Try placing RTL folded constants in the constant pool.
My recent attempts to come up with a testcase for my patch to evaluate
ss_plus in simplify-rtx.c, identified a missed optimization opportunity
(that's potentially a long-time regression): The RTL optimizers no longer
place constants in the constant pool.
The motivating x86_64 example is the simple program:
typedef char v8qi __attribute__ ((vector_size (8)));
v8qi foo()
{
v8qi tx = { 1, 0, 0, 0, 0, 0, 0, 0 };
v8qi ty = { 2, 0, 0, 0, 0, 0, 0, 0 };
v8qi t = __builtin_ia32_paddsb(tx, ty);
return t;
}
which (with my previous patch) currently results in:
foo: movq .LC0(%rip), %xmm0
movq .LC1(%rip), %xmm1
paddsb %xmm1, %xmm0
ret
even though the RTL contains the result in a REG_EQUAL note:
(insn 7 6 12 2 (set (reg:V8QI 83)
(ss_plus:V8QI (reg:V8QI 84)
(reg:V8QI 85))) "ssaddqi3.c":7:12 1419 {*mmx_ssaddv8qi3}
(expr_list:REG_DEAD (reg:V8QI 85)
(expr_list:REG_DEAD (reg:V8QI 84)
(expr_list:REG_EQUAL (const_vector:V8QI [
(const_int 3 [0x3])
(const_int 0 [0]) repeated x7
])
(nil)))))
Together with the patch below, GCC will now generate the much
more sensible:
foo: movq .LC2(%rip), %xmm0
ret
My first approach was to look in cse.c (where the REG_EQUAL note gets
added) and notice that the constant pool handling functionality has been
unreachable for a while. A quick search for constant_pool_entries_cost
shows that it's initialized to zero, but never set to a non-zero value,
meaning that force_const_mem is never called. This functionality used
to work way back in 2003, but has been lost over time:
https://gcc.gnu.org/pipermail/gcc-patches/2003-October/116435.html
The changes to cse.c below restore this functionality (placing suitable
constants in the constant pool) with two significant refinements;
(i) it only attempts to do this if the function already uses a constant
pool (thanks to the availability of crtl->uses_constant_pool since 2003).
(ii) it allows different constants (i.e. modes) to have different costs,
so that floating point "doubles" and 64-bit, 128-bit, 256-bit and 512-bit
vectors don't all have the share the same cost. Back in 2003, the
assumption was that everything in a constant pool had the same
cost, hence the global variable constant_pool_entries_cost.
Although this is a useful CSE fix, it turns out that it doesn't cure
my motivating problem above. CSE only considers a single instruction,
so determines that it's cheaper to perform the ss_plus (COSTS_N_INSNS(1))
than read the result from the constant pool (COSTS_N_INSNS(2)). It's
only when the other reads from the constant pool are also eliminated,
that this transformation is a win. Hence a better place to perform
this transformation is in combine, where after failing to "recog" the
load of a suitable constant, it can retry after calling force_const_mem.
This achieves the desired transformation and allows the backend insn_cost
call-back to control whether or not using the constant pool is preferrable.
Alas, it's rare to change code generation without affecting something in
GCC's testsuite. On x86_64-pc-linux-gnu there were two families of new
failures (and I'd predict similar benign fallout on other platforms).
One failure was gcc.target/i386/387-12.c (aka PR target/26915), where
the test is missing an explicit -m32 flag. On i686, it's very reasonable
to materialize -1.0 using "fld1; fchs", but on x86_64-pc-linux-gnu we
currently generate the awkward:
testm1: fld1
fchs
fstpl -8(%rsp)
movsd -8(%rsp), %xmm0
ret
which combine now very reasonably simplifies to just:
testm1: movsd .LC3(%rip), %xmm0
ret
The other class of x86_64-pc-linux-gnu failure was from materialization
of vector constants using vpbroadcast (e.g. gcc.target/i386/pr90773-17.c)
where the decision is finely balanced; the load of an integer register
with an immediate constant, followed by a vpbroadcast is deemed to be
COSTS_N_INSNS(2), whereas a load from the constant pool is also reported
as COSTS_N_INSNS(2). My solution is to tweak the i386.c's rtx_costs
so that all other things being equal, an instruction (sequence) that
accesses memory is fractionally more expensive than one that doesn't.
2021-10-18 Roger Sayle <roger@nextmovesoftware.com>
gcc/ChangeLog
* combine.c (recog_for_combine): For an unrecognized move/set of
a constant, try force_const_mem to place it in the constant pool.
* cse.c (constant_pool_entries_cost, constant_pool_entries_regcost):
Delete global variables (that are no longer assigned a cost value).
(cse_insn): Simplify logic for deciding whether to place a folded
constant in the constant pool using force_const_mem.
(cse_main): Remove zero initialization of constant_pool_entries_cost
and constant_pool_entries_regcost.
* config/i386/i386.c (ix86_rtx_costs): Make memory accesses
fractionally more expensive, when optimizing for speed.
gcc/testsuite/ChangeLog
* gcc.target/i386/387-12.c: Add explicit -m32 option.
