ARC: Improved ARC rtx_costs/insn_cost for SHIFTs and ROTATEs.

This patch overhauls the ARC backend's insn_cost target hook, and makes
some related improvements to rtx_costs, BRANCH_COST, etc.  The primary
goal is to allow the backend to indicate that shifts and rotates are
slow (discouraged) when the CPU doesn't have a barrel shifter. I should
also acknowledge Richard Sandiford for inspiring the use of set_cost
in this rewrite of arc_insn_cost; this implementation borrows heavily
for the target hooks for AArch64 and ARM.

The motivating example is derived from PR rtl-optimization/110717.

struct S { int a : 5; };
unsigned int foo (struct S *p) {
  return p->a;
}

With a barrel shifter, GCC -O2 generates the reasonable:

foo:    ldb_s   r0,[r0]
        asl_s   r0,r0,27
        j_s.d   [blink]
        asr_s   r0,r0,27

What's interesting is that during combine, the middle-end actually
has two shifts by three bits, and a sign-extension from QI to SI.

Trying 8, 9 -> 11:
    8: r158:SI=r157:QI#0<<0x3
      REG_DEAD r157:QI
    9: r159:SI=sign_extend(r158:SI#0)
      REG_DEAD r158:SI
   11: r155:SI=r159:SI>>0x3
      REG_DEAD r159:SI

Whilst it's reasonable to simplify this to two shifts by 27 bits when
the CPU has a barrel shifter, it's actually a significant pessimization
when these shifts are implemented by loops.  This combination can be
prevented if the backend provides accurate-ish estimates for insn_cost.

Previously, without a barrel shifter, GCC -O2 -mcpu=em generates:

foo:	ldb_s   r0,[r0]
        mov     lp_count,27
        lp      2f
        add     r0,r0,r0
        nop
2:      # end single insn loop
        mov     lp_count,27
        lp      2f
        asr     r0,r0
        nop
2:      # end single insn loop
        j_s     [blink]

which contains two loops and requires about ~113 cycles to execute.
With this patch to rtx_cost/insn_cost, GCC -O2 -mcpu=em generates:

foo:	ldb_s   r0,[r0]
        mov_s   r2,0    ;3
        add3    r0,r2,r0
        sexb_s  r0,r0
        asr_s   r0,r0
        asr_s   r0,r0
        j_s.d   [blink]
        asr_s   r0,r0

which requires only ~6 cycles, for the shorter shifts by 3 and sign
extension.

2023-10-30  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/arc/arc.cc (arc_rtx_costs): Improve cost estimates.
	Provide reasonable values for SHIFTS and ROTATES by constant
	bit counts depending upon TARGET_BARREL_SHIFTER.
	(arc_insn_cost): Use insn attributes if the instruction is
	recognized.  Avoid calling get_attr_length for type "multi",
	i.e. define_insn_and_split patterns without explicit type.
	Fall-back to set_rtx_cost for single_set and pattern_cost
	otherwise.
	* config/arc/arc.h (COSTS_N_BYTES): Define helper macro.
	(BRANCH_COST): Improve/correct definition.
	(LOGICAL_OP_NON_SHORT_CIRCUIT): Preserve previous behavior.

2 files changed, 56 insertions, 43 deletions

diff --git a/gcc/config/arc/arc.cc b/gcc/config/arc/arc.cc
index e98692a..e209ad2 100644
--- a/gcc/config/arc/arc.cc
+++ b/gcc/config/arc/arc.cc
@@ -5545,7 +5545,7 @@ arc_rtx_costs (rtx x, machine_mode mode, int outer_code,
     case CONST:
     case LABEL_REF:
     case SYMBOL_REF:
-      *total = speed ? COSTS_N_INSNS (1) : COSTS_N_INSNS (4);
+      *total = speed ? COSTS_N_INSNS (1) : COSTS_N_BYTES (4);
       return true;
 
     case CONST_DOUBLE:
@@ -5569,26 +5569,32 @@ arc_rtx_costs (rtx x, machine_mode mode, int outer_code,
     case ASHIFT:
     case ASHIFTRT:
     case LSHIFTRT:
+    case ROTATE:
+    case ROTATERT:
+      if (mode == DImode)
+	return false;
       if (TARGET_BARREL_SHIFTER)
 	{
-	  if (CONSTANT_P (XEXP (x, 0)))
+	  *total = COSTS_N_INSNS (1);
+	  if (CONSTANT_P (XEXP (x, 1)))
 	    {
-	      *total += rtx_cost (XEXP (x, 1), mode, (enum rtx_code) code,
+	      *total += rtx_cost (XEXP (x, 0), mode, (enum rtx_code) code,
 				  0, speed);
 	      return true;
 	    }
-	  *total = COSTS_N_INSNS (1);
 	}
       else if (GET_CODE (XEXP (x, 1)) != CONST_INT)
-	*total = COSTS_N_INSNS (16);
+	*total = speed ? COSTS_N_INSNS (16) : COSTS_N_INSNS (4);
       else
 	{
-	  *total = COSTS_N_INSNS (INTVAL (XEXP ((x), 1)));
-	  /* ??? want_to_gcse_p can throw negative shift counts at us,
-	     and then panics when it gets a negative cost as result.
-	     Seen for gcc.c-torture/compile/20020710-1.c -Os .  */
-	  if (*total < 0)
-	    *total = 0;
+	  int n = INTVAL (XEXP (x, 1)) & 31;
+	  if (n < 4)
+	    *total = COSTS_N_INSNS (n);
+	  else
+	    *total = speed ? COSTS_N_INSNS (n + 2) : COSTS_N_INSNS (4);
+	  *total += rtx_cost (XEXP (x, 0), mode, (enum rtx_code) code,
+			      0, speed);
+	  return true;
 	}
       return false;
 
