Re: Why Git is so fast

From: James Pickens <jepicken@gmail.com>
Date: May 1, 2009, 01:25 UTC
Message-ID: <885649360904301825i40b6b7b7o9874ee3df2809a21@mail.gmail.com>
In-Reply-To: <f488382f0904301723i37ef03d9w4e93848e603ed28b@mail.gmail.com>

On Thu, Apr 30, 2009, Steven Noonan <steven@uplinklabs.net> wrote:

Show 13 quoted lines

> A bit off topic, but the results are rather interesting to me, and I
> think I see a weakness in how GCC is doing this on Intel. Someone
> please correct me if I'm wrong, but the PowerPC code seems much better
> because it can yield very high instruction-level parallelism. It does
> 5 loads and then 5 stores, using 4 registers for temporary storage and
> 2 registers for pointers.
>
> I realize the Intel x86 architecture is quite constrained in that it
> has so few general purpose registers, but there has to be better code
> than what GCC emitted above. It seems like the processor would stall
> because of the quantity of sequential inter-dependent instructions
> that can't be done in parallel (mov to memory that depends on a mov to
> eax, etc).

There aren't any unnecessary dependencies.  Take this sequence:

1: movl (%edx), %eax 2: movl %eax, (%ecx) 3: movl 4(%edx), %eax 4: movl %eax, 4(%ecx)

There are two unavoidable dependencies - #2 depends on #1, and #4 depends on #3. #3 does not depend on #2, even though they both use %eax, because #3 is a write to %eax. So whatever was in %eax before #3 is irrelevant. The processor knows this and will use register renaming to execute #1 and #3 in parallel, and #2 and #4 in parallel.

James