Re: regrename and odd behaviour with early clobber operands

[Richard Sandiford <richard.sandiford@linaro.org>]

Ramana Radhakrishnan <ramana.radhakrishnan@linaro.org> writes:
> I can't see how it is right to construct essentially 2 chains for the
> same register that have overlapping live ranges without an intervening
> conditional branch and since regrename sort of works inside a bb .
> Ideally the chain for 122 should have been terminated at the end of
> 123 rather than allowing this to remain open and have the use in insn
> 141 available for use in both chains starting at 122 and 140 . What
> I'm not sure is which part of regrename makes sure that this part of
> the comment for Stage 5 is ensured.
>
>             `and earlier
> 	     chains they would overlap with must have been closed at
> 	     the previous insn at the latest, as such operands cannot
> 	     possibly overlap with any input operands.  */'

Just to summarise on-list what we talked about on IRC: this is supposed
to happen through REG_DEAD notes.  The bug in this case appears to be
that the required note is missing.

The patch below seems to fix things.  If it's right, I'm very surprised
we hadn't noticed until now.  There must be something else going on...

Richard


Index: gcc/df-problems.c
===================================================================
--- gcc/df-problems.c	2011-07-11 12:21:33.000000000 +0100
+++ gcc/df-problems.c	2011-08-16 16:18:52.333237669 +0100
@@ -3376,7 +3376,7 @@ df_note_bb_compute (unsigned int bb_inde
       while (*mws_rec)
 	{
 	  struct df_mw_hardreg *mws = *mws_rec;
-	  if ((DF_MWS_REG_DEF_P (mws))
+	  if ((DF_MWS_REG_USE_P (mws))
 	      && !df_ignore_stack_reg (mws->start_regno))
 	    {
 	      bool really_add_notes = debug_insn != 0;