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James Gregory 2013-12-31 11:06:31 +11:00
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@ -22,9 +22,13 @@ the branch or fall-through takes 4 or 5 cycles (if it executes in the U-
or V-pipe, respectively)—1 or 2 cycles more than a branch and 3 or 4
cycles more than a fall-through on the 486.
------------------- -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
![](images/i.jpg) *Branch prediction is unprecedented in the x86, and fundamentally alters the nature of pedal-to-the-metal optimization, for the simple reason that it renders unrolled loops largely obsolete. Rare indeed is the loop that can't afford to spare even 1 or 0 (yes, zero!) cycles per iteration for loop counting, and that's how low the cost can go for maintaining a loop on the Pentium.*
------------------- -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> ![](images/i.jpg)
> Branch prediction is unprecedented in the x86, and fundamentally alters
> the nature of pedal-to-the-metal optimization, for the simple reason
> that it renders unrolled loops largely obsolete. Rare indeed is the loop
> that can't afford to spare even 1 or 0 (yes, zero!) cycles per iteration
> for loop counting, and that's how low the cost can go for maintaining a
> loop on the Pentium.
Also, unrolled loops are bigger than normal loops, so there are extra
(and expensive) cache misses the first time through the loop if the
@ -58,9 +62,13 @@ count on a branch to take 1 cycle when it falls through, but on the
Pentium you can't be sure whether it will take 1 or either 4 or 5 cycles
on any given iteration.
------------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
![](images/i.jpg) *As things currently stand, branch prediction is an annoyance for assembly language optimization because it's impossible to be certain exactly how code will perform until you measure it, and even then it's difficult to be sure exactly where the cycles went. All I can say is try to fall through branches if possible, and try to be consistent in your branching if not.*
------------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> ![](images/i.jpg)
> As things currently stand, branch prediction is an annoyance for
> assembly language optimization because it's impossible to be certain
> exactly how code will perform until you measure it, and even then it's
> difficult to be sure exactly where the cycles went. All I can say is try
> to fall through branches if possible, and try to be consistent in your
> branching if not.
### Miscellaneous Pentium Topics {#Heading9}