cmd/internal/obj/riscv: simplify addition with constant

This CL simplifies riscv addition (add r, imm) to
(ADDI (ADDI r, imm/2), imm-imm/2) if imm is in specific
ranges (-4096 <= imm < -2048 or 2047 < imm <= 4094).

There is little impact to the go1 benchmark, while the total
size of pkg/linux_riscv64 decreased by about 11KB.

src/cmd/internal/obj/riscv/obj.go

@@ -774,6 +774,27 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 				break // no need to split
 			}
 
+			// Split into two additions if possible.
+			imm := q.From.Offset
+			const minInt12, maxInt12 = -(1 << 11), (1 << 11) - 1
+			if q.As == AADDI && 2*minInt12 <= imm && imm <= 2*maxInt12 {
+				imm0, imm1 := imm/2, imm-imm/2
+				// ADDI $(imm/2), REG, TO
+				p.Spadj = 0 // needed if TO is SP
+				p.As = AADDI
+				p.From = obj.Addr{Type: obj.TYPE_CONST, Offset: imm0}
+				p.Reg = q.Reg
+				p.To = q.To
+				p = obj.Appendp(p, newprog)
+				// ADDI $(imm-imm/2), TO, TO
+				p.Spadj = q.Spadj
+				p.As = AADDI
+				p.From = obj.Addr{Type: obj.TYPE_CONST, Offset: imm1}
+				p.Reg = q.To.Reg
+				p.To = q.To
+				break
+			}
+
 			p.As = ALUI
 			p.From = obj.Addr{Type: obj.TYPE_CONST, Offset: high}
 			p.Reg = 0

test/codegen/arithmetic.go

@@ -575,3 +575,11 @@ func constantFold3(i, j int) int {
 	r := (5 * i) * (6 * j)
 	return r
 }
+
+func addConst(i int64) (int64, int64) {
+	// riscv64:`ADDI`,-`LUI`
+	a := i + 3001
+	// riscv64:`LUI`,`ADDIW`
+	b := i + 5009
+	return a, b
+}