Re: 2.2.18Pre Lan Performance Rocks!

Jeff V. Merkey
2000-10-31T23:22:15Z

Ingo Molnar wrote:
>
> On Tue, 31 Oct 2000, Jeff V. Merkey wrote:
>
> > [...] These types of optimizations are possible when people have
> > acccess to Intel Red Cover documents, [...]
>
> optimizing away AGIs has been documented by public Intel PDFs for years:
>
> [...] Since the Pentium processor has two integer pipelines, a register
> used as the base or index component of an effective address calculation
> (in either pipe) causes an additional clock cycle if that register is the
> destination of either instruction from the immediately preceding clock
> cycle. This effect is known as Address Generation Interlock (AGI).
>
> (ditto for the p6 core CPUs), and GCC (IIRC) tries to avoid AGI conflicts
> as much as possible. (and this kind of stuff belongs into the compiler)

Odd. When I profile Linux with EMON, I see tons of them. Anywhere code
does

mov eax, addr
mov [addr], ebx

one will get generated. Even something as simple as:

mov eax, addr
inc eax
dec eax
mov [addr]. ebx

will avoid an AGI (since the other pipeline has now been allowed a few
clocks to fetch the address and load it).

Jeff

>
> Ingo