interp: replace many panics with error messages

This commit replaces most panics in interp/frame.go and interp/scan.go
with real error messages. The remaining ones are panics that should not
happen when working with valid IR.

interp/errors.go

@@ -78,6 +78,9 @@ func errorAt(inst llvm.Value, msg string) scanner.Error {
 // getPosition returns the position information for the given instruction, as
 // far as it is available.
 func getPosition(inst llvm.Value) token.Position {
+	if inst.IsAInstruction().IsNil() {
+		return token.Position{}
+	}
 	loc := inst.InstructionDebugLoc()
 	if loc.IsNil() {
 		return token.Position{}

interp/frame.go

@@ -97,7 +97,7 @@ func (fr *frame) evalBasicBlock(bb, incoming llvm.BasicBlock, indent string) (re
 				value = operand.Load()
 			}
 			if value.Type() != inst.Type() {
-				panic("interp: load: type does not match")
+				return nil, nil, fr.errorAt(inst, "interp: load: type does not match")
 			}
 			fr.locals[inst] = fr.getValue(value)
 		case !inst.IsAStoreInst().IsNil():
@@ -121,15 +121,13 @@ func (fr *frame) evalBasicBlock(bb, incoming llvm.BasicBlock, indent string) (re
 					// Not a constant operation.
 					// This should be detected by the scanner, but isn't at the
 					// moment.
-					panic("todo: non-const gep")
+					return nil, nil, fr.errorAt(inst, "todo: non-const gep")
 				}
 				indices[i] = uint32(operand.Value().ZExtValue())
 			}
 			result := value.GetElementPtr(indices)
 			if result.Type() != inst.Type() {
-				println(" expected:", inst.Type().String())
-				println(" actual:  ", result.Type().String())
-				panic("interp: gep: type does not match")
+				return nil, nil, fr.errorAt(inst, "interp: gep: type does not match")
 			}
 			fr.locals[inst] = result
 
@@ -184,7 +182,7 @@ func (fr *frame) evalBasicBlock(bb, incoming llvm.BasicBlock, indent string) (re
 					}
 				}
 				// It is not possible in Go to bitcast a map value to a pointer.
-				panic("unimplemented: bitcast of map")
+				return nil, nil, fr.errorAt(inst, "unimplemented: bitcast of map")
 			}
 			value := fr.getLocal(operand).(*LocalValue)
 			fr.locals[inst] = &LocalValue{fr.Eval, fr.builder.CreateBitCast(value.Value(), inst.Type(), "")}
@@ -397,16 +395,16 @@ func (fr *frame) evalBasicBlock(bb, incoming llvm.BasicBlock, indent string) (re
 				typecode := fr.getLocal(inst.Operand(0)).(*LocalValue).Underlying
 				interfaceMethodSet := fr.getLocal(inst.Operand(1)).(*LocalValue).Underlying
 				if typecode.IsAConstantExpr().IsNil() || typecode.Opcode() != llvm.PtrToInt {
-					panic("interp: expected typecode to be a ptrtoint")
+					return nil, nil, fr.errorAt(inst, "interp: expected typecode to be a ptrtoint")
 				}
 				typecode = typecode.Operand(0)
 				if interfaceMethodSet.IsAConstantExpr().IsNil() || interfaceMethodSet.Opcode() != llvm.GetElementPtr {
-					panic("interp: expected method set in runtime.interfaceImplements to be a constant gep")
+					return nil, nil, fr.errorAt(inst, "interp: expected method set in runtime.interfaceImplements to be a constant gep")
 				}
 				interfaceMethodSet = interfaceMethodSet.Operand(0).Initializer()
 				methodSet := llvm.ConstExtractValue(typecode.Initializer(), []uint32{1})
 				if methodSet.IsAConstantExpr().IsNil() || methodSet.Opcode() != llvm.GetElementPtr {
-					panic("interp: expected method set to be a constant gep")
+					return nil, nil, fr.errorAt(inst, "interp: expected method set to be a constant gep")
 				}
 				methodSet = methodSet.Operand(0).Initializer()
 
@@ -476,7 +474,10 @@ func (fr *frame) evalBasicBlock(bb, incoming llvm.BasicBlock, indent string) (re
 					params = append(params, local)
 				}
 				var ret Value
-				scanResult := fr.Eval.hasSideEffects(callee)
+				scanResult, err := fr.hasSideEffects(callee)
+				if err != nil {
+					return nil, nil, err
+				}
 				if scanResult.severity == sideEffectLimited || dirtyParams && scanResult.severity != sideEffectAll {
 					// Side effect is bounded. This means the operation invokes
 					// side effects (like calling an external function) but it
@@ -545,7 +546,7 @@ func (fr *frame) evalBasicBlock(bb, incoming llvm.BasicBlock, indent string) (re
 			// conditional branch (if/then/else)
 			cond := fr.getLocal(inst.Operand(0)).Value()
 			if cond.Type() != fr.Mod.Context().Int1Type() {
-				panic("expected an i1 in a branch instruction")
+				return nil, nil, fr.errorAt(inst, "expected an i1 in a branch instruction")
 			}
 			thenBB := inst.Operand(1)
 			elseBB := inst.Operand(2)
@@ -563,7 +564,7 @@ func (fr *frame) evalBasicBlock(bb, incoming llvm.BasicBlock, indent string) (re
 			case llvm.ConstInt(fr.Mod.Context().Int1Type(), 1, false): // true
 				return nil, []llvm.Value{elseBB}, nil // else
 			default:
-				panic("branch was not true or false")
+				return nil, nil, fr.errorAt(inst, "branch was not true or false")
 			}
 		case !inst.IsABranchInst().IsNil() && inst.OperandsCount() == 1:
 			// unconditional branch (goto)
@@ -589,6 +590,7 @@ func (fr *frame) getLocal(v llvm.Value) Value {
 	} else if value := fr.getValue(v); value != nil {
 		return value
 	} else {
+		// This should not happen under normal circumstances.
 		panic("cannot find value")
 	}
 }

interp/interp.go

@@ -27,7 +27,6 @@ type Eval struct {
 type evalPackage struct {
 	*Eval
 	packagePath string
-	errors      []error
 }
 
