[PATCH v2] RISC-V: costs: support shift-and-add in strength-reduction

[Philipp Tomsich <philipp.tomsich@vrull.eu>]

The strength-reduction implementation in expmed.cc will assess the
profitability of using shift-and-add using a RTL expression that wraps
a MULT (with a power-of-2) in a PLUS.  Unless the RISC-V rtx_costs
function recognizes this as expressing a sh[123]add instruction, we
will return an inflated cost---thus defeating the optimization.

This change adds the necessary idiom recognition to provide an
accurate cost for this for of expressing sh[123]add.

Instead on expanding to
	li	a5,200
	mulw	a0,a5,a0
with this change, the expression 'a * 200' is sythesized as:
	sh2add	a0,a0,a0   // *5 = a + 4 * a
	sh2add	a0,a0,a0   // *5 = a + 4 * a
	slli	a0,a0,3    // *8

gcc/ChangeLog:

	* config/riscv/riscv.c (riscv_rtx_costs): Recognize shNadd,
	if expressed as a plus and multiplication with a power-of-2.
	Split costing for MINUS from PLUS.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/zba-shNadd-07.c: New test.

Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
---

Changes in v2:
- Split rtx_costs calculation for MINUS from PLUS to ensure that
  (minus reg (ashift reg SHAMT)) is not mistaken for a shNadd
- Add testcase

 gcc/config/riscv/riscv.cc                     | 19 ++++++++++++
 .../gcc.target/riscv/zba-shNadd-07.c          | 31 +++++++++++++++++++
 2 files changed, 50 insertions(+)
 create mode 100644 gcc/testsuite/gcc.target/riscv/zba-shNadd-07.c

diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
index 3e2dc8192e4..2a94482b8ed 100644
--- a/gcc/config/riscv/riscv.cc
+++ b/gcc/config/riscv/riscv.cc
@@ -2428,6 +2428,12 @@ riscv_rtx_costs (rtx x, machine_mode mode, int outer_code, int opno ATTRIBUTE_UN
       return false;
 
     case MINUS:
+      if (float_mode_p)
+	*total = tune_param->fp_add[mode == DFmode];
+      else
+	*total = riscv_binary_cost (x, 1, 4);
+      return false;
+
     case PLUS:
       /* add.uw pattern for zba.  */
       if (TARGET_ZBA
@@ -2451,6 +2457,19 @@ riscv_rtx_costs (rtx x, machine_mode mode, int outer_code, int opno ATTRIBUTE_UN
 	  *total = COSTS_N_INSNS (1);
 	  return true;
 	}
+      /* Before strength-reduction, the shNadd can be expressed as the addition
+	 of a multiplication with a power-of-two.  If this case is not handled,
+	 the strength-reduction in expmed.c will calculate an inflated cost. */
+      if (TARGET_ZBA
+	  && mode == word_mode
+	  && GET_CODE (XEXP (x, 0)) == MULT
+	  && REG_P (XEXP (XEXP (x, 0), 0))
+	  && CONST_INT_P (XEXP (XEXP (x, 0), 1))
+	  && IN_RANGE (pow2p_hwi (INTVAL (XEXP (XEXP (x, 0), 1))), 1, 3))
+	{
+	  *total = COSTS_N_INSNS (1);
+	  return true;
+	}
       /* shNadd.uw pattern for zba.
 	 [(set (match_operand:DI 0 "register_operand" "=r")
 	       (plus:DI
diff --git a/gcc/testsuite/gcc.target/riscv/zba-shNadd-07.c b/gcc/testsuite/gcc.target/riscv/zba-shNadd-07.c
new file mode 100644
index 00000000000..98d35e1da9b
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/zba-shNadd-07.c
@@ -0,0 +1,31 @@
+/* { dg-do compile } */
+/* { dg-options "-march=rv64gc_zba -mabi=lp64 -O2" } */
+
+unsigned long
+f1 (unsigned long i)
+{
+  return i * 200;
+}
+
+unsigned long
+f2 (unsigned long i)
+{
+  return i * 783;
+}
+
+unsigned long
+f3 (unsigned long i)
+{
+  return i * 784;
+}
+
+unsigned long
+f4 (unsigned long i)
+{
+  return i * 1574;
+}
+
+/* { dg-final { scan-assembler-times "sh2add" 2 } } */
+/* { dg-final { scan-assembler-times "sh1add" 2 } } */
+/* { dg-final { scan-assembler-times "slli" 5 } } */
+/* { dg-final { scan-assembler-times "mul" 1 } } */
-- 
2.34.1