[dev.ssa] cmd/compile: split decompose pass in two

A first pass to decompose user types (structs, maybe
arrays someday), and a second pass to decompose builtin
types (strings, interfaces, slices, complex).  David wants
this for value range analysis so he can have structs decomposed
but slices and friends will still be intact and he can deduce
things like the length of a slice is >= 0.

Change-Id: Ia2300d07663329b51ed6270cfed21d31980daa7c
Reviewed-on: https://go-review.googlesource.com/19340
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -99,7 +99,8 @@ var passes = [...]pass{
 	{"early copyelim", copyelim, false},
 	{"early deadcode", deadcode, false}, // remove generated dead code to avoid doing pointless work during opt
 	{"short circuit", shortcircuit, false},
-	{"decompose", decompose, true},
+	{"decompose user", decomposeUser, true},
+	{"decompose builtin", decomposeBuiltIn, true},
 	{"opt", opt, true},                // TODO: split required rules and optimizing rules
 	{"opt deadcode", deadcode, false}, // remove any blocks orphaned during opt
 	{"generic cse", cse, true},
@@ -148,8 +149,8 @@ var passOrder = [...]constraint{
 	// tighten will be most effective when as many values have been removed as possible
 	{"generic deadcode", "tighten"},
 	{"generic cse", "tighten"},
-	// don't run optimization pass until we've decomposed compound objects
+	// don't run optimization pass until we've decomposed builtin objects
-	{"decompose", "opt"},
+	{"decompose builtin", "opt"},
 	// don't layout blocks until critical edges have been removed
 	{"critical", "layout"},
 	// regalloc requires the removal of all critical edges

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

@@ -4,16 +4,16 @@
 
 package ssa
 
-// decompose converts phi ops on compound types into phi
+// decompose converts phi ops on compound builtin types into phi
 // ops on simple types.
 // (The remaining compound ops are decomposed with rewrite rules.)
-func decompose(f *Func) {
+func decomposeBuiltIn(f *Func) {
 	for _, b := range f.Blocks {
 		for _, v := range b.Values {
 			if v.Op != OpPhi {
 				continue
 			}
-			decomposePhi(v)
+			decomposeBuiltInPhi(v)
 		}
 	}
 
@@ -78,22 +78,13 @@ func decompose(f *Func) {
 				f.NamedValues[typeName] = append(f.NamedValues[typeName], typ)
 				f.NamedValues[dataName] = append(f.NamedValues[dataName], data)
 			}
-		case t.IsStruct():
-			n := t.NumFields()
-			for _, v := range f.NamedValues[name] {
-				for i := int64(0); i < n; i++ {
-					fname := LocalSlot{name.N, t.FieldType(i), name.Off + t.FieldOff(i)} // TODO: use actual field name?
-					x := v.Block.NewValue1I(v.Line, OpStructSelect, t.FieldType(i), i, v)
-					f.NamedValues[fname] = append(f.NamedValues[fname], x)
-				}
-			}
 		case t.Size() > f.Config.IntSize:
 			f.Unimplementedf("undecomposed named type %s", t)
 		}
 	}
 }
 
-func decomposePhi(v *Value) {
+func decomposeBuiltInPhi(v *Value) {
 	// TODO: decompose 64-bit ops on 32-bit archs?
 	switch {
 	case v.Type.IsComplex():
@@ -104,8 +95,6 @@ func decomposePhi(v *Value) {
 		decomposeSlicePhi(v)
 	case v.Type.IsInterface():
 		decomposeInterfacePhi(v)
-	case v.Type.IsStruct():
-		decomposeStructPhi(v)
 	case v.Type.Size() > v.Block.Func.Config.IntSize:
 		v.Unimplementedf("undecomposed type %s", v.Type)
 	}
@@ -182,6 +171,50 @@ func decomposeInterfacePhi(v *Value) {
 	v.AddArg(itab)
 	v.AddArg(data)
 }
+
+func decomposeUser(f *Func) {
+	for _, b := range f.Blocks {
+		for _, v := range b.Values {
+			if v.Op != OpPhi {
+				continue
+			}
+			decomposeUserPhi(v)
+		}
+	}
+	// Split up named values into their components.
+	// NOTE: the component values we are making are dead at this point.
+	// We must do the opt pass before any deadcode elimination or we will
+	// lose the name->value correspondence.
+	i := 0
+	for _, name := range f.Names {
+		t := name.Type
+		switch {
+		case t.IsStruct():
+			n := t.NumFields()
+			for _, v := range f.NamedValues[name] {
+				for i := int64(0); i < n; i++ {
+					fname := LocalSlot{name.N, t.FieldType(i), name.Off + t.FieldOff(i)} // TODO: use actual field name?
+					x := v.Block.NewValue1I(v.Line, OpStructSelect, t.FieldType(i), i, v)
+					f.NamedValues[fname] = append(f.NamedValues[fname], x)
+				}
+			}
+			delete(f.NamedValues, name)
+		default:
+			f.Names[i] = name
+			i++
+		}
+	}
+	f.Names = f.Names[:i]
+}
+
+func decomposeUserPhi(v *Value) {
+	switch {
+	case v.Type.IsStruct():
+		decomposeStructPhi(v)
+	}
+	// TODO: Arrays of length 1?
+}
+
 func decomposeStructPhi(v *Value) {
 	t := v.Type
 	n := t.NumFields()
@@ -199,7 +232,9 @@ func decomposeStructPhi(v *Value) {
 
 	// Recursively decompose phis for each field.
 	for _, f := range fields[:n] {
-		decomposePhi(f)
+		if f.Type.IsStruct() {
+			decomposeStructPhi(f)
+		}
 	}
 }