[PATCH ] x86/lib: Optimise copy loop for long buffers in csum-partial_64.c
From: David Laight <hidden>
Date: 2022-01-06 16:20:38
Also in:
lkml
Subsystem:
the rest, x86 architecture (32-bit and 64-bit) · Maintainers:
Linus Torvalds, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen
gcc converts the loop into one that only increments the pointer
but makes a mess of calculating the limit and gcc 9.1+ completely
refuses to use the final value of 'buff' from the last iteration.
Explicitly code a pointer comparison and don't bother changing len.
Signed-off-by: David Laight <redacted>
---
The asm("" : "+r" (buff)); forces gcc to use the loop-updated
value of 'buff' and removes at least 6 instructions.
The gcc folk really ought to look at why gcc 9.1 onwards is so
much worse that gcc 8.
See https://godbolt.org/z/T39PcnvfE
arch/x86/lib/csum-partial_64.c | 33 ++++++++++++++++++---------------
1 file changed, 18 insertions(+), 15 deletions(-)
diff --git a/arch/x86/lib/csum-partial_64.c b/arch/x86/lib/csum-partial_64.c
index edd3e579c2a7..342de5f24fcb 100644
--- a/arch/x86/lib/csum-partial_64.c
+++ b/arch/x86/lib/csum-partial_64.c@@ -27,21 +27,24 @@ __wsum csum_partial(const void *buff, int len, __wsum sum) u64 temp64 = (__force u64)sum; unsigned result; - while (unlikely(len >= 64)) { - asm("addq 0*8(%[src]),%[res]\n\t" - "adcq 1*8(%[src]),%[res]\n\t" - "adcq 2*8(%[src]),%[res]\n\t" - "adcq 3*8(%[src]),%[res]\n\t" - "adcq 4*8(%[src]),%[res]\n\t" - "adcq 5*8(%[src]),%[res]\n\t" - "adcq 6*8(%[src]),%[res]\n\t" - "adcq 7*8(%[src]),%[res]\n\t" - "adcq $0,%[res]" - : [res] "+r" (temp64) - : [src] "r" (buff) - : "memory"); - buff += 64; - len -= 64; + if (unlikely(len >= 64)) { + const void *lim = buff + (len & ~63u); + do { + asm("addq 0*8(%[src]),%[res]\n\t" + "adcq 1*8(%[src]),%[res]\n\t" + "adcq 2*8(%[src]),%[res]\n\t" + "adcq 3*8(%[src]),%[res]\n\t" + "adcq 4*8(%[src]),%[res]\n\t" + "adcq 5*8(%[src]),%[res]\n\t" + "adcq 6*8(%[src]),%[res]\n\t" + "adcq 7*8(%[src]),%[res]\n\t" + "adcq $0,%[res]" + : [res] "+r" (temp64) + : [src] "r" (buff) + : "memory"); + asm("" : "+r" (buff)); + buff += 64; + } while (buff < lim); } if (len & 32) {
--
2.17.1
-
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