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CRIS: Make rtx-cost 0 for many CONST_INT "quick" operands

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Stepping through a gdb session inspecting costs that cause
gcc.dg/tree-ssa/slsr-13.c to fail, exposed that before this
patch, cris_rtx_costs told that a shift of 1 of a register
costs 5, while adding two registers costs 4.

Making the cost of a quick-immediate constant equal to using
a register (default 0) reflects actual performance and
size-cost better.  It also happens to make
gcc.dg/tree-ssa/slsr-13.c pass with what looks like better
code being generated, and improves coremark performance by
0.4%.

But, blindly doing this for *all* valid operands that fit
the "quick-immediate" addressing mode, trips interaction
with other factors*, with the end result mixed at best.  So,
do this only for MINUS and logical operations for the time
being, and only for modes that fit in one register.

*) Examples of "other factors":

- A bad default implementation of insn_cost or actually,
pattern_cost, that looks only at the set_src_cost and
furthermore sees such a cost of 0 as invalid.  (Compare to
the more sane set_rtx_cost.)  This naturally tripped up
combine and ifcvt, causing all sorts of changes, good and
bad.

- Having the same cost, to compare a register with 0 as with
-31..31, means a compare insn of an eliminable form no
longer looks preferable.

	* config/cris/cris.cc (cris_rtx_costs) [CONST_INT]: Return 0
	for many quick operands, for register-sized modes.
This commit is contained in:
Hans-Peter Nilsson 2023-03-30 00:56:13 +02:00 committed by Hans-Peter Nilsson
parent 3be4e43a6a
commit 68982b98d2
1 changed files with 22 additions and 1 deletions
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23
gcc/config/cris/cris.cc
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@ -1884,7 +1884,28 @@ cris_rtx_costs (rtx x, machine_mode mode, int outer_code, int opno,
if (val == 0)
*total = 0;
else if (val < 32 && val >= -32)
*total = 1;
switch (outer_code)
{
/* For modes that fit in one register we tell they cost
the same as with register operands. DImode operations
needs careful consideration for more basic reasons:
shifting by a non-word-size amount needs more
operations than an addition by a register pair.
Deliberately excluding SET, PLUS and comparisons and
also not including the full -64..63 range for (PLUS
and) MINUS. */
case MINUS: case ASHIFT: case LSHIFTRT:
case ASHIFTRT: case AND: case IOR:
if (GET_MODE_SIZE(mode) <= UNITS_PER_WORD)
{
*total = 0;
break;
}
/* FALL THROUGH. */
default:
*total = 1;
break;
}
/* Eight or 16 bits are a word and cycle more expensive. */
else if (val <= 32767 && val >= -32768)
*total = 2;