Re: [PATCH v2] x86emul: adjust BSF/BSR/LZCNT/TZCNT behavior as to EFLAGS

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.07.2025 18:19, Andrew Cooper wrote:
> On 14/07/2025 5:02 pm, Jan Beulich wrote:
>> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
>> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
>> @@ -5270,62 +5270,26 @@ x86_emulate(
>>          break;
>>  
>>      case X86EMUL_OPC(0x0f, 0xbc): /* bsf or tzcnt */
>> -    {
>> -        bool zf;
>> -
>> -        asm ( "bsf %2,%0" ASM_FLAG_OUT(, "; setz %1")
>> -              : "=r" (dst.val), ASM_FLAG_OUT("=@ccz", "=qm") (zf)
>> -              : "rm" (src.val) );
>> -        _regs.eflags &= ~X86_EFLAGS_ZF;
>> -        if ( (vex.pfx == vex_f3) && vcpu_has_bmi1() )
>> -        {
>> -            _regs.eflags &= ~X86_EFLAGS_CF;
>> -            if ( zf )
>> -            {
>> -                _regs.eflags |= X86_EFLAGS_CF;
>> -                dst.val = op_bytes * 8;
>> -            }
>> -            else if ( !dst.val )
>> -                _regs.eflags |= X86_EFLAGS_ZF;
>> -        }
>> -        else if ( zf )
>> +        if ( vex.pfx == vex_f3 )
>> +            emulate_2op_SrcV_srcmem("rep; bsf", src, dst, _regs.eflags);
> 
> Do we need the ; ?
> 
> We surely don't on 4.21, but I presume there are bugs in older
> binutils?  (All Clangs back to 3.5 seem happy)

Actually we can use TZCNT here and LZCNT below with gas 2.25 (and Clang
looks to be happy too, even version 3.0). I expect that's preferable over
merely omitting the semicolons?

Jan