cris: try to generate zero-based comparisons
* config/cris/cris.c (cris_reduce_compare): New function.
* config/cris/cris-protos.h (cris_reduce_compare): Add prototype.
* config/cris/cris.md ("cbranch<mode>4", "cbranchdi4", "cstoredi4")
(cstore<mode>4"): Apply cris_reduce_compare in expanders.
The decc0ration work of the CRIS port made me look closer at the
code for trivial comparisons, as in the condition for branches
and conditional-stores, like in:
void g(short int a, short int b)
{
short int c = a + b;
if (c >= 0)
foo ();
}
At -O2, the cc0 version of the CRIS port has an explicit
*uneliminated* compare instruction ("cmp.w -1,$r10") instead of
an (eliminated) compare against 0 (which below I'll call a
zero-compare). This for the CRIS-cc0 version, but I see this
also for a much older gcc, at 4.7. For the decc0rated port, the
compare *is* a test against 0, eventually eliminated. To wit,
for cc0 (mind the delay-slot):
_g:
subq 4,$sp
add.w $r11,$r10
cmp.w -1,$r10
ble .L9
move $srp,[$sp]
jsr _foo
.L9:
jump [$sp+]
The compare instruction is expected to be eliminated, i.e. the
following diff to the above is desired, modulo the missing
sibling call, which corresponds to what I get from 4.7 and for
the decc0rated port:
!--- a Wed Feb 5 15:22:27 2020
!+++ b Wed Feb 5 15:22:51 2020
!@@ -1,8 +1,7 @@
! _g:
! subq 4,$sp
! add.w $r11,$r10
!- cmp.w -1,$r10
!- ble .L9
!+ bmi .L9
! move $srp,[$sp]
!
! jsr _foo
Tracking this difference, I see that for both cc0-CRIS and the
decc0rated CRIS, the comparison actually starts out as a compare
against -1 at "expand" time, but is transformed for decc0rated
CRIS to a zero-compare in "cse1".
For CRIS-cc0 "cse1" does try to replace the compare with a
zero-compare, but fails because at the same time it tries to
replace the c operand with (a + b). Or some such; it fails and
no other pass succeeds. I was not into fixing cc0-handling in
core gcc, so I didn't look closer.
BTW, at first, I was a bit surprised to see that for compares
against a constant, a zero-compare is not canonical RTX for
*all* conditions, and that instead only a subset of all RTX
conditions against a constant are canonical, transforming one
condition to the canonical one by adding 1 or -1 to the
constant. It does makes sense at a closer look, but still not
so much when emitting RTL.
There are several places that mention in comments that emitting
RTX as zero-compare is preferable, but nothing is done about it.
Some generic code instead seems confused that the *target* is
helped by seeing canonical RTX, or perhaps it (its authors) like
me, confused about what a canonical comparison is. For example,
prepare_cmp_insn calls canonicalize_comparison last before
emitting the actual instructions. I see most ports for various
port-specific reasons does their own massaging in their cbranch
and cstore expanders. Still, the suboptimal compares *should*
be fixed at expand time; better start out right than just
relying on later optimizations.
This kind of change is not acceptable in the current gcc
development stage, at least as a change in generic code.
However, it's problematic enough that I chose to fix this right
now in the CRIS port. For that, I claim a possibly
long-standing regression. After this, code before and after
decc0ration is similar enough that I can spot
compare-elimination-efforts and apply regression test-cases
without them drowning in cc0-specific xfailing.
I hope to eventually lift out cris_reduce_compare (renamed) into
say expmed.c, called in e.g. emit_store_flag_1 (replacing the
in-line code) and prepare_cmp_insn. Later.
This commit is contained in:
Hans-Peter Nilsson
parent
cf0f07ef0e
commit
cfaf520486
@ -1,3 +1,11 @@
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2020-02-10 Hans-Peter Nilsson <hp@axis.com>
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Try to generate zero-based comparisons.
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* config/cris/cris.c (cris_reduce_compare): New function.
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* config/cris/cris-protos.h (cris_reduce_compare): Add prototype.
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* config/cris/cris.md ("cbranch<mode>4", "cbranchdi4", "cstoredi4")
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(cstore<mode>4"): Apply cris_reduce_compare in expanders.
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2020-02-10 Richard Earnshaw <rearnsha@arm.com>
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PR target/91913
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@ -38,6 +38,7 @@ extern bool cris_constant_index_p (const_rtx);
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extern bool cris_base_p (const_rtx, bool);
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extern bool cris_base_or_autoincr_p (const_rtx, bool);
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extern bool cris_bdap_index_p (const_rtx, bool);
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extern void cris_reduce_compare (rtx *, rtx *, rtx *);
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extern bool cris_biap_index_p (const_rtx, bool);
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extern bool cris_legitimate_address_p (machine_mode, rtx, bool);
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extern bool cris_store_multiple_op_p (rtx);
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@ -3053,6 +3053,63 @@ cris_split_movdx (rtx *operands)
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return val;
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}
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/* Try to change a comparison against a constant to be against zero, and
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an unsigned compare against zero to be an equality test. Beware:
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only valid for compares of integer-type operands. Also, note that we
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don't use operand 0 at the moment. */
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void
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cris_reduce_compare (rtx *relp, rtx *, rtx *op1p)
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{
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rtx op1 = *op1p;
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rtx_code code = GET_CODE (*relp);
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/* Code lifted mostly from emit_store_flag_1. */
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switch (code)
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{
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case LT:
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if (op1 == const1_rtx)
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code = LE;
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break;
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case LE:
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if (op1 == constm1_rtx)
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code = LT;
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break;
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case GE:
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if (op1 == const1_rtx)
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code = GT;
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break;
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case GT:
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if (op1 == constm1_rtx)
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code = GE;
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break;
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case GEU:
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if (op1 == const1_rtx)
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code = NE;
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break;
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case LTU:
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if (op1 == const1_rtx)
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code = EQ;
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break;
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case GTU:
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if (op1 == const0_rtx)
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code = NE;
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break;
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case LEU:
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if (op1 == const0_rtx)
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code = EQ;
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break;
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default:
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break;
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}
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if (code != GET_CODE (*relp))
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{
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*op1p = const0_rtx;
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PUT_CODE (*relp, code);
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}
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}
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/* The expander for the prologue pattern name. */
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void
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@ -3539,7 +3539,7 @@
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(label_ref (match_operand 3 "" ""))
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(pc)))]
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""
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"")
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"cris_reduce_compare (&operands[0], &operands[1], &operands[2]);")
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(define_expand "cbranchdi4"
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[(set (cc0)
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@ -3552,6 +3552,7 @@
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(pc)))]
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""
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{
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cris_reduce_compare (&operands[0], &operands[1], &operands[2]);
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if (TARGET_V32 && !REG_P (operands[1]))
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operands[1] = force_reg (DImode, operands[1]);
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if (TARGET_V32 && MEM_P (operands[2]))
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@ -3652,6 +3653,7 @@
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[(cc0) (const_int 0)]))]
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""
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{
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cris_reduce_compare (&operands[1], &operands[2], &operands[3]);
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if (TARGET_V32 && !REG_P (operands[2]))
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operands[2] = force_reg (DImode, operands[2]);
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if (TARGET_V32 && MEM_P (operands[3]))
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@ -3666,7 +3668,7 @@
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(match_operator:SI 1 "ordered_comparison_operator"
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[(cc0) (const_int 0)]))]
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""
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"")
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"cris_reduce_compare (&operands[1], &operands[2], &operands[3]);")
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;; Like bCC, we have to check the overflow bit for
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;; signed conditions.
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