Re: [Qemu-devel] [PATCH 2/7] tcg: Optimize add2 + sub2

[Richard Henderson]

On 09/27/2012 04:20 PM, Aurelien Jarno wrote:
> I understand that we can't easily insert an instruction, so the
> limitation comes from here, but is it really something happening often?

It will certainly appear sometimes.  E.g. s390x has an add immediate
instruction that does exactly: r1 += imm16 << 32.

Or did you mean specifically the full constant being folded?  That
would happen quite a bit more often.  That you can see with most any
64-bit RISC guest when they attempt to generate a constant from 
addition primitives instead of logical primitives.

For a 32-bit host, we've already decomposed logical primitives to 32-bit
operations.  And we can constant-fold through all of those.  But when
addition comes into play, we can't constant-fold through add2.


r~