Il 26/05/2013 18:35, Lior Vernia ha scritto:
> What about no to the first bullet but yes to the second (just x86 on
> ARM)? Any room for significant improvement in that case, starting from
> the foundations of QEMU?
You could write a target-specific translator, yes. But first of all I
would answer whether you're using 32- or 64-bit, and run some profiling
to see what is the hotspot in your case.
I know that in some scenarios helpers for SSE take a considerable amount
of time (5-10%). You could look at adding SIMD data types to TCG, and
map them to Neon operations or even to fully-unrolled loops.
As other works, ahead-of-time translation can also do a lot more
optimizations, including very aggressive dead-code elimination. For
example, again considering SSE, something like
pcmpeqw %xmm0, %xmm1
pmovmskb %xmm1, %eax
test %eax, %eax
jz ...
will be translated to a slow sequence in QEMU due to the expensive
pmovmskb. A custom code generator can observe that %eax is dead and use
a better translation of this idiom.
Also, floating-point emulation is always done in software in QEMU due to
different representations (and due to the 80-bit floating-point
registers mostly used by 32-bit x86). This is going to be slow no
matter what.
Paolo