 // Run evaluates the function with the given name and then eliminates all

interp/scan.go

@@ -38,33 +38,33 @@ type sideEffectResult struct {
 // hasSideEffects scans this function and all descendants, recursively. It
 // returns whether this function has side effects and if it does, which globals
 // it mentions anywhere in this function or any called functions.
-func (e *Eval) hasSideEffects(fn llvm.Value) *sideEffectResult {
+func (e *evalPackage) hasSideEffects(fn llvm.Value) (*sideEffectResult, error) {
 	switch fn.Name() {
 	case "runtime.alloc":
 		// Cannot be scanned but can be interpreted.
-		return &sideEffectResult{severity: sideEffectNone}
+		return &sideEffectResult{severity: sideEffectNone}, nil
 	case "runtime.nanotime":
 		// Fixed value at compile time.
-		return &sideEffectResult{severity: sideEffectNone}
+		return &sideEffectResult{severity: sideEffectNone}, nil
 	case "runtime._panic":
-		return &sideEffectResult{severity: sideEffectLimited}
+		return &sideEffectResult{severity: sideEffectLimited}, nil
 	case "runtime.interfaceImplements":
-		return &sideEffectResult{severity: sideEffectNone}
+		return &sideEffectResult{severity: sideEffectNone}, nil
 	case "runtime.sliceCopy":
-		return &sideEffectResult{severity: sideEffectNone}
+		return &sideEffectResult{severity: sideEffectNone}, nil
 	case "runtime.trackPointer":
-		return &sideEffectResult{severity: sideEffectNone}
+		return &sideEffectResult{severity: sideEffectNone}, nil
 	case "llvm.dbg.value":
-		return &sideEffectResult{severity: sideEffectNone}
+		return &sideEffectResult{severity: sideEffectNone}, nil
 	}
 	if fn.IsDeclaration() {
-		return &sideEffectResult{severity: sideEffectLimited}
+		return &sideEffectResult{severity: sideEffectLimited}, nil
 	}
 	if e.sideEffectFuncs == nil {
 		e.sideEffectFuncs = make(map[llvm.Value]*sideEffectResult)
 	}
 	if se, ok := e.sideEffectFuncs[fn]; ok {
-		return se
+		return se, nil
 	}
 	result := &sideEffectResult{
 		severity:        sideEffectInProgress,
@@ -75,6 +75,7 @@ func (e *Eval) hasSideEffects(fn llvm.Value) *sideEffectResult {
 	for bb := fn.EntryBasicBlock(); !bb.IsNil(); bb = llvm.NextBasicBlock(bb) {
 		for inst := bb.FirstInstruction(); !inst.IsNil(); inst = llvm.NextInstruction(inst) {
 			if inst.IsAInstruction().IsNil() {
+				// Should not happen in valid IR.
 				panic("not an instruction")
 			}
 
@@ -91,7 +92,7 @@ func (e *Eval) hasSideEffects(fn llvm.Value) *sideEffectResult {
 			switch inst.InstructionOpcode() {
 			case llvm.IndirectBr, llvm.Invoke:
 				// Not emitted by the compiler.
-				panic("unknown instructions")
+				return nil, e.errorAt(inst, "unknown instructions")
 			case llvm.Call:
 				child := inst.CalledValue()
 				if !child.IsAInlineAsm().IsNil() {
@@ -117,7 +118,10 @@ func (e *Eval) hasSideEffects(fn llvm.Value) *sideEffectResult {
 					}
 					continue
 				}
-				childSideEffects := e.hasSideEffects(child)
+				childSideEffects, err := e.hasSideEffects(child)
+				if err != nil {
+					return nil, err
+				}
 				switch childSideEffects.severity {
 				case sideEffectInProgress, sideEffectNone:
 					// no side effects or recursive function - continue scanning
@@ -159,7 +163,7 @@ func (e *Eval) hasSideEffects(fn llvm.Value) *sideEffectResult {
 		// No side effect was reported for this function.
 		result.severity = sideEffectNone
 	}
-	return result
+	return result, nil
 }
 
 // hasLocalSideEffects checks whether the given instruction flows into a branch
@@ -174,6 +178,7 @@ func (e *Eval) hasLocalSideEffects(dirtyLocals map[llvm.Value]struct{}, inst llv
 	for use := inst.FirstUse(); !use.IsNil(); use = use.NextUse() {
 		user := use.User()
 		if user.IsAInstruction().IsNil() {
+			// Should not happen in valid IR.
 			panic("user not an instruction")
 		}
 		switch user.InstructionOpcode() {

interp/scan_test.go

@@ -59,7 +59,11 @@ func TestScan(t *testing.T) {
 			t.Errorf("scan test: could not find tested function %s in the IR", tc.name)
 			continue
 		}
-		result := e.hasSideEffects(fn)
+		evalPkg := &evalPackage{e, "testdata"}
+		result, err := evalPkg.hasSideEffects(fn)
+		if err != nil {
+			t.Errorf("scan test: failed to scan %s for side effects: %v", fn.Name(), err)
+		}
 
 		// Check whether the result is what we expect.
 		if result.severity != tc.severity {