Re: [PATCH] [i386] GLC tuning: Break false dependency for dest register.

[Uros Bizjak <ubizjak@gmail.com>]

On Fri, Jan 14, 2022 at 2:44 PM Hongyu Wang <wwwhhhyyy333@gmail.com> wrote:
>
> > Are there any technical obstacles to introduce subst to
> > define_{,insn_and_}split?
>
> gccint says: define_subst can be used only in define_insn and
> define_expand, it cannot be used in other expressions (e.g. in
> define_insn_and_split).

Hm, hm ... annoying ...

> I have no idea how to implement it in current infrastructure.
>
> > In the proposed patch, if the output register is also mentioned in the
> > input, then clearing before insn will clear the value in the input
> > register. The solution in the i386.md also takes care of this issue.
> >
>
> For this, I think we can add REGNO checks for operands in condition
> (which means there is true dependency).

Let's go in your direction, considering the limitations of current
infrastructure.
+{
+  if (TARGET_DEST_FALSE_DEPENDENCY
+      && get_attr_dest_false_dep (insn) ==
+        DEST_FALSE_DEP_TRUE)
+    output_asm_insn ("vxorps\t{%x0, %x0, %x0}", operands);
+  return "vgetmant<ssemodesuffix>\t{%2, <round_saeonly_mask_op3>%1,
%0<mask_operand3>|%0<mask_operand3>, %1<round_saeonly_mask_op3>, %2}";
+}

There is no need to pass the information via attributes. IMO, you
shoud use subst attribute directly in the condition:

{
  if (TARGET_DEST_FALSE_DEPENDENCY
      && <mask3_dest_false_dep_attr>
      && !reg_mentioned_p (operands[0], operands[1]))
   output_asm_insn ("vxorps\t{%x0, %x0, %x0}", operands);
  return ".....";
}

Assume the above works, so please:

- rename TARGET_DEST_FALSE_DEPENDENCY to something less generic, maybe
following existing BMI example with TARGET_AVOID_FALSE_DEP_FOR_AVX512F
- rename "mask3_dest_false_dep_attr" to "mask3_false_dep_for_avx512f_cond"

Please note reg_mentioned_p in the above condition. This function
returns nonzero if register op0 appears somewhere within op1 and is
critical for the correct operation of your patch.

Uros.