Re: [Qemu-devel] [PATCH 16/25] tcg/i386: Handle ctz and clz opcodes

[Richard Henderson]

On 11/17/2016 05:50 PM, Bastian Koppelmann wrote:
> On 11/16/2016 08:25 PM, Richard Henderson wrote:
> > +
> > +    OP_32_64(clz):
> > +        if (const_args[2]) {
> > +            tcg_debug_assert(have_bmi1);
> > +            tcg_debug_assert(args[2] == (rexw ? 64 : 32));
> > +            tcg_out_modrm(s, OPC_LZCNT + rexw, args[0], args[1]);
> > +        } else {
> > +            /* ??? See above.  */
> > +            tcg_out_modrm(s, OPC_BSR + rexw, args[0], args[1]);
>
> The Intel ISA manual states that it find the bit index of the most
> significant bit, where the least significant bit is index 0. So for the
> input 0x2 this should return 1. However this is not the number of
> leading zeros.

Oh, of course you're right.  I thought I was testing this, but while alpha does 
have this operation, it turns out it isn't used much.


r~