[dev.ssa] cmd/compile/internal/ssa: transform degenerate control blocks

* In cases where we end up with empty branches like in
if a then jmp b else jmp b;
the flow can be replaced by a; jmp b.

The following functions is optimized as follows:
func f(a bool, x int) int {
        v := 0
        if a {
                v = -1
        } else {
                v = -1
        }
        return x | v
}

Before this change:
02819 (arith_ssa.go:362)  VARDEF "".~r2+16(FP)
02820 (arith_ssa.go:362)  MOVQ  $0, "".~r2+16(FP)
02821 (arith_ssa.go:362)  MOVB  "".a(FP), AX
02822 (arith_ssa.go:362)  TESTB AX, AX
02823 (arith_ssa.go:364)  JEQ 2824
02824 (arith_ssa.go:369)  VARDEF "".~r2+16(FP)
02825 (arith_ssa.go:369)  MOVQ  $-1, "".~r2+16(FP)
02826 (arith_ssa.go:369)  RET

After this change:
02819 (arith_ssa.go:362)  VARDEF "".~r2+16(FP)
02820 (arith_ssa.go:369)  VARDEF "".~r2+16(FP)
02821 (arith_ssa.go:369)  MOVQ  $-1, "".~r2+16(FP)
02822 (arith_ssa.go:369)  RET

Updates #14277

Change-Id: Ibe7d284f43406c704903632a4fcf2a4a64059686
Reviewed-on: https://go-review.googlesource.com/19464
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -6,16 +6,104 @@ package ssa
 
 // fuse simplifies control flow by joining basic blocks.
 func fuse(f *Func) {
+	for changed := true; changed; {
+		changed = false
 		for _, b := range f.Blocks {
-		if b.Kind != BlockPlain {
+			changed = fuseBlockIf(b) || changed
-			continue
+			changed = fuseBlockPlain(b) || changed
 		}
-		c := b.Succs[0]
+	}
-		if len(c.Preds) != 1 {
+}
-			continue
+
+// fuseBlockIf handles the following cases where s0 and s1 are empty blocks.
+//
+//   b       b          b
+//  / \      | \      / |
+// s0  s1    |  s1   s0 |
+//  \ /      | /      \ |
+//   ss      ss        ss
+//
+// If ss doesn't contain any Phi ops and s0 & s1 are empty then the branch
+// can be dropped.
+// TODO: If ss doesn't contain any Phi ops, are s0 and s1 dead code anyway?
+func fuseBlockIf(b *Block) bool {
+	if b.Kind != BlockIf {
+		return false
 	}
 
-		// move all of b's values to c.
+	var ss0, ss1 *Block
+	s0 := b.Succs[0]
+	if s0.Kind != BlockPlain || len(s0.Preds) != 1 || len(s0.Values) != 0 {
+		s0, ss0 = nil, s0
+	} else {
+		ss0 = s0.Succs[0]
+	}
+	s1 := b.Succs[1]
+	if s1.Kind != BlockPlain || len(s1.Preds) != 1 || len(s1.Values) != 0 {
+		s1, ss1 = nil, s1
+	} else {
+		ss1 = s1.Succs[0]
+	}
+
+	if ss0 != ss1 {
+		return false
+	}
+	ss := ss0
+
+	// TODO: Handle OpPhi operations. We can still replace OpPhi if the
+	// slots corresponding to b, s0 and s1 point to the same variable.
+	for _, v := range ss.Values {
+		if v.Op == OpPhi {
+			return false
+		}
+	}
+
+	// Now we have two following b->ss, b->s0->ss and b->s1->ss,
+	// with s0 and s1 empty if exist.
+	// We can replace it with b->ss without if ss has no phis
+	// which is checked above.
+	// No critical edge is introduced because b will have one successor.
+	if s0 != nil {
+		ss.removePred(s0)
+	}
+	if s1 != nil {
+		ss.removePred(s1)
+	}
+	if s0 != nil && s1 != nil {
+		// Add an edge if both edges are removed, otherwise b is no longer connected to ss.
+		ss.Preds = append(ss.Preds, b)
+	}
+	b.Kind = BlockPlain
+	b.Control = nil
+	b.Succs = append(b.Succs[:0], ss)
+
+	// Trash the empty blocks s0 & s1.
+	if s0 != nil {
+		s0.Kind = BlockInvalid
+		s0.Values = nil
+		s0.Succs = nil
+		s0.Preds = nil
+	}
+	if s1 != nil {
+		s1.Kind = BlockInvalid
+		s1.Values = nil
+		s1.Succs = nil
+		s1.Preds = nil
+	}
+	return true
+}
+
+func fuseBlockPlain(b *Block) bool {
+	if b.Kind != BlockPlain {
+		return false
+	}
+
+	c := b.Succs[0]
+	if len(c.Preds) != 1 {
+		return false
+	}
+
+	// move all of b'c values to c.
 	for _, v := range b.Values {
 		v.Block = c
 		c.Values = append(c.Values, v)
@@ -35,7 +123,7 @@ func fuse(f *Func) {
 			}
 		}
 	}
-		if f.Entry == b {
+	if f := b.Func; f.Entry == b {
 		f.Entry = c
 	}
 
