Re: [Qemu-devel] [PATCH 5/5] tcg/i386: Use SHLX/SHRX/SARX instructions

[Richard Henderson]

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~