Re: [PATCH 0/7] block-sha1: improved SHA1 hashing

[Linus Torvalds <torvalds@linux-foundation.org>]

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)