Re: [Qemu-devel] Re: branches are expensive

[Paul Brook]

> I've tested Qemu 0.10.0 and with i386-softmmu on a i386 host I get the
> following numbers:
> direct jump count 70%, 2 jumps 54%
>
> For completeness I tested qemu-system-arm on a i386 host as well:
> direct jump count 44%, 2 jumps 37%
>
> So it looks like the chaining on ARM targets is not as effective as on i386
> targets (regardless of the guest, I used the same guest setup, compiled for
> different architectures, on all tests). Do you have any ideas why this is
> the case?

A couple of likely reasons:

- ARM uses 1k pages, i386 uses 4k pages, so there's greater probability of 
spanning a page boundary.
- ARM has conditional execution, so code will tend to have less conditional 
branches. The number of function calls (in particular function returns, which 
are indirect branches) is likely to be about the same, so the proportion of 
direct jumps is less.

Paul