cmd/internal/obj/riscv: support MOVD with floating point constants

Currently, we only support loading of values from memory (or other
registers). Add floating point constant support to MOVD. This is
implemented by storing the floating point constant to a symbol,
which is then loaded into the floating point register.

Change-Id: I6db242d27f606f0d5d084a3ab93538698d3a4f8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/631876
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
Reviewed-by: Mark Ryan <markdryan@rivosinc.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/asm/internal/asm/testdata/riscv64.s

@@ -510,6 +510,9 @@ start:
 	MOVD	F0, 4(X5)				// 27b20200
 	MOVD	F0, F1					// d3000022
 
+	// Convert to load of symbol (AUIPC + FLD)
+	MOVD	$(709.78271289338397), F3		// 970f000087b10f00
+
 	// TLS load with local-exec (LUI + ADDIW + ADD of TP + load)
 	MOV	tls(SB), X5				// b70f00009b8f0f00b38f4f0083b20f00
 	MOVB	tls(SB), X5				// b70f00009b8f0f00b38f4f0083820f00

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

@@ -147,6 +147,15 @@ func progedit(ctxt *obj.Link, p *obj.Prog, newprog obj.ProgAlloc) {
 			p.From.Name = obj.NAME_EXTERN
 			p.From.Offset = 0
 		}
+
+	case AMOVD:
+		if p.From.Type == obj.TYPE_FCONST && p.From.Name == obj.NAME_NONE && p.From.Reg == obj.REG_NONE {
+			f64 := p.From.Val.(float64)
+			p.From.Type = obj.TYPE_MEM
+			p.From.Sym = ctxt.Float64Sym(f64)
+			p.From.Name = obj.NAME_EXTERN
+			p.From.Offset = 0
+		}
 	}
 }
 
@@ -2322,12 +2331,19 @@ func instructionsForMOV(p *obj.Prog) []*instruction {
 			}
 
 			// Note that the values for $off_hi and $off_lo are currently
-			// zero and will be assigned during relocation.
+			// zero and will be assigned during relocation. If the destination
+			// is an integer register then we can use the same register for the
+			// address computation, otherwise we need to use the temporary register.
 			//
 			// AUIPC $off_hi, Rd
 			// L $off_lo, Rd, Rd
-			insAUIPC := &instruction{as: AAUIPC, rd: ins.rd}
+			//
-			ins.as, ins.rs1, ins.rs2, ins.imm = movToLoad(p.As), ins.rd, obj.REG_NONE, 0
+			addrReg := ins.rd
+			if addrReg < REG_X0 || addrReg > REG_X31 {
+				addrReg = REG_TMP
+			}
+			insAUIPC := &instruction{as: AAUIPC, rd: addrReg}
+			ins.as, ins.rs1, ins.rs2, ins.imm = movToLoad(p.As), addrReg, obj.REG_NONE, 0
 			inss = []*instruction{insAUIPC, ins}
 
 		default: