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Re: [PATCH] Change replace_rtx if from is a REG (PR target/70245, take 2)
- From: Jakub Jelinek <jakub at redhat dot com>
- To: Joern Rennecke <gnu at amylaar dot uk>
- Cc: Alan Modra <amodra at gmail dot com>, Segher Boessenkool <segher at kernel dot crashing dot org>, Bernd Schmidt <bschmidt at redhat dot com>, Richard Biener <rguenther at suse dot de>, Eric Botcazou <ebotcazou at adacore dot com>, Jeff Law <law at redhat dot com>, Uros Bizjak <ubizjak at gmail dot com>, gcc-patches at gcc dot gnu dot org, Oleg Endo <olegendo at gcc dot gnu dot org>
- Date: Thu, 17 Mar 2016 14:42:06 +0100
- Subject: Re: [PATCH] Change replace_rtx if from is a REG (PR target/70245, take 2)
- Authentication-results: sourceware.org; auth=none
- References: <20160317124407 dot GJ3017 at tucnak dot redhat dot com> <1458221535436 dot 101872 dot 30058 at webmail5>
- Reply-to: Jakub Jelinek <jakub at redhat dot com>
On Thu, Mar 17, 2016 at 01:32:15PM +0000, Joern Rennecke wrote:
> > Note that during register allocation / reload, REGNO equivalence is generally wrong,
> as it fails to distinguish between the frame pointer and a hard register that has the
> same regno as the frame pointer which has been created as a register allocation for
> a pseudo register when the frame pointer is being eliminated.
> lra_get_elimination_hard_regno actually gets this wrong (as well as the interfaces -
> there is no rtx value available, so it'd be very hard to fix), which means that targets
> can't use safely lra with frame pointer elimination and matching constraints, unless
> they use a separate soft frame pointer.
> If you want to decide automagically if you'd run into an ambiguity with register
> elimination, you have to check if either side of the comparison appears in
> regs_eliminate_1[i].from for 0 <= i < ARRAY_SIZE(regs_eliminate_1) .
> Unfortunately, although rarely encountered, it is possible to use the frame pointer
> in a mode other than Pmode, and once cleanup_subreg_operands is done with such
> a reference, an equality check with frame_pointer_rtx will fail.
Sure. Which is why the trunk now uses the old behavior of replace_rtx
unless you ask for different one through extra argument.
In epiphany.md, I see
(define_peephole2
[(parallel [(set (match_operand:WMODE 0 "gpr_operand" "")
(match_operand:WMODE 1 "" ""))
(clobber (match_operand 8 "cc_operand"))])
(match_operand 2 "" "")
(set (match_operand:WMODE2 3 "gpr_operand" "")
(match_operand:WMODE2 9 "gpr_operand" ""))
(set (match_dup 3)
(if_then_else:WMODE2 (match_operator 5 "proper_comparison_operator"
[(match_operand 6 "cc_operand")
(match_operand 7 "const0_operand")])
(match_operand:WMODE2 4 "nonmemory_operand" "")
(match_dup 3)))]
"REGNO (operands[0]) == REGNO (operands[9])
&& peep2_reg_dead_p (3, operands[0])
&& !reg_set_p (operands[0], operands[2])
&& !reg_set_p (operands[3], operands[2])
&& !reg_overlap_mentioned_p (operands[3], operands[2])"
[(parallel [(set (match_dup 10) (match_dup 1))
(clobber (match_dup 8))])
(match_dup 2)
(set (match_dup 3)
(if_then_else:WMODE2 (match_dup 5) (match_dup 4) (match_dup 3)))]
{
operands[10] = simplify_gen_subreg (<WMODE:MODE>mode, operands[3],
<WMODE2:MODE>mode, 0);
replace_rtx (operands[2], operands[9], operands[3]);
replace_rtx (operands[2], operands[0], operands[10]);
gcc_assert (!reg_overlap_mentioned_p (operands[0], operands[2]));
})
and peephole2s are after RA, and I bet you rely on the replacements to be done no matter
if there is pointer equality or just REGNO equality (and, can there be different modes
or just one?). sh.md has something similar.
Jakub