@@ -44,5 +132,5 @@ func fuse(f *Func) {
 	b.Values = nil
 	b.Preds = nil
 	b.Succs = nil
-	}
+	return true
 }

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

@@ -0,0 +1,95 @@
+package ssa
+
+import (
+	"testing"
+)
+
+func TestFuseEliminatesOneBranch(t *testing.T) {
+	ptrType := &TypeImpl{Size_: 8, Ptr: true, Name: "testptr"} // dummy for testing
+	c := NewConfig("amd64", DummyFrontend{t}, nil, true)
+	fun := Fun(c, "entry",
+		Bloc("entry",
+			Valu("mem", OpInitMem, TypeMem, 0, nil),
+			Valu("sb", OpSB, TypeInvalid, 0, nil),
+			Goto("checkPtr")),
+		Bloc("checkPtr",
+			Valu("ptr1", OpLoad, ptrType, 0, nil, "sb", "mem"),
+			Valu("nilptr", OpConstNil, ptrType, 0, nil, "sb"),
+			Valu("bool1", OpNeqPtr, TypeBool, 0, nil, "ptr1", "nilptr"),
+			If("bool1", "then", "exit")),
+		Bloc("then",
+			Goto("exit")),
+		Bloc("exit",
+			Exit("mem")))
+
+	CheckFunc(fun.f)
+	fuse(fun.f)
+
+	for _, b := range fun.f.Blocks {
+		if b == fun.blocks["then"] && b.Kind != BlockInvalid {
+			t.Errorf("then was not eliminated, but should have")
+		}
+	}
+}
+
+func TestFuseEliminatesBothBranches(t *testing.T) {
+	ptrType := &TypeImpl{Size_: 8, Ptr: true, Name: "testptr"} // dummy for testing
+	c := NewConfig("amd64", DummyFrontend{t}, nil, true)
+	fun := Fun(c, "entry",
+		Bloc("entry",
+			Valu("mem", OpInitMem, TypeMem, 0, nil),
+			Valu("sb", OpSB, TypeInvalid, 0, nil),
+			Goto("checkPtr")),
+		Bloc("checkPtr",
+			Valu("ptr1", OpLoad, ptrType, 0, nil, "sb", "mem"),
+			Valu("nilptr", OpConstNil, ptrType, 0, nil, "sb"),
+			Valu("bool1", OpNeqPtr, TypeBool, 0, nil, "ptr1", "nilptr"),
+			If("bool1", "then", "else")),
+		Bloc("then",
+			Goto("exit")),
+		Bloc("else",
+			Goto("exit")),
+		Bloc("exit",
+			Exit("mem")))
+
+	CheckFunc(fun.f)
+	fuse(fun.f)
+
+	for _, b := range fun.f.Blocks {
+		if b == fun.blocks["then"] && b.Kind != BlockInvalid {
+			t.Errorf("then was not eliminated, but should have")
+		}
+		if b == fun.blocks["else"] && b.Kind != BlockInvalid {
+			t.Errorf("then was not eliminated, but should have")
+		}
+	}
+}
+
+func TestFuseEliminatesEmptyBlocks(t *testing.T) {
+	c := NewConfig("amd64", DummyFrontend{t}, nil, true)
+	fun := Fun(c, "entry",
+		Bloc("entry",
+			Valu("mem", OpInitMem, TypeMem, 0, nil),
+			Valu("sb", OpSB, TypeInvalid, 0, nil),
+			Goto("z0")),
+		Bloc("z1",
+			Goto("z2")),
+		Bloc("z3",
+			Goto("exit")),
+		Bloc("z2",
+			Goto("z3")),
+		Bloc("z0",
+			Goto("z1")),
+		Bloc("exit",
+			Exit("mem"),
+		))
+
+	CheckFunc(fun.f)
+	fuse(fun.f)
+
+	for k, b := range fun.blocks {
+		if k[:1] == "z" && b.Kind != BlockInvalid {
+			t.Errorf("%s was not eliminated, but should have", k)
+		}
+	}
+}

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

@@ -403,8 +403,11 @@ func TestNilcheckBug(t *testing.T) {
 			Valu("bool2", OpIsNonNil, TypeBool, 0, nil, "ptr1"),
 			If("bool2", "extra", "exit")),
 		Bloc("extra",
+			// prevent fuse from eliminating this block
+			Valu("store", OpStore, TypeMem, 8, nil, "ptr1", "nilptr", "mem"),
 			Goto("exit")),
 		Bloc("exit",
+			Valu("phi", OpPhi, TypeMem, 0, nil, "mem", "store"),
 			Exit("mem")))
 
 	CheckFunc(fun.f)