http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56151
Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |ebotcazou at gcc dot
| |gnu.org
--- Comment #5 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2013-02-04
10:34:25 UTC ---
> Patch to help explain the problem:
>
> Index: optabs.c
> ===================================================================
> --- optabs.c (revision 195687)
> +++ optabs.c (working copy)
> @@ -1452,8 +1452,13 @@ expand_binop_directly (enum machine_mode
> {
> /* If PAT is composed of more than one insn, try to add an appropriate
> REG_EQUAL note to it. If we can't because TEMP conflicts with an
> - operand, call expand_binop again, this time without a target. */
> + operand, call expand_binop again, this time without a target.
> +
> + However, if target is a MEM, just accept the lossage of not having
> + a REG_EQUAL note. This avoids splitting up insns of the form
> + "MEM=MEM op X", a form we may not be able to reconstruct later. */
> if (INSN_P (pat) && NEXT_INSN (pat) != NULL_RTX
> + && ! (target && MEM_P (target))
> && ! add_equal_note (pat, ops[0].value, optab_to_code (binoptab),
> ops[1].value, ops[2].value))
> {
I agree that we should try to do something like that. Generating inferior code
to be able to add a REG_EQUAL that doesn't help in the end isn't very clever.
There is also the same pattern in expand_unop_direct.
Is that the "maximal" test that can be added to catch this case or could it be
made more precise?
