[PATCH 2/2] loongarch: avoid unnecessary sign-extend after 32-bit division

[Xi Ruoyao <xry111@xry111.site>]

Like add.w/sub.w/mul.w, div.w/mod.w/div.wu/mod.wu also sign-extend the
output on LA64.  But, LoongArch v1.00 mandates that the inputs of 32-bit
division to be sign-extended so we have to expand 32-bit division into
RTL sequences.

We defined div.w/mod.w/div.wu/mod.wu as a (DI, DI) -> SI instruction.
This definition does not indicate the fact that these instructions will
store the result as sign-extended value in a 64-bit GR.  Then the
compiler would emit unnecessary sign-extend operations.  For example:

    int div(int a, int b) { return a / b; }

was compiled to:

    div.w  $r4, $r4, $r5
    slli.w $r4, $r4, 0    # this is unnecessary
    jr     $r1

To remove this unnecessary operation, we change the division
instructions to (DI, DI) -> DI and describe the sign-extend behavior
explicitly in the RTL template.  In the expander for 32-bit division we
then use simplify_gen_subreg to extract the lower 32 bits.

gcc/ChangeLog:

	* config/loongarch/loongarch.md (<any_div>di3_fake): Describe
	the sign-extend of result in the RTL template.
	(<any_div><mode>3): Adjust for <any_div>di3_fake change.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/div-4.c: New test.
---
 gcc/config/loongarch/loongarch.md          | 12 ++++++++----
 gcc/testsuite/gcc.target/loongarch/div-4.c |  9 +++++++++
 2 files changed, 17 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/gcc.target/loongarch/div-4.c

diff --git a/gcc/config/loongarch/loongarch.md b/gcc/config/loongarch/loongarch.md
index b002eb2ac22..0202f73efae 100644
--- a/gcc/config/loongarch/loongarch.md
+++ b/gcc/config/loongarch/loongarch.md
@@ -752,6 +752,7 @@ (define_expand "<optab><mode>3"
   {
     rtx reg1 = gen_reg_rtx (DImode);
     rtx reg2 = gen_reg_rtx (DImode);
+    rtx rd = gen_reg_rtx (DImode);
 
     operands[1] = gen_rtx_SIGN_EXTEND (word_mode, operands[1]);
     operands[2] = gen_rtx_SIGN_EXTEND (word_mode, operands[2]);
@@ -759,7 +760,9 @@ (define_expand "<optab><mode>3"
     emit_insn (gen_rtx_SET (reg1, operands[1]));
     emit_insn (gen_rtx_SET (reg2, operands[2]));
 
-    emit_insn (gen_<optab>di3_fake (operands[0], reg1, reg2));
+    emit_insn (gen_<optab>di3_fake (rd, reg1, reg2));
+    emit_insn (gen_rtx_SET (operands[0],
+			    simplify_gen_subreg (SImode, rd, DImode, 0)));
     DONE;
   }
 })
@@ -781,9 +784,10 @@ (define_insn "*<optab><mode>3"
 	(const_string "no")))])
 
 (define_insn "<optab>di3_fake"
-  [(set (match_operand:SI 0 "register_operand" "=r,&r,&r")
-	(any_div:SI (match_operand:DI 1 "register_operand" "r,r,0")
-		    (match_operand:DI 2 "register_operand" "r,r,r")))]
+  [(set (match_operand:DI 0 "register_operand" "=r,&r,&r")
+	(sign_extend:DI
+	  (any_div:SI (match_operand:DI 1 "register_operand" "r,r,0")
+		      (match_operand:DI 2 "register_operand" "r,r,r"))))]
   ""
 {
   return loongarch_output_division ("<insn>.w<u>\t%0,%1,%2", operands);
diff --git a/gcc/testsuite/gcc.target/loongarch/div-4.c b/gcc/testsuite/gcc.target/loongarch/div-4.c
new file mode 100644
index 00000000000..a52f87d6caf
--- /dev/null
+++ b/gcc/testsuite/gcc.target/loongarch/div-4.c
@@ -0,0 +1,9 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+/* { dg-final { scan-assembler-not "slli" } } */
+
+int
+div(int a, int b)
+{
+  return a / b;
+}
-- 
2.37.0