@@ -5620,6 +5626,8 @@ arc_rtx_costs (rtx x, machine_mode mode, int outer_code,
       return false;
 
     case PLUS:
+      if (mode == DImode)
+	return false;
       if (outer_code == MEM && CONST_INT_P (XEXP (x, 1))
 	  && RTX_OK_FOR_OFFSET_P (mode, XEXP (x, 1)))
 	{
@@ -11154,35 +11162,37 @@ static int
 arc_insn_cost (rtx_insn *insn, bool speed)
 {
   int cost;
-  if (recog_memoized (insn) < 0)
-    return 0;
-
-  /* If optimizing for size, we want the insn size.  */
-  if (!speed)
-    return get_attr_length (insn);
-
-  /* Use cost if provided.  */
-  cost = get_attr_cost (insn);
-  if (cost > 0)
-    return cost;
-
-  /* For speed make a simple cost model: memory access is more
-     expensive than any other instruction.  */
-  enum attr_type type = get_attr_type (insn);
-
-  switch (type)
+  enum attr_type type;
+  if (recog_memoized (insn) >= 0)
     {
-    case TYPE_LOAD:
-    case TYPE_STORE:
-      cost = COSTS_N_INSNS (2);
-      break;
-
-    default:
-      cost = COSTS_N_INSNS (1);
-      break;
+      if (speed)
+	{
+	  /* Use cost if provided.  */
+	  cost = get_attr_cost (insn);
+	  if (cost > 0)
+	    return cost;
+	  /* For speed make a simple cost model: memory access is more
+	     expensive than any other instruction.  */
+	  type = get_attr_type (insn);
+	  if (type == TYPE_LOAD || type == TYPE_STORE)
+	    return COSTS_N_INSNS (2);
+	}
+      else
+	{
+	  /* If optimizing for size, we want the insn size.  */
+	  type = get_attr_type (insn);
+	  if (type != TYPE_MULTI)
+	    return get_attr_length (insn);
+	}
     }
 
-  return cost;
+  if (rtx set = single_set (insn))
+    cost = set_rtx_cost (set, speed);
+  else
+    cost = pattern_cost (PATTERN (insn), speed);
+  /* If the cost is zero, then it's likely a complex insn.  We don't
+     want the cost of these to be less than something we know about.  */
+  return cost ? cost : COSTS_N_INSNS (2);
 }
 
 static unsigned
diff --git a/gcc/config/arc/arc.h b/gcc/config/arc/arc.h
index 5877389..b34f0b2 100644
--- a/gcc/config/arc/arc.h
+++ b/gcc/config/arc/arc.h
@@ -956,10 +956,16 @@ arc_select_cc_mode (OP, X, Y)
 
 /* Costs.  */
 
+/* Analog of COSTS_N_INSNS when optimizing for size.  */
+#ifndef COSTS_N_BYTES
+#define COSTS_N_BYTES(N) (N)
+#endif
+
 /* The cost of a branch insn.  */
 /* ??? What's the right value here?  Branches are certainly more
    expensive than reg->reg moves.  */
-#define BRANCH_COST(speed_p, predictable_p) 2
+#define BRANCH_COST(speed_p, predictable_p) \
+	(speed_p ? COSTS_N_INSNS (2) : COSTS_N_INSNS (1))
 
 /* Scc sets the destination to 1 and then conditionally zeroes it.
    Best case, ORed SCCs can be made into clear - condset - condset.
@@ -971,11 +977,8 @@ arc_select_cc_mode (OP, X, Y)
    beging decisive of p0, we want:
    p0 * (branch_cost - 4) > (1 - p0) * 5
    ??? We don't get to see that probability to evaluate, so we can
-   only wildly guess that it might be 50%.
-   ??? The compiler also lacks the notion of branch predictability.  */
-#define LOGICAL_OP_NON_SHORT_CIRCUIT \
-  (BRANCH_COST (optimize_function_for_speed_p (cfun), \
-		false) > 9)
+   only wildly guess that it might be 50%.  */
+#define LOGICAL_OP_NON_SHORT_CIRCUIT false
 
 /* Nonzero if access to memory by bytes is slow and undesirable.
    For RISC chips, it means that access to memory by bytes is no