[dev.ssa] cmd/compile/ssa: don't nil check phis with non-nil arguments

Move the known-non-nil scan outside the work loop to resolve an issue
with values that were declared outside the block being operated on.
Also consider phis whose arguments are all non-nil, as non-nil.

Change-Id: I4d5b840042de9eb181f2cb918f36913fb5d517a2
Reviewed-on: https://go-review.googlesource.com/13441Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -45,6 +45,30 @@ func nilcheckelim(f *Func) {
 	// walkStates to maintain the known non-nil values.
 	nonNilValues := make([]bool, f.NumValues())
 
+	// make an initial pass identifying any non-nil values
+	for _, b := range f.Blocks {
+		// a value resulting from taking the address of a
+		// value, or a value constructed from an offset of a
+		// non-nil ptr (OpAddPtr) implies it is non-nil
+		for _, v := range b.Values {
+			if v.Op == OpAddr || v.Op == OpAddPtr {
+				nonNilValues[v.ID] = true
+			} else if v.Op == OpPhi {
+				// phis whose arguments are all non-nil
+				// are non-nil
+				argsNonNil := true
+				for _, a := range v.Args {
+					if !nonNilValues[a.ID] {
+						argsNonNil = false
+					}
+				}
+				if argsNonNil {
+					nonNilValues[v.ID] = true
+				}
+			}
+		}
+	}
+
 	// perform a depth first walk of the dominee tree
 	for len(work) > 0 {
 		node := work[len(work)-1]
@@ -53,19 +77,6 @@ func nilcheckelim(f *Func) {
 		var pushRecPtr bool
 		switch node.op {
 		case Work:
-			// a value resulting from taking the address of a
-			// value, or a value constructed from an offset of a
-			// non-nil ptr (OpAddPtr) implies it is non-nil
-			for _, v := range node.block.Values {
-				if v.Op == OpAddr || v.Op == OpAddPtr {
-					// set this immediately instead of
-					// using SetPtr so we can potentially
-					// remove an OpIsNonNil check in the
-					// current work block
-					nonNilValues[v.ID] = true
-				}
-			}
-
 			if node.ptr != nil {
 				// already have a nilcheck in the dominator path
 				if nonNilValues[node.ptr.ID] {

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

@@ -200,7 +200,51 @@ func TestNilcheckAddPtr(t *testing.T) {
 	}
 }
 
-// TestNilcheckKeepRemove verifies that dupliate checks of the same pointer
+// TestNilcheckPhi tests that nil checks of phis, for which all values are known to be
+// non-nil are removed.
+func TestNilcheckPhi(t *testing.T) {
+	ptrType := &TypeImpl{Size_: 8, Ptr: true, Name: "testptr"} // dummy for testing
+	c := NewConfig("amd64", DummyFrontend{t})
+	fun := Fun(c, "entry",
+		Bloc("entry",
+			Valu("mem", OpArg, TypeMem, 0, ".mem"),
+			Valu("sb", OpSB, TypeInvalid, 0, nil),
+			Valu("sp", OpSP, TypeInvalid, 0, nil),
+			Valu("baddr", OpAddr, TypeBool, 0, "b", "sp"),
+			Valu("bool1", OpLoad, TypeBool, 0, nil, "baddr", "mem"),
+			If("bool1", "b1", "b2")),
+		Bloc("b1",
+			Valu("ptr1", OpAddr, ptrType, 0, nil, "sb"),
+			Goto("checkPtr")),
+		Bloc("b2",
+			Valu("ptr2", OpAddr, ptrType, 0, nil, "sb"),
+			Goto("checkPtr")),
+		// both ptr1 and ptr2 are guaranteed non-nil here
+		Bloc("checkPtr",
+			Valu("phi", OpPhi, ptrType, 0, nil, "ptr1", "ptr2"),
+			Valu("bool2", OpIsNonNil, TypeBool, 0, nil, "phi"),
+			If("bool2", "extra", "exit")),
+		Bloc("extra",
+			Goto("exit")),
+		Bloc("exit",
+			Exit("mem")))
+
+	CheckFunc(fun.f)
+	nilcheckelim(fun.f)
+
+	// clean up the removed nil check
+	fuse(fun.f)
+	deadcode(fun.f)
+
+	CheckFunc(fun.f)
+	for _, b := range fun.f.Blocks {
+		if b == fun.blocks["checkPtr"] && isNilCheck(b) {
+			t.Errorf("checkPtr was not eliminated")
+		}
+	}
+}
+
+// TestNilcheckKeepRemove verifies that duplicate checks of the same pointer
 // are removed, but checks of different pointers are not.
 func TestNilcheckKeepRemove(t *testing.T) {
 	ptrType := &TypeImpl{Size_: 8, Ptr: true, Name: "testptr"} // dummy for testing