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Re: [PATCH] Add explicit VIS intrinsics for addition and subtraction.

On 09/28/2011 06:38 PM, Eric Botcazou wrote:
[Vlad, if you have a few minutes, would you mind having a look at the couple of
questions at the end of the message?  Thanks in advance].

No problem.
Here are the results of the investigation.  Pseudo 116 needs to be assigned a
hard register.  It is used mostly in vector instructions so we would like it
to be assigned a FP reg, but it is initialized in insn 2:

(insn 2 5 3 2 (set (reg/v:V4HI 116 [ a ])
         (reg:V4HI 24 %i0 [ a ])) combined-1.c:7 93 {*movdf_insn_sp32_v9}
      (expr_list:REG_DEAD (reg:V4HI 24 %i0 [ a ])

so it ends up being assigned the (integer) argument register %i0 instead.  It
used to be assigned a FP reg as expected with the GCC 4.6.x series.

The register class preference discovery is OK:

     r116: preferred EXTRA_FP_REGS, alternative GENERAL_OR_EXTRA_FP_REGS,
     a2 (r116,l0) best EXTRA_FP_REGS, allocno GENERAL_OR_EXTRA_FP_REGS

i.e. EXTRA_FP_REGS is "preferred"/"best".  Then it seems that this preference
is dropped and only the class of the allocno, GENERAL_OR_EXTRA_FP_REGS, is
handed down to the coloring stage.  By contrast, in the GCC 4.6 series, the
cover_class of the allocno is EXTRA_FP_REGS.

The initial cost for %i0 is twice as high (24000) as the cost of FP regs.  But
then it is reduced by 12000 when process_bb_node_for_hard_reg_moves sees insn
2 above and then again by 12000 when process_regs_for_copy sees the same insn.
So, in the end, %i0 is given cost 0 and thus beats every other register.  This
doesn't happen in the GCC 4.6 series because %i0 isn't in the cover_class.

This is at -O1.  At -O2, there is an extra pass at the discovery stage and it
sets the class of the allocno to EXTRA_FP_REGS, like with the GCC 4.6 series,
so a simple workaround is

---       (revision 179316)
+++       (working copy)
@@ -1,5 +1,5 @@
  /* { dg-do compile } */
-/* { dg-options "-O -mcpu=ultrasparc -mvis" } */
+/* { dg-options "-O2 -mcpu=ultrasparc -mvis" } */
  typedef short vec16 __attribute__((vector_size(8)));
  typedef int vec32 __attribute__((vector_size(8)));

Finally the couple of questions:

1. Is it expected that the register class preference be dropped at -O1?

  2. Is it expected that a single insn be processed by 2 different mechanisms
that independently halve the initial cost of a hard register?

Sorry for the delay with the answer. I missed this email.

About the 1st question. Before gcc4.7, the only class (allocno class) used for coloring can be a cover class. So it was not possible to use GENERAL_OR_EXTRA_FP_REGS in gcc4.6 and older versions. Starting gcc4.7, class used for coloring can be any class which is more profitable than memory. Although there is inaccuracy in cost calculations for -O1 because only one pass for cost calculations is used (it is very expensive pass). To get better cost evaluations, more passes should be used. But again we don't do more 2 passes because even one pass is not cheap.

In brief, I don't see any criminal that the class calculation is different for -O1 and -O2.

About the 2nd question. It seems to me wrong. I'd remove function process_bb_node_for_hard_reg_moves and its call from setup_allocno_cover_class_and_costs because function process_regs_for_copy is more accurate (it works with subreg). Although, I might be miss something here. There were a lot of problems and tunings of cost calculation code. Generated code *performance* (and even generation of *valid* code) is very sensitive to changes in ira-costs.c. So even if such change looks obvious, a lot of testing and benchmarking should be done. I could do that but it will take a week or two before committing such change if everything is ok.

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