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

Linus Torvalds wrote:

On Thu, 6 Aug 2009, Artur Skawina wrote:

quoted

Oh, i noticed that '-mtune' makes quite a difference, it can change
the relative performance of the functions significantly, in unobvious
ways; depending on which cpu gcc tunes for (build config or -mtune);
some implementations slow down, others become a bit faster.

That probably is mainly true for P4, although it's quite possible that it 
has an effect for just what the register allocator does, and then for 
spilling.

And it looks like _all_ the tweakability is in the spilling. Nothing else 
matters.

How does this patch work for you? It avoids doing that C-level register 
rotation, and instead rotates the register names with the preprocessor.

I realize it's ugly as hell, but it does make it easier for gcc to see 
what's going on.

The patch is against my git patches, but I think it should apply pretty 
much as-is to your sha1bench sources too. Does it make any difference for 
you?

it's a bit slower (P4):

before: linus          0.6288       97.06
after:  linus          0.6604       92.42

i was trying similar things, like the example below, too, but it wasn't a
win on 32 bit...

artur

[the iteration below is functionally correct, but scheduling is most likely
 fubared as it wasn't a win and i was checking how much a difference it made
 on P4 -- ~-20..~0%, but never faster (relative to linusas2; it _is_ faster
 than 'linus'. Dropped this version when merging your new preprocessor macros.]

@@ -125,6 +127,8 @@
 #define W(x) (array[(x)&15])
 #define SHA_XOR(t) \
        TEMP = SHA_ROL(W(t+13) ^ W(t+8) ^ W(t+2) ^ W(t), 1); W(t) = TEMP;
+#define SHA_XOR2(t) \
+       SHA_ROL(W(t+13) ^ W(t+8) ^ W(t+2) ^ W(t), 1)
 
 #define T_16_19(t) \
         { unsigned TEMP;\

@@ -139,10 +143,27 @@
 #endif
 
 #define T_20_39(t) \
-        { unsigned TEMP;\
-       SHA_XOR(t); \
-       TEMP += (B^C^D) + E + 0x6ed9eba1; \
-       E = D; D = C; C = SHA_ROR(B, 2); B = A; TEMP += SHA_ROL(A,5); A = TEMP; }
+        if (t%2==0) {\
+               unsigned TEMP;\
+               unsigned TEMP2;\
+               \
+               TEMP   = SHA_XOR2(t); \
+               TEMP2  = SHA_XOR2(t+1); \
+               W(t)   = TEMP;\
+               W(t+1) = TEMP2;\
+               TEMP   += E + 0x6ed9eba1; \
+               E      = C;\
+               TEMP   += (B^E^D); \
+               TEMP2  += D + 0x6ed9eba1; \
+               D      = SHA_ROR(B, 2);\
+               B      = SHA_ROL(A, 5);\
+               B      += TEMP;\
+               C      = SHA_ROR(A, 2);\
+               A      ^= E; \
+               A      ^= D; \
+               A      += TEMP2;\
+               A      += SHA_ROL(B, 5);\
+       }
 
 #if UNROLL
        T_20_39(20); T_20_39(21); T_20_39(22); T_20_39(23); T_20_39(24);