This commit is contained in:
Roger Sayle
parent
815f15d338
commit
247c407c83
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@ -11567,7 +11567,27 @@ recog_for_combine (rtx *pnewpat, rtx_insn *insn, rtx *pnotes)
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bool changed = false;
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if (GET_CODE (pat) == SET)
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changed = change_zero_ext (pat);
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{
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/* For an unrecognized single set of a constant, try placing it in
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the constant pool, if this function already uses one. */
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rtx src = SET_SRC (pat);
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if (CONSTANT_P (src)
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&& !CONST_INT_P (src)
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&& crtl->uses_const_pool)
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{
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machine_mode mode = GET_MODE (src);
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if (mode == VOIDmode)
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mode = GET_MODE (SET_DEST (pat));
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src = force_const_mem (mode, src);
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if (src)
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{
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SUBST (SET_SRC (pat), src);
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changed = true;
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}
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}
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else
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changed = change_zero_ext (pat);
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}
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else if (GET_CODE (pat) == PARALLEL)
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{
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int i;
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@ -20801,6 +20801,13 @@ ix86_rtx_costs (rtx x, machine_mode mode, int outer_code_i, int opno,
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*total = cost->sse_op;
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return true;
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case MEM:
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/* An insn that accesses memory is slightly more expensive
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than one that does not. */
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if (speed)
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*total += 1;
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return false;
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default:
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return false;
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}
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48
gcc/cse.c
48
gcc/cse.c
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@ -491,14 +491,6 @@ static struct table_elt *table[HASH_SIZE];
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static struct table_elt *free_element_chain;
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/* Set to the cost of a constant pool reference if one was found for a
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symbolic constant. If this was found, it means we should try to
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convert constants into constant pool entries if they don't fit in
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the insn. */
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static int constant_pool_entries_cost;
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static int constant_pool_entries_regcost;
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/* Trace a patch through the CFG. */
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struct branch_path
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@ -4609,9 +4601,6 @@ cse_insn (rtx_insn *insn)
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int src_folded_regcost = MAX_COST;
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int src_related_regcost = MAX_COST;
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int src_elt_regcost = MAX_COST;
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/* Set nonzero if we need to call force_const_mem on with the
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contents of src_folded before using it. */
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int src_folded_force_flag = 0;
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scalar_int_mode int_mode;
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dest = SET_DEST (sets[i].rtl);
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@ -5166,15 +5155,7 @@ cse_insn (rtx_insn *insn)
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src_related_cost, src_related_regcost) <= 0
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&& preferable (src_folded_cost, src_folded_regcost,
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src_elt_cost, src_elt_regcost) <= 0)
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{
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trial = src_folded, src_folded_cost = MAX_COST;
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if (src_folded_force_flag)
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{
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rtx forced = force_const_mem (mode, trial);
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if (forced)
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trial = forced;
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}
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}
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trial = src_folded, src_folded_cost = MAX_COST;
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else if (src
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&& preferable (src_cost, src_regcost,
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src_eqv_cost, src_eqv_regcost) <= 0
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@ -5361,23 +5342,24 @@ cse_insn (rtx_insn *insn)
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break;
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}
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/* If we previously found constant pool entries for
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constants and this is a constant, try making a
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pool entry. Put it in src_folded unless we already have done
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this since that is where it likely came from. */
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/* If the current function uses a constant pool and this is a
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constant, try making a pool entry. Put it in src_folded
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unless we already have done this since that is where it
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likely came from. */
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else if (constant_pool_entries_cost
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else if (crtl->uses_const_pool
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&& CONSTANT_P (trial)
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&& (src_folded == 0
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|| (!MEM_P (src_folded)
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&& ! src_folded_force_flag))
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&& !CONST_INT_P (trial)
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&& (src_folded == 0 || !MEM_P (src_folded))
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&& GET_MODE_CLASS (mode) != MODE_CC
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&& mode != VOIDmode)
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{
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src_folded_force_flag = 1;
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src_folded = trial;
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src_folded_cost = constant_pool_entries_cost;
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src_folded_regcost = constant_pool_entries_regcost;
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src_folded = force_const_mem (mode, trial);
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if (src_folded)
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{
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src_folded_cost = COST (src_folded, mode);
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src_folded_regcost = approx_reg_cost (src_folded);
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}
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}
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}
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@ -6630,8 +6612,6 @@ cse_main (rtx_insn *f ATTRIBUTE_UNUSED, int nregs)
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cse_cfg_altered = false;
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cse_jumps_altered = false;
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recorded_label_ref = false;
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constant_pool_entries_cost = 0;
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constant_pool_entries_regcost = 0;
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ebb_data.path_size = 0;
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ebb_data.nsets = 0;
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rtl_hooks = cse_rtl_hooks;
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@ -1,6 +1,6 @@
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/* PR target/26915 */
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/* { dg-do compile } */
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/* { dg-options "-O -mfpmath=387 -mfancy-math-387" } */
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/* { dg-options "-O -m32 -mfpmath=387 -mfancy-math-387" } */
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double testm0(void)
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{
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