Linus Torvalds wrote:
>
> On Thu, 6 Aug 2009, Artur Skawina wrote:
>> 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);
--
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