LoongArch: Fix cost model for alsl

Our cost model for alsl was wrong: it matches (a + b * imm) where imm is
1, 2, 3, or 4 (should be 2, 4, 8, or 16), and it does not match
(a + (b << imm)) at all.  For the test case:

    a += c << 3;
    b += c << 3;

it caused the compiler to perform a CSE and make one slli and two add,
but we just want two alsl.

Also add a "code == PLUS" check to prevent matching a - (b << imm) as we
don't have any "slsl" instruction.

gcc/ChangeLog:

	* config/loongarch/loongarch.cc (loongarch_rtx_costs): Fix the
	cost for (a + b * imm) and (a + (b << imm)) which can be
	implemented with a single alsl instruction.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/alsl-cost.c: New test.

2 files changed, 36 insertions, 5 deletions

diff --git a/gcc/config/loongarch/loongarch.cc b/gcc/config/loongarch/loongarch.cc
index 9d97f02..3a8e129 100644
--- a/gcc/config/loongarch/loongarch.cc
+++ b/gcc/config/loongarch/loongarch.cc
@@ -3929,14 +3929,31 @@ loongarch_rtx_costs (rtx x, machine_mode mode, int outer_code,
 
       /* If it's an add + mult (which is equivalent to shift left) and
 	 it's immediate operand satisfies const_immalsl_operand predicate.  */
-      if ((mode == SImode || (TARGET_64BIT && mode == DImode))
-	  && GET_CODE (XEXP (x, 0)) == MULT)
+      if (code == PLUS
+	  && (mode == SImode || (TARGET_64BIT && mode == DImode)))
 	{
-	  rtx op2 = XEXP (XEXP (x, 0), 1);
-	  if (const_immalsl_operand (op2, mode))
+	  HOST_WIDE_INT shamt = -1;
+	  rtx lhs = XEXP (x, 0);
+	  rtx_code code_lhs = GET_CODE (lhs);
+
+	  switch (code_lhs)
+	    {
+	    case ASHIFT:
+	      if (CONST_INT_P (XEXP (lhs, 1)))
+		shamt = INTVAL (XEXP (lhs, 1));
+	      break;
+	    case MULT:
+	      if (CONST_INT_P (XEXP (lhs, 1)))
+		shamt = exact_log2 (INTVAL (XEXP (lhs, 1)));
+	      break;
+	    default:
+	      break;
+	    }
+
+	  if (IN_RANGE (shamt, 1, 4))
 	    {
 	      *total = (COSTS_N_INSNS (1)
-			+ set_src_cost (XEXP (XEXP (x, 0), 0), mode, speed)
+			+ set_src_cost (XEXP (lhs, 0), mode, speed)
 			+ set_src_cost (XEXP (x, 1), mode, speed));
 	      return true;
 	    }
diff --git a/gcc/testsuite/gcc.target/loongarch/alsl-cost.c b/gcc/testsuite/gcc.target/loongarch/alsl-cost.c
new file mode 100644
index 0000000..a182279
--- /dev/null
+++ b/gcc/testsuite/gcc.target/loongarch/alsl-cost.c
@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -mtune=loongarch64" } */
+/* { dg-final { scan-assembler-times "alsl\\\.\[wd\]" 2 } } */
+
+struct P
+{
+  long a, b;
+};
+
+struct P
+t (struct P x, long n)
+{
+  return (struct P){.a = x.a + n * 8, .b = x.b + n * 8};
+}