[PATCH] RISC-V: Removed unnecessary sign-extend for vsetvl

[Lehua Ding <lehua.ding@rivai.ai>]

Hi,

This patch try to combine bellow two insns and then further remove
unnecessary sign_extend operations. This optimization is borrowed
from LLVM (https://godbolt.org/z/4f6v56xej):
  (set (reg:DI 134 [ _1 ])
       (unspec:DI [
               (const_int 19 [0x13])
               (const_int 8 [0x8])
               (const_int 5 [0x5])
               (const_int 2 [0x2]) repeated x2
           ] UNSPEC_VSETVL))
  (set (reg/v:DI 135 [ <retval> ])
          (sign_extend:DI (subreg:SI (reg:DI 134 [ _1 ]) 0)))

The reason we can remove signe_extend is because currently the vl value
returned by the vsetvl instruction ranges from 0 to 65536 (uint16_t), and
bits 17 to 63 (including 31) are always 0, so there is no change after
sign_extend. Note that for HI and QI modes we cannot do this.
Of course, if the range of instructions returned by vsetvl later expands
to 32bits, then this combine pattern needs to be removed. But that could be
a long time from now.

gcc/ChangeLog:

	* config/riscv/vector.md (*vsetvldi_no_side_effects_si_extend):
	New combine pattern.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/vsetvl/vsetvl_int.c: New test.

---
 gcc/config/riscv/vector.md                    | 41 +++++++++++++++++++
 .../gcc.target/riscv/rvv/vsetvl/vsetvl_int.c  | 31 ++++++++++++++
 2 files changed, 72 insertions(+)
 create mode 100644 gcc/testsuite/gcc.target/riscv/rvv/vsetvl/vsetvl_int.c

diff --git a/gcc/config/riscv/vector.md b/gcc/config/riscv/vector.md
index e23f64938b7..d1499d330ff 100644
--- a/gcc/config/riscv/vector.md
+++ b/gcc/config/riscv/vector.md
@@ -1604,6 +1604,47 @@
   [(set_attr "type" "vsetvl")
    (set_attr "mode" "SI")])
 
+;; This pattern use to combine bellow two insns and then further remove
+;; unnecessary sign_extend operations:
+;;   (set (reg:DI 134 [ _1 ])
+;;        (unspec:DI [
+;;                (const_int 19 [0x13])
+;;                (const_int 8 [0x8])
+;;                (const_int 5 [0x5])
+;;                (const_int 2 [0x2]) repeated x2
+;;            ] UNSPEC_VSETVL))
+;;   (set (reg/v:DI 135 [ <retval> ])
+;;           (sign_extend:DI (subreg:SI (reg:DI 134 [ _1 ]) 0)))
+;;
+;; The reason we can remove signe_extend is because currently the vl value
+;; returned by the vsetvl instruction ranges from 0 to 65536 (uint16_t), and
+;; bits 17 to 63 (including 31) are always 0, so there is no change after
+;; sign_extend. Note that for HI and QI modes we cannot do this.
+;; Of course, if the range of instructions returned by vsetvl later expands
+;; to 32bits, then this combine pattern needs to be removed. But that could be
+;; a long time from now.
+(define_insn_and_split "*vsetvldi_no_side_effects_si_extend"
+  [(set (match_operand:DI 0 "register_operand")
+        (sign_extend:DI
+          (subreg:SI
+	    (unspec:DI [(match_operand:P 1 "csr_operand")
+		        (match_operand 2 "const_int_operand")
+		        (match_operand 3 "const_int_operand")
+		        (match_operand 4 "const_int_operand")
+		        (match_operand 5 "const_int_operand")] UNSPEC_VSETVL) 0)))]
+  "TARGET_VECTOR && TARGET_64BIT"
+  "#"
+  "&& 1"
+  [(set (match_dup 0)
+        (unspec:DI [(match_dup 1)
+                    (match_dup 2)
+                    (match_dup 3)
+                    (match_dup 4)
+                    (match_dup 5)] UNSPEC_VSETVL))]
+  ""
+  [(set_attr "type" "vsetvl")
+   (set_attr "mode" "SI")])
+
 ;; RVV machine description matching format
 ;; (define_insn ""
 ;;   [(set (match_operand:MODE 0)
diff --git a/gcc/testsuite/gcc.target/riscv/rvv/vsetvl/vsetvl_int.c b/gcc/testsuite/gcc.target/riscv/rvv/vsetvl/vsetvl_int.c
new file mode 100644
index 00000000000..4cdd5877742
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/rvv/vsetvl/vsetvl_int.c
@@ -0,0 +1,31 @@
+/* { dg-do compile } */
+/* { dg-options "-march=rv64gcv -mabi=lp64d" } */
+
+#include "riscv_vector.h"
+
+void bar1 (int32_t a);
+
+int32_t
+foo1 ()
+{
+  int32_t a = __riscv_vsetvl_e8mf8(19);
+  bar1 (a);
+  return a;
+}
+
+void bar2 (uint32_t a);
+
+uint32_t
+foo2 ()
+{
+  uint32_t a = __riscv_vsetvl_e8mf8(19);
+  bar2 (a);
+  return a;
+}
+
+int32_t foo3 ()
+{
+  return __riscv_vsetvl_e8mf8(19);
+}
+
+/* { dg-final { scan-assembler-not {sext\.w} { target { no-opts "-O0" "-g" } } } } */
-- 
2.36.3