On Sat, 8 Aug 2009, Artur Skawina wrote:
>
> i was seeing such large variations depending on the -mtune flags that
> i gave up and now do just -march=i686; that's what i would expect
> for generic x86 binaries.
I think I have found a way to avoid the gcc crazyness.
Lookie here:
# TIME[s] SPEED[MB/s]
rfc3174 5.094 119.8
rfc3174 5.098 119.7
linus 1.462 417.5
linusas 2.008 304
linusas2 1.878 325
mozilla 5.566 109.6
mozillaas 5.866 104.1
openssl 1.609 379.3
spelvin 1.675 364.5
spelvina 1.601 381.3
nettle 1.591 383.6
notice? I outperform all the hand-tuned asm on 32-bit too. By quite a
margin, in fact.
Now, I didn't try a P4, and it's possible that it won't do that there, but
the 32-bit code generation sure looks impressive on my Nehalem box. The
magic? I force the stores to the 512-bit hash bucket to be done in order.
That seems to help a lot.
The diff is trivial (on top of the "rename registers with cpp" patch), as
appended. And it does seem to fix the P4 issues too, although I can
obviously (once again) only test Prescott, and only in 64-bit mode:
# TIME[s] SPEED[MB/s]
rfc3174 1.662 36.73
rfc3174 1.64 37.22
linus 0.2523 241.9
linusas 0.4367 139.8
linusas2 0.4487 136
mozilla 0.9704 62.9
mozillaas 0.9399 64.94
that's some really impressive improvement. All from just saying "do the
stores in the order I told you to, dammit!" to the compiler.
Linus
---
block-sha1/sha1.c | 3 ++-
1 files changed, 2 insertions(+), 1 deletions(-)
diff --git a/block-sha1/sha1.c b/block-sha1/sha1.c
index 19dc41d..f70e1ba 100644
--- a/block-sha1/sha1.c
+++ b/block-sha1/sha1.c
@@ -93,6 +93,7 @@ void blk_SHA1_Final(unsigned char hashout[20], blk_SHA_CTX *ctx)
/* This "rolls" over the 512-bit array */
#define W(x) (array[(x)&15])
+#define setW(x, val) (*(volatile unsigned int *)&W(x) = (val))
/*
* Where do we get the source from? The first 16 iterations get it from
@@ -102,7 +103,7 @@ void blk_SHA1_Final(unsigned char hashout[20], blk_SHA_CTX *ctx)
#define SHA_MIX(t) SHA_ROL(W(t+13) ^ W(t+8) ^ W(t+2) ^ W(t), 1)
#define SHA_ROUND(t, input, fn, constant, A, B, C, D, E) do { \
- unsigned int TEMP = input(t); W(t) = TEMP; \
+ unsigned int TEMP = input(t); setW(t, TEMP); \
E += TEMP + SHA_ROL(A,5) + (fn) + (constant); \
B = SHA_ROR(B, 2); } while (0)
--
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