interp: bail out of loops that iterate too many times (#5395)

* interp: bail out of loops that iterate too many times

The existing loop guard (errLoopUnrolled) only fires when a loop body
emits runtime instructions. Loops that are fully evaluable at compile
time, such as inserting thousands of entries into a map, were not
caught and could hang the compiler.

Add a per-basic-block iteration counter that triggers a recoverable
error (errLoopTooLong) when any block is entered more than 1000 times
in a single function call. This defers the init function to runtime,
which is the same behavior as other interp bailouts.

Profiling showed that 83% of CPU time was spent in GC, caused by
allocation pressure from the interp memory cloning on each map
mutation. The iteration limit avoids this entirely by bailing out
before the quadratic cost becomes significant.

Performance on the reproducer from #2090 (map init with strconv.Itoa):

    entries  before       after
    5,000    7.4s         2.1s
    10,000   17.5s        2.1s
    20,000   48.0s        2.8s
    65,536   >180s (OOM)  3.2s

* Add -interp-loop-limit

interp/interp.go

@@ -19,35 +19,37 @@ const checks = true
 
 // runner contains all state related to one interp run.
 type runner struct {
-	mod           llvm.Module
-	targetData    llvm.TargetData
-	builder       llvm.Builder
-	pointerSize   uint32                   // cached pointer size from the TargetData
-	dataPtrType   llvm.Type                // often used type so created in advance
-	uintptrType   llvm.Type                // equivalent to uintptr in Go
-	maxAlign      int                      // maximum alignment of an object, alignment of runtime.alloc() result
-	byteOrder     binary.ByteOrder         // big-endian or little-endian
-	debug         bool                     // log debug messages
-	pkgName       string                   // package name of the currently executing package
-	functionCache map[llvm.Value]*function // cache of compiled functions
-	objects       []object                 // slice of objects in memory
-	globals       map[llvm.Value]int       // map from global to index in objects slice
-	start         time.Time
-	timeout       time.Duration
-	callsExecuted uint64
+	mod               llvm.Module
+	targetData        llvm.TargetData
+	builder           llvm.Builder
+	pointerSize       uint32                   // cached pointer size from the TargetData
+	dataPtrType       llvm.Type                // often used type so created in advance
+	uintptrType       llvm.Type                // equivalent to uintptr in Go
+	maxAlign          int                      // maximum alignment of an object, alignment of runtime.alloc() result
+	byteOrder         binary.ByteOrder         // big-endian or little-endian
+	debug             bool                     // log debug messages
+	pkgName           string                   // package name of the currently executing package
+	functionCache     map[llvm.Value]*function // cache of compiled functions
+	objects           []object                 // slice of objects in memory
+	globals           map[llvm.Value]int       // map from global to index in objects slice
+	start             time.Time
+	timeout           time.Duration
+	maxLoopIterations int
+	callsExecuted     uint64
 }
 
-func newRunner(mod llvm.Module, timeout time.Duration, debug bool) *runner {
+func newRunner(mod llvm.Module, timeout time.Duration, maxLoopIterations int, debug bool) *runner {
 	r := runner{
-		mod:           mod,
-		targetData:    llvm.NewTargetData(mod.DataLayout()),
-		byteOrder:     llvmutil.ByteOrder(mod.Target()),
-		debug:         debug,
-		functionCache: make(map[llvm.Value]*function),
-		objects:       []object{{}},
-		globals:       make(map[llvm.Value]int),
-		start:         time.Now(),
-		timeout:       timeout,
+		mod:               mod,
+		targetData:        llvm.NewTargetData(mod.DataLayout()),
+		byteOrder:         llvmutil.ByteOrder(mod.Target()),
+		debug:             debug,
+		functionCache:     make(map[llvm.Value]*function),
+		objects:           []object{{}},
+		globals:           make(map[llvm.Value]int),
+		start:             time.Now(),
+		timeout:           timeout,
+		maxLoopIterations: maxLoopIterations,
 	}
 	r.pointerSize = uint32(r.targetData.PointerSize())
 	r.dataPtrType = llvm.PointerType(mod.Context().Int8Type(), 0)
@@ -64,8 +66,8 @@ func (r *runner) dispose() {
 
 // Run evaluates runtime.initAll function as much as possible at compile time.
 // Set debug to true if it should print output while running.
-func Run(mod llvm.Module, timeout time.Duration, debug bool) error {
-	r := newRunner(mod, timeout, debug)
+func Run(mod llvm.Module, timeout time.Duration, maxLoopIterations int, debug bool) error {
+	r := newRunner(mod, timeout, maxLoopIterations, debug)
 	defer r.dispose()
 
 	initAll := mod.NamedFunction("runtime.initAll")
@@ -204,10 +206,10 @@ func Run(mod llvm.Module, timeout time.Duration, debug bool) error {
 
 // RunFunc evaluates a single package initializer at compile time.
 // Set debug to true if it should print output while running.
-func RunFunc(fn llvm.Value, timeout time.Duration, debug bool) error {
+func RunFunc(fn llvm.Value, timeout time.Duration, maxLoopIterations int, debug bool) error {
 	// Create and initialize *runner object.
 	mod := fn.GlobalParent()
-	r := newRunner(mod, timeout, debug)
+	r := newRunner(mod, timeout, maxLoopIterations, debug)
 	defer r.dispose()
 	initName := fn.Name()
 	if !strings.HasSuffix(initName, ".init") {