On 02/16/2014 08:21 AM, Paolo Bonzini wrote:
> Il 31/01/2014 15:43, Richard Henderson ha scritto:
>> + gen_shift_maybe_vex:
>> + if (have_bmi2 && !const_args[2]) {
>> + tcg_out_vex_modrm(s, vexop + rexw, args[0], args[2], args[1]);
>> + break;
>> + }
>> + /* FALLTHRU */
>
> What if args[2] happens to be ECX?
I ran some measurements and as I expected this basically never happens. For
64-bit, I never saw it occur. For 32-bit, 1/800 of all shifts used ecx.
For 64-bit, the use of shlx et al is always a size win. The mov and shift,
including their rex prefixes, are 3 bytes each, while the shlx is 5 byes.
For 32-bit, things are more complicated. The mov and shift are 2 bytes each,
so the use of shlx is by itself a 1 byte size penalty. Except that sometimes
the avoidance of the mov results in fewer spills, and thus fewer bytes overall.
So overall I see the barest fraction (< 0.01%) size decrease across all TBs.
r~
