2014-08-14 18:00 GMT+04:00 Alexander Monakov <amona...@ispras.ru>:
>
>
> On Thu, 14 Aug 2014, Yuri Rumyantsev wrote:
>
>> Hi All,
>>
>> Here is a fix for PR 62011 - remove false dependency for unary
>> bit-manipulation instructions for latest BigCore chips (Sandybridge
>> and Haswell) by outputting in assembly file zeroing destination
>> register before bmi instruction. I checked that performance restored
>> for popcnt, lzcnt and tzcnt instructions.
>
> I am not an x86 reviewer, but one thing looks a bit superfluous to me:
>
>> +/* Retirn true if we need to insert before bit-manipulation instruction
> note typo^
>
>> + zeroing of its destination register. */
>> +bool
>> +ix86_avoid_false_dep_for_bm (rtx insn, rtx operands[])
>> +{
>> + unsigned int regno0;
>> + df_ref use;
>> + if (!TARGET_AVOID_FALSE_DEP_FOR_BM || optimize_function_for_size_p (cfun))
>> + return false;
>> + regno0 = true_regnum (operands[0]);
>> + /* Check if insn does not use REGNO0. */
>> + FOR_EACH_INSN_USE (use, insn)
>> + if (regno0 == DF_REF_REGNO (use))
>> + return false;
>> + return true;
>> +}
>
> The loop is to prevent adding the xor when the dest operand is also the source
> operand. Looks like a simpler "reg_or_subregno (operands[0]) ==
> reg_or_subregno (operands[1])" could be used here, as long as the assumption
> that this is called only for two-operand instruction holds?
This wouldn't cover memory operand case.
Ilya
>
> Alexander
