cmd/compile: Fix float32 constant generation

Fixes golang#13

math.Float32bits returns a uint32, and then a cast to int64 used to convert
it into a 64bit immediate which overflowed the signed int32 bounds.
Casting it into a signed int32 first forces the compiler to sign extend
the value returned by math.Float32bits which correctly generates the 32bit
signed immediate for the MOV instruction.

Change-Id: Ia531a26eeb4e344aff490ab73a35b8cf31c9e6dc

riscvtest/fmv.go

@@ -2,5 +2,14 @@ package main
 
 func main() {
 	a := float32(5)
-	riscvexit(int(a))
+	if int(a) != 5 {
+		riscvexit(1)
+	}
+
+	b := float32(1e6)
+	if b < -1e7 || b > 1e7 {
+		riscvexit(2)
+	}
+
+	riscvexit(0)
 }

src/cmd/compile/internal/riscv/ssa.go

@@ -288,7 +288,7 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
 		// Convert the float to the equivalent integer literal so we can
 		// move it using existing infrastructure.
 		p.From.Type = obj.TYPE_CONST
-		p.From.Offset = int64(math.Float32bits(float32(math.Float64frombits(uint64(v.AuxInt)))))
+		p.From.Offset = int64(int32(math.Float32bits(float32(math.Float64frombits(uint64(v.AuxInt))))))
 		p.To.Type = obj.TYPE_REG
 		p.To.Reg = v.Reg()
 	case ssa.OpRISCVMOVaddr: