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.

1 files changed, 14 insertions, 34 deletions

diff --git a/gcc/cse.c b/gcc/cse.c
index 330c1e9..4c3988e 100644
--- a/gcc/cse.c
+++ b/gcc/cse.c
@@ -491,14 +491,6 @@ static struct table_elt *table[HASH_SIZE];
 
 static struct table_elt *free_element_chain;
 
-/* Set to the cost of a constant pool reference if one was found for a
-   symbolic constant.  If this was found, it means we should try to
-   convert constants into constant pool entries if they don't fit in
-   the insn.  */
-
-static int constant_pool_entries_cost;
-static int constant_pool_entries_regcost;
-
 /* Trace a patch through the CFG.  */
 
 struct branch_path
@@ -4609,9 +4601,6 @@ cse_insn (rtx_insn *insn)
       int src_folded_regcost = MAX_COST;
       int src_related_regcost = MAX_COST;
       int src_elt_regcost = MAX_COST;
-      /* Set nonzero if we need to call force_const_mem on with the
-	 contents of src_folded before using it.  */
-      int src_folded_force_flag = 0;
       scalar_int_mode int_mode;
 
       dest = SET_DEST (sets[i].rtl);
@@ -5166,15 +5155,7 @@ cse_insn (rtx_insn *insn)
 			     src_related_cost, src_related_regcost) <= 0
 	      && preferable (src_folded_cost, src_folded_regcost,
 			     src_elt_cost, src_elt_regcost) <= 0)
-	    {
-	      trial = src_folded, src_folded_cost = MAX_COST;
-	      if (src_folded_force_flag)
-		{
-		  rtx forced = force_const_mem (mode, trial);
-		  if (forced)
-		    trial = forced;
-		}
-	    }
+	    trial = src_folded, src_folded_cost = MAX_COST;
 	  else if (src
 		   && preferable (src_cost, src_regcost,
 				  src_eqv_cost, src_eqv_regcost) <= 0
@@ -5361,23 +5342,24 @@ cse_insn (rtx_insn *insn)
 	      break;
 	    }
 
-	  /* If we previously found constant pool entries for
-	     constants and this is a constant, try making a
-	     pool entry.  Put it in src_folded unless we already have done
-	     this since that is where it likely came from.  */
+	  /* If the current function uses a constant pool and this is a
+	     constant, try making a pool entry. Put it in src_folded
+	     unless we already have done this since that is where it
+	     likely came from.  */
 
-	  else if (constant_pool_entries_cost
+	  else if (crtl->uses_const_pool
 		   && CONSTANT_P (trial)
-		   && (src_folded == 0
-		       || (!MEM_P (src_folded)
-			   && ! src_folded_force_flag))
+		   && !CONST_INT_P (trial)
+		   && (src_folded == 0 || !MEM_P (src_folded))
 		   && GET_MODE_CLASS (mode) != MODE_CC
 		   && mode != VOIDmode)
 	    {
-	      src_folded_force_flag = 1;
-	      src_folded = trial;
-	      src_folded_cost = constant_pool_entries_cost;
-	      src_folded_regcost = constant_pool_entries_regcost;
+	      src_folded = force_const_mem (mode, trial);
+	      if (src_folded)
+		{
+		  src_folded_cost = COST (src_folded, mode);
+		  src_folded_regcost = approx_reg_cost (src_folded);
+		}
 	    }
 	}
 
@@ -6630,8 +6612,6 @@ cse_main (rtx_insn *f ATTRIBUTE_UNUSED, int nregs)
   cse_cfg_altered = false;
   cse_jumps_altered = false;
   recorded_label_ref = false;
-  constant_pool_entries_cost = 0;
-  constant_pool_entries_regcost = 0;
   ebb_data.path_size = 0;
   ebb_data.nsets = 0;
   rtl_hooks = cse_rtl_hooks;