[Bug rtl-optimization/42691] problematic REG_EQUAL note added to SUBREG

["ebotcazou at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #10 from ebotcazou at gcc dot gnu dot org  2010-01-13 23:27 -------
> Otherwise, I would simply remove this comment.

Fine with me, it's indeed a little confusing.

> I does hunk #3, because I am not sure if all combination results from
> case2 would pass "recog_for_combine". If any case fail, gcc may try
> other combination patterns, where I am afraid changed_i3_dest=1 may
> cause problem. I haven't read the code after validate_replacement
> label thoroughly. To be conservative, I use hunk#3 to guarantee that
> if the combination result from case2 is not recognized, nothing has
> been changed. gcc is free to go ahead and try other combination
> patterns. This is conservative. If anyone confirms that the cases that
> I concern of will never happen, I am happy to remove hunk#3. Can you
> confirm?

Yes, jumping to validate_replacement means that the other combinations are not
tried.  If the new pattern is not valid, the code will try to massage it and
not try other combinations.  And I think that, in practice, it won't actually
do anything since the various subsequent tricks are incompatible with case2.

> Index: combine.c
> ===================================================================
> --- combine.c   (revision 155801)
> +++ combine.c   (working copy)
> @@ -2663,10 +2663,17 @@ try_combine (rtx i3, rtx i2, rtx i1, int
>           i2dest = SET_DEST (temp);
>           i2dest_killed = dead_or_set_p (i2, i2dest);
> 
> +         /* Replace the source in I2 with our dest and make the resulting
> +            insn the new pattern for I3.  Then skip to where we
> +            validate the pattern.  Everything was set up above.  */

"Replace the source in I2 with the new constant and..."

>           SUBST (SET_SRC (temp),
>                  immed_double_const (olo, ohi, GET_MODE (SET_DEST (temp))));
>
>           newpat = PATTERN (i2);
> +
> +          /* If the result of combination is recognized, the note of I3 needs
> +           * to be removed. */
> +          changed_i3_dest = 1;

No need to invoke the consequences, just state the cause: "The dest of I3 has
been replaced with the dest of I2."


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42691