which showed a 27% improvement: https://lore.kernel.org/git/alpine.LFD.2.01.0908051545000.3390@localhost.localdomain/ However, the unaligned loads were either not the useful part of that speedup, or perhaps compilers and processors have changed since then. Here are times for computing the sha1 of 4GB of random data, with and without -DNO_UNALIGNED_LOADS (and BLK_SHA1=1, of course). This is with gcc 10, -O2, and the processor is a Core i9-9880H. [stock] Benchmark #1: t/helper/test-tool sha1 <foo.rand Time (mean ± σ): 6.638 s ± 0.081 s [User: 6.269 s, System: 0.368 s] Range (min … max): 6.550 s … 6.841 s 10 runs [-DNO_UNALIGNED_LOADS] Benchmark #1: t/helper/test-tool sha1 <foo.rand Time (mean ± σ): 6.418 s ± 0.015 s [User: 6.058 s, System: 0.360 s] Range (min … max): 6.394 s … 6.447 s 10 runs And here's the same test run on an AMD A8-7600, using gcc 8. [stock] Benchmark #1: t/helper/test-tool sha1 <foo.rand Time (mean ± σ): 11.721 s ± 0.113 s [User: 10.761 s, System: 0.951 s] Range (min … max): 11.509 s … 11.861 s 10 runs [-DNO_UNALIGNED_LOADS] Benchmark #1: t/helper/test-tool sha1 <foo.rand Time (mean ± σ): 11.744 s ± 0.066 s [User: 10.807 s, System: 0.928 s] Range (min … max): 11.637 s … 11.863 s 10 runs So the unaligned loads don't seem to help much, and actually make things worse. It's possible there are platforms where they provide more benefit, but: - the non-x86 platforms for which we use this code are old and obscure (powerpc and s390). - the main caller that cares about performance is block-sha1. But these days it is rarely used anyway, in favor of sha1dc (which is already much slower, and nobody seems to have cared that much). Let's just drop unaligned versions entirely in the name of simplicity. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'Makefile')
-rw-r--r--Makefile1
1 files changed, 0 insertions, 1 deletions
diff --git a/Makefile b/Makefile
index 372139f..a8bedd4 100644
--- a/Makefile
+++ b/Makefile
@@ -1214,7 +1214,6 @@ SANITIZERS := $(foreach flag,$(subst $(comma),$(space),$(SANITIZE)),$(flag))
BASIC_CFLAGS += -fsanitize=$(SANITIZE) -fno-sanitize-recover=$(SANITIZE)
BASIC_CFLAGS += -fno-omit-frame-pointer
ifneq ($(filter undefined,$(SANITIZERS)),)
-BASIC_CFLAGS += -DNO_UNALIGNED_LOADS
BASIC_CFLAGS += -DSHA1DC_FORCE_ALIGNED_ACCESS
endif
ifneq ($(filter leak,$(SANITIZERS)),)