[release-branch.go1.8] cmd/compile: remove unnecessary type conversions on s390x

Some rules insert MOVDreg ops to ensure that type changes are kept.
If there is no type change (or the input is constant) then the MOVDreg
can be omitted, allowing further optimization.

Reduces the size of the .text section in the asm tool by ~33KB.

For #19227.

Change-Id: I0f7b40d8dbcda73bca96eb6d2bf13f9ffa88f4b6
Reviewed-on: https://go-review.googlesource.com/37535
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/compile/internal/ssa/gen/S390X.rules

@@ -448,7 +448,15 @@
 // ***************************
 // TODO: Should the optimizations be a separate pass?
 
-// if a register move has only 1 use, just use the same register without emitting instruction
+// Fold unnecessary type conversions.
+(MOVDreg <t> x) && t.Compare(x.Type) == CMPeq -> x
+(MOVDnop <t> x) && t.Compare(x.Type) == CMPeq -> x
+
+// Propagate constants through type conversions.
+(MOVDreg (MOVDconst [c])) -> (MOVDconst [c])
+(MOVDnop (MOVDconst [c])) -> (MOVDconst [c])
+
+// If a register move has only 1 use, just use the same register without emitting instruction.
 // MOVDnop doesn't emit instruction, only for ensuring the type.
 (MOVDreg x) && x.Uses == 1 -> (MOVDnop x)
 

src/cmd/compile/internal/ssa/rewriteS390X.go

@@ -524,6 +524,8 @@ func rewriteValueS390X(v *Value, config *Config) bool {
 		return rewriteValueS390X_OpS390XMOVDload(v, config)
 	case OpS390XMOVDloadidx:
 		return rewriteValueS390X_OpS390XMOVDloadidx(v, config)
+	case OpS390XMOVDnop:
+		return rewriteValueS390X_OpS390XMOVDnop(v, config)
 	case OpS390XMOVDreg:
 		return rewriteValueS390X_OpS390XMOVDreg(v, config)
 	case OpS390XMOVDstore:
@@ -10195,9 +10197,68 @@ func rewriteValueS390X_OpS390XMOVDloadidx(v *Value, config *Config) bool {
 	}
 	return false
 }
+func rewriteValueS390X_OpS390XMOVDnop(v *Value, config *Config) bool {
+	b := v.Block
+	_ = b
+	// match: (MOVDnop <t> x)
+	// cond: t.Compare(x.Type) == CMPeq
+	// result: x
+	for {
+		t := v.Type
+		x := v.Args[0]
+		if !(t.Compare(x.Type) == CMPeq) {
+			break
+		}
+		v.reset(OpCopy)
+		v.Type = x.Type
+		v.AddArg(x)
+		return true
+	}
+	// match: (MOVDnop (MOVDconst [c]))
+	// cond:
+	// result: (MOVDconst [c])
+	for {
+		v_0 := v.Args[0]
+		if v_0.Op != OpS390XMOVDconst {
+			break
+		}
+		c := v_0.AuxInt
+		v.reset(OpS390XMOVDconst)
+		v.AuxInt = c
+		return true
+	}
+	return false
+}
 func rewriteValueS390X_OpS390XMOVDreg(v *Value, config *Config) bool {
 	b := v.Block
 	_ = b
+	// match: (MOVDreg <t> x)
+	// cond: t.Compare(x.Type) == CMPeq
+	// result: x
+	for {
+		t := v.Type
+		x := v.Args[0]
+		if !(t.Compare(x.Type) == CMPeq) {
+			break
+		}
+		v.reset(OpCopy)
+		v.Type = x.Type
+		v.AddArg(x)
+		return true
+	}
+	// match: (MOVDreg (MOVDconst [c]))
+	// cond:
+	// result: (MOVDconst [c])
+	for {
+		v_0 := v.Args[0]
+		if v_0.Op != OpS390XMOVDconst {
+			break
+		}
+		c := v_0.AuxInt
+		v.reset(OpS390XMOVDconst)
+		v.AuxInt = c
+		return true
+	}
 	// match: (MOVDreg x)
 	// cond: x.Uses == 1
 	// result: (MOVDnop x)