RISC-V: Support IMM for operand 0 of ussub pattern

This patch would like to allow IMM for the operand 0 of ussub pattern.
Aka .SAT_SUB(1023, y) as the below example.

Form 1:
  #define DEF_SAT_U_SUB_IMM_FMT_1(T, IMM) \
  T __attribute__((noinline))             \
  sat_u_sub_imm##IMM##_##T##_fmt_1 (T y)  \
  {                                       \
    return (T)IMM >= y ? (T)IMM - y : 0;  \
  }

DEF_SAT_U_SUB_IMM_FMT_1(uint64_t, 1023)

Before this patch:
  10   │ sat_u_sub_imm82_uint64_t_fmt_1:
  11   │     li  a5,82
  12   │     bgtu    a0,a5,.L3
  13   │     sub a0,a5,a0
  14   │     ret
  15   │ .L3:
  16   │     li  a0,0
  17   │     ret

After this patch:
  10   │ sat_u_sub_imm82_uint64_t_fmt_1:
  11   │     li  a5,82
  12   │     sltu    a4,a5,a0
  13   │     addi    a4,a4,-1
  14   │     sub a0,a5,a0
  15   │     and a0,a4,a0
  16   │     ret

The below test suites are passed for this patch:
1. The rv64gcv fully regression test.

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_gen_unsigned_xmode_reg): Add new
	func impl to gen xmode rtx reg from operand rtx.
	(riscv_expand_ussub): Gen xmode reg for operand 1.
	* config/riscv/riscv.md: Allow const_int for operand 1.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/sat_arith.h: Add test helper macro.
	* gcc.target/riscv/sat_u_sub_imm-1.c: New test.
	* gcc.target/riscv/sat_u_sub_imm-1_1.c: New test.
	* gcc.target/riscv/sat_u_sub_imm-1_2.c: New test.
	* gcc.target/riscv/sat_u_sub_imm-2.c: New test.
	* gcc.target/riscv/sat_u_sub_imm-2_1.c: New test.
	* gcc.target/riscv/sat_u_sub_imm-2_2.c: New test.
	* gcc.target/riscv/sat_u_sub_imm-3.c: New test.
	* gcc.target/riscv/sat_u_sub_imm-3_1.c: New test.
	* gcc.target/riscv/sat_u_sub_imm-3_2.c: New test.
	* gcc.target/riscv/sat_u_sub_imm-4.c: New test.
	* gcc.target/riscv/sat_u_sub_imm-run-1.c: New test.
	* gcc.target/riscv/sat_u_sub_imm-run-2.c: New test.
	* gcc.target/riscv/sat_u_sub_imm-run-3.c: New test.
	* gcc.target/riscv/sat_u_sub_imm-run-4.c: New test.

Signed-off-by: Pan Li <pan2.li@intel.com>

2 files changed, 46 insertions, 2 deletions

diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
index 8538d40..90a6e93 100644
--- a/gcc/config/riscv/riscv.cc
+++ b/gcc/config/riscv/riscv.cc
@@ -11907,6 +11907,50 @@ riscv_expand_usadd (rtx dest, rtx x, rtx y)
   emit_move_insn (dest, gen_lowpart (mode, xmode_dest));
 }
 
+/* Generate a REG rtx of Xmode from the given rtx and mode.
+   The rtx x can be REG (QI/HI/SI/DI) or const_int.
+   The machine_mode mode is the original mode from define pattern.
+
+   If rtx is REG,  the gen_lowpart of Xmode will be returned.
+
+   If rtx is const_int,  a new REG rtx will be created to hold the value of
+   const_int and then returned.
+
+   According to the gccint doc, the constants generated for modes with fewer
+   bits than in HOST_WIDE_INT must be sign extended to full width.  Thus there
+   will be two cases here,  take QImode as example.
+
+   For .SAT_SUB (127, y) in QImode, we have (const_int 127) and one simple
+   mov from const_int to the new REG rtx is good enough here.
+
+   For .SAT_SUB (254, y) in QImode, we have (const_int -2) after define_expand.
+   Aka 0xfffffffffffffffe in Xmode of RV64 but we actually need 0xfe in Xmode
+   of RV64.  So we need to cleanup the highest 56 bits of the new REG rtx moved
+   from the (const_int -2).
+
+   Then the underlying expanding can perform the code generation based on
+   the REG rtx of Xmode,  instead of taking care of these in expand func.  */
+
+static rtx
+riscv_gen_unsigned_xmode_reg (rtx x, machine_mode mode)
+{
+  if (!CONST_INT_P (x))
+    return gen_lowpart (Xmode, x);
+
+  rtx xmode_x = gen_reg_rtx (Xmode);
+
+  if (mode == Xmode)
+    emit_move_insn (xmode_x, x);
+  else
+    {
+      rtx reg_x = gen_reg_rtx (mode);
+      emit_move_insn (reg_x, x);
+      riscv_emit_unary (ZERO_EXTEND, xmode_x, reg_x);
+    }
+
+  return xmode_x;
+}
+
 /* Implements the unsigned saturation sub standard name usadd for int mode.
 
    z = SAT_SUB(x, y).
@@ -11920,7 +11964,7 @@ void
 riscv_expand_ussub (rtx dest, rtx x, rtx y)
 {
   machine_mode mode = GET_MODE (dest);
-  rtx xmode_x = gen_lowpart (Xmode, x);
+  rtx xmode_x = riscv_gen_unsigned_xmode_reg (x, mode);
   rtx xmode_y = gen_lowpart (Xmode, y);
   rtx xmode_lt = gen_reg_rtx (Xmode);
   rtx xmode_minus = gen_reg_rtx (Xmode);
diff --git a/gcc/config/riscv/riscv.md b/gcc/config/riscv/riscv.md
index a9cbcca..a94705a 100644
--- a/gcc/config/riscv/riscv.md
+++ b/gcc/config/riscv/riscv.md
@@ -4369,7 +4369,7 @@
 
 (define_expand "ussub<mode>3"
   [(match_operand:ANYI 0 "register_operand")
-   (match_operand:ANYI 1 "register_operand")
+   (match_operand:ANYI 1 "reg_or_int_operand")
    (match_operand:ANYI 2 "register_operand")]
   ""
   {