[dev.ssa] cmd/compile: clean up nilcheck logic

Be more clear about the two conditions that we care about:
1) a block that performs a nil check (OpIsNonNil), which may be removed
2) a block that is the non-nil sucessor for an OpIsNonNil block

Now we only care about removing nilchecks for two scenarios:
- a type 1 block is dominated by a type 2 block for the same value
- a block is both type 1 and type 2 for the same value

Fixes math/big.

Change-Id: I50018a4014830461ddfe2a2daf588468e4a8f0b4
Reviewed-on: https://go-review.googlesource.com/14325
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 45 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/ssa/nilcheck_test.go b/src/cmd/compile/internal/ssa/nilcheck_test.go
index 1d048fbb34..cbd17e0093 100644
--- a/src/cmd/compile/internal/ssa/nilcheck_test.go
+++ b/src/cmd/compile/internal/ssa/nilcheck_test.go
@@ -382,3 +382,48 @@ func TestNilcheckUser(t *testing.T) {
 		}
 	}
 }
+
+// TestNilcheckBug reproduces a bug in nilcheckelim found by compiling math/big
+func TestNilcheckBug(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),
+			Goto("checkPtr")),
+		Bloc("checkPtr",
+			Valu("ptr1", OpConstPtr, ptrType, 0, nil, "sb"),
+			Valu("nilptr", OpConstNil, ptrType, 0, nil, "sb"),
+			Valu("bool1", OpNeqPtr, TypeBool, 0, nil, "ptr1", "nilptr"),
+			If("bool1", "secondCheck", "couldBeNil")),
+		Bloc("couldBeNil",
+			Goto("secondCheck")),
+		Bloc("secondCheck",
+			Valu("bool2", OpIsNonNil, TypeBool, 0, nil, "ptr1"),
+			If("bool2", "extra", "exit")),
+		Bloc("extra",
+			Goto("exit")),
+		Bloc("exit",
+			Exit("mem")))
+
+	CheckFunc(fun.f)
+	// we need the opt here to rewrite the user nilcheck
+	opt(fun.f)
+	nilcheckelim(fun.f)
+
+	// clean up the removed nil check
+	fuse(fun.f)
+	deadcode(fun.f)
+
+	CheckFunc(fun.f)
+	foundSecondCheck := false
+	for _, b := range fun.f.Blocks {
+		if b == fun.blocks["secondCheck"] && isNilCheck(b) {
+			foundSecondCheck = true
+		}
+	}
+	if !foundSecondCheck {
+		t.Errorf("secondCheck was eliminated, but shouldn't have")
+	}
+}