Re: [PATCH, PR61605, 1/2] Handle copy cycles in pass_cprop_hardreg

[Jeff Law]

On 10/16/14 03:13, Tom de Vries wrote:

Eric,

this patch is the first half of the fix for PR61605.

The problem it addresses is the following: Consider this copy cycle (a =
b; b = a):
...
(insn 2 18 3 2 (set (reg/v:SI 1 dx [orig:86 yD.1749 ] [86])
         (reg:SI 5 di [ yD.1749 ])) test.c:9 90 {*movsi_internal}
      (expr_list:REG_DEAD (reg:SI 5 di [ yD.1749 ])
         (nil)))
(note 3 2 6 2 NOTE_INSN_FUNCTION_BEG)
(insn 6 3 7 2 (set (reg:SI 5 di)
         (reg/v:SI 1 dx [orig:86 yD.1749 ] [86])) test.c:10 90
{*movsi_internal}
      (nil))
...

cprop_hardreg handles this currently in the following way:
- it processes the first copy, and sets up di as representant of dx.
- it then processes the second copy, and replaces the dx with di:
   ...
   (insn 6 3 7 2 (set (reg:SI 5 di)
     (reg:SI 5 di [orig:86 yD.1749 ] [86])) test.c:10 90 {*movsi_internal}
      (nil))
   ...
   turning it into a noop.

pass_fast_rtl_dce subsequently removes the noop.

However, while processing the second copy, it considers the set of di in
insn 6 as killing, and removes di as representant of dx. So a use of dx
in a following insn is not replaced by di.

By running pass_cprop_hardreg once more after pass_fast_rtl_dce, we do
manage to replace the use of dx in a following insn by di.

This patch achieves the same, without rerunning pass_cprop_hardreg. It
ensures in copyprop_hardreg_forward_1 that the set of a dest by a noop
is not considered killing.

Bootstrapped and reg-tested on x86_64.

OK for trunk?

Thanks,
- Tom

0001-Don-t-regard-a-copy-with-identical-src-and-dest-as-k.patch


2014-10-13  Tom de Vries<t...@codesourcery.com>

        PR rtl-optimization/61605
        * regcprop.c (copyprop_hardreg_forward_1): Add copy_p and noop_p.  Don't
        notice stores for noops.  Don't regard noops as copies.

OK.
Jeff