Loop unrolling + fusion ?
twhitehead at gmail.com
Mon Mar 2 14:42:33 EST 2009
On March 1, 2009 17:31:13 Max Bolingbroke wrote:
> I am no assembly guru and haven't seen that last form of leaq either,
> but I'm going to guess that:
> leaq (%rsi,%rsi,4), %rax
> Says that rax is rsi * ((1 + 1) * 2 ^ 4) = rsi * 32
> leaq 0(,%rax,8), %rsi
If I recall correctly, leaq is load effective address (i.e., write the address
into the destination register instead of the data at the address).
The address form is i(b,o,s) = i+b+o*s. You have (%rsi,%rsi,4) = %rsi+%rsi*4
into %rax followed by 0(,%rax,8) = rax*8 into %rsi, ultimately giving %rsi*40
into %rsi (which is the multiplication you have in the ghc generated loop).
(the restrictions on the address form is that s must be one of 1, 2, 4, or 8)
Interesting discussion by the way. : )
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