[PATCH v2] LoongArch: Remove redundant sign extension instructions caused by SLT instructions.

[Lulu Cheng <chenglulu@loongson.cn>]

v1 -> v2:
	1. Modify description information


Since the SLT instruction does not distinguish between 64-bit operations and 32-bit
operations under the 64-bit LoongArch architecture, if the operand of slt is SImode,
the sign extension of the operand needs to be displayed.

But similar to the test case below, the sign extension is redundant:

	extern int src1, src2, src3;

	int
	test (void)
	{
	  int data1 = src1 + src2;
	  int data2 = src1 + src3;
	  return data1 > data2 ? data1 : data2;
	}
Assembly code before optimization:
 	...
	add.w	$r4,$r4,$r14
	add.w	$r13,$r13,$r14
	slli.w	$r12,$r4,0
	slli.w	$r14,$r13,0
	slt	$r12,$r12,$r14
	masknez	$r4,$r4,$r12
	maskeqz	$r12,$r13,$r12
	or	$r4,$r4,$r12
	slli.w	$r4,$r4,0
	...

After optimization:
	...
	add.w	$r12,$r12,$r14
	add.w	$r13,$r13,$r14
	slt	$r4,$r12,$r13
	masknez	$r12,$r12,$r4
	maskeqz	$r4,$r13,$r4
	or	$r4,$r12,$r4
	...

Similar to this test example, the two operands of SLT are obtained by the
addition operation, and add.w implicitly sign-extends, so the two operands
of SLT do not require sign-extend.

gcc/ChangeLog:

	* config/loongarch/loongarch.cc (loongarch_expand_conditional_move):
	Optimize the function implementation.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/slt-sign-extend.c: New test.
---
 gcc/config/loongarch/loongarch.cc             | 53 +++++++++++++++++--
 .../gcc.target/loongarch/slt-sign-extend.c    | 14 +++++
 2 files changed, 63 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/gcc.target/loongarch/slt-sign-extend.c

diff --git a/gcc/config/loongarch/loongarch.cc b/gcc/config/loongarch/loongarch.cc
index 86d58784113..1905599b9e8 100644
--- a/gcc/config/loongarch/loongarch.cc
+++ b/gcc/config/loongarch/loongarch.cc
@@ -4384,14 +4384,30 @@ loongarch_expand_conditional_move (rtx *operands)
   enum rtx_code code = GET_CODE (operands[1]);
   rtx op0 = XEXP (operands[1], 0);
   rtx op1 = XEXP (operands[1], 1);
+  rtx op0_extend = op0;
+  rtx op1_extend = op1;
+
+  /* Record whether operands[2] and operands[3] modes are promoted to word_mode.  */
+  bool promote_p = false;
+  machine_mode mode = GET_MODE (operands[0]);
 
   if (FLOAT_MODE_P (GET_MODE (op1)))
     loongarch_emit_float_compare (&code, &op0, &op1);
   else
     {
+      if ((REGNO (op0) == REGNO (operands[2])
+	   || (REGNO (op1) == REGNO (operands[3]) && (op1 != const0_rtx)))
+	  && (GET_MODE_SIZE (GET_MODE (op0)) < word_mode))
+	{
+	  mode = word_mode;
+	  promote_p = true;
+	}
+
       loongarch_extend_comparands (code, &op0, &op1);
 
       op0 = force_reg (word_mode, op0);
+      op0_extend = op0;
+      op1_extend = force_reg (word_mode, op1);
 
       if (code == EQ || code == NE)
 	{
@@ -4418,23 +4434,52 @@ loongarch_expand_conditional_move (rtx *operands)
       && register_operand (operands[2], VOIDmode)
       && register_operand (operands[3], VOIDmode))
     {
-      machine_mode mode = GET_MODE (operands[0]);
+      rtx op2 = operands[2];
+      rtx op3 = operands[3];
+
+      if (promote_p)
+	{
+	  if (REGNO (XEXP (operands[1], 0)) == REGNO (operands[2]))
+	    op2 = op0_extend;
+	  else
+	    {
+	      loongarch_extend_comparands (code, &op2, &const0_rtx);
+	      op2 = force_reg (mode, op2);
+	    }
+
+	  if (REGNO (XEXP (operands[1], 1)) == REGNO (operands[3]))
+	    op3 = op1_extend;
+	  else
+	    {
+	      loongarch_extend_comparands (code, &op3, &const0_rtx);
+	      op3 = force_reg (mode, op3);
+	    }
+	}
+
       rtx temp = gen_reg_rtx (mode);
       rtx temp2 = gen_reg_rtx (mode);
 
       emit_insn (gen_rtx_SET (temp,
 			      gen_rtx_IF_THEN_ELSE (mode, cond,
-						    operands[2], const0_rtx)));
+						    op2, const0_rtx)));
 
       /* Flip the test for the second operand.  */
       cond = gen_rtx_fmt_ee ((code == EQ) ? NE : EQ, GET_MODE (op0), op0, op1);
 
       emit_insn (gen_rtx_SET (temp2,
 			      gen_rtx_IF_THEN_ELSE (mode, cond,
-						    operands[3], const0_rtx)));
+						    op3, const0_rtx)));
 
       /* Merge the two results, at least one is guaranteed to be zero.  */
-      emit_insn (gen_rtx_SET (operands[0], gen_rtx_IOR (mode, temp, temp2)));
+      if (promote_p)
+	{
+	  rtx temp3 = gen_reg_rtx (mode);
+	  emit_insn (gen_rtx_SET (temp3, gen_rtx_IOR (mode, temp, temp2)));
+	  temp3 = gen_lowpart (GET_MODE (operands[0]), temp3);
+	  loongarch_emit_move (operands[0], temp3);
+	}
+      else
+	emit_insn (gen_rtx_SET (operands[0], gen_rtx_IOR (mode, temp, temp2)));
     }
   else
     emit_insn (gen_rtx_SET (operands[0],
diff --git a/gcc/testsuite/gcc.target/loongarch/slt-sign-extend.c b/gcc/testsuite/gcc.target/loongarch/slt-sign-extend.c
new file mode 100644
index 00000000000..ea6b28b7c45
--- /dev/null
+++ b/gcc/testsuite/gcc.target/loongarch/slt-sign-extend.c
@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-options "-mabi=lp64d -O2" } */
+/* { dg-final { scan-assembler-not "slli.w" } } */
+
+extern int src1, src2, src3;
+
+int
+test (void)
+{
+  int data1 = src1 + src2;
+  int data2 = src1 + src3;
+
+  return data1 > data2 ? data1 : data2;
+}
-- 
2.31.1