On Fri, Nov 18, 2022 at 02:35:30PM +0800, HAO CHEN GUI wrote:
> 在 2022/11/17 21:24, David Edelsohn 写道:
> > Why are you using zero_constant predicate instead of matching (const_int 0)
> > for operand 2?
> The "const_int 0" is an operand other than a predicate. We need a predicate
> here.
Said differently, it is passed as an operand to this named pattern or
optab, so you need a match_operand here.
> > Why does this need the new all_branch_comparison_operator? Can the ifcvt
> > optimization correctly elide the 2 insn sequence?
> Because rs6000 defines "*cbranch_2insn" insn, such insns are generated after
> expand.
>
> (jump_insn 50 47 51 11 (set (pc)
> (if_then_else (ge (reg:CCFP 156)
> (const_int 0 [0]))
> (label_ref 53)
> (pc)))
> "/home/guihaoc/gcc/gcc-mainline-base/gmp/mpz/cmpabs_d.c":80:7 884
> {*cbranch_2insn}
> (expr_list:REG_DEAD (reg:CCFP 156)
> (int_list:REG_BR_PROB 633507684 (nil)))
> -> 53)
But notice the cost of *cbranch_2insn -- ifcvt should never generate
cbranchcc4 with such composite conditions!
> In prepare_cmp_insn, the comparison is verified by insn_operand_matches. If
> extra_insn_branch_comparison_operator is not included in "cbranchcc4"
> predicate,
> it hits ICE here.
>
> if (GET_MODE_CLASS (mode) == MODE_CC)
> {
> enum insn_code icode = optab_handler (cbranch_optab, CCmode);
> test = gen_rtx_fmt_ee (comparison, VOIDmode, x, y);
> gcc_assert (icode != CODE_FOR_nothing
> && insn_operand_matches (icode, 0, test));
> *ptest = test;
> return;
> }
>
> The real conditional move is generated by emit_conditional_move_1. Commonly
> "*cbranch_2insn" can't be optimized out and it returns NULL_RTX.
>
> if (COMPARISON_P (comparison))
> {
> saved_pending_stack_adjust save;
> save_pending_stack_adjust (&save);
> last = get_last_insn ();
> do_pending_stack_adjust ();
> machine_mode cmpmode = comp.mode;
> prepare_cmp_insn (XEXP (comparison, 0), XEXP (comparison, 1),
> GET_CODE (comparison), NULL_RTX, unsignedp,
> OPTAB_WIDEN, &comparison, &cmpmode);
> if (comparison)
> {
> rtx res = emit_conditional_move_1 (target, comparison,
> op2, op3, mode);
> if (res != NULL_RTX)
> return res;
> }
> delete_insns_since (last);
> restore_pending_stack_adjust (&save);
>
> I think that extra_insn_branch_comparison_operator should be included in
> "cbranchcc4" predicates as such insns exist. And leave it to
> emit_conditional_move which decides whether it can be optimized or not.
I don't think we should pretend we have any conditional jumps the
machine does not actually have, in cbranchcc4. When would this ever be
useful? cror;beq can be quite expensive, compared to the code it would
replace anyway.
If something generates those here (which then ICEs later), that is
wrong, fix *that*? Is it ifcvt doing it?
Segher
