Optimize/eliminate bounds checking

TODO: do better at it by tracking min/max values of integers. The
following straightforward code doesn't have its bounds checks removed:

    for _, n := range slice {
        println(n)
    }

compiler.go

@@ -1610,6 +1610,26 @@ func (c *Compiler) parseCall(frame *Frame, instr *ssa.CallCommon, parentHandle l
 	}
 }
 
+func (c *Compiler) emitBoundsCheck(frame *Frame, arrayLen, index llvm.Value) {
+	if frame.fn.nobounds {
+		// The //go:nobounds pragma was added to the function to avoid bounds
+		// checking.
+		return
+	}
+	// Optimize away trivial cases.
+	// LLVM would do this anyway with interprocedural optimizations, but it
+	// helps to see cases where bounds checking would really help.
+	if index.IsConstant() && arrayLen.IsConstant() {
+		index := index.SExtValue()
+		arrayLen := arrayLen.SExtValue()
+		if index >= 0 && index < arrayLen {
+			return
+		}
+	}
+	lookupBoundsCheck := c.mod.NamedFunction("runtime.lookupBoundsCheck")
+	c.builder.CreateCall(lookupBoundsCheck, []llvm.Value{arrayLen, index}, "")
+}
+
 func (c *Compiler) parseExpr(frame *Frame, expr ssa.Value) (llvm.Value, error) {
 	if value, ok := frame.locals[expr]; ok {
 		// Value is a local variable that has already been computed.
@@ -1730,8 +1750,7 @@ func (c *Compiler) parseExpr(frame *Frame, expr ssa.Value) (llvm.Value, error) {
 		// Check bounds.
 		arrayLen := expr.X.Type().(*types.Array).Len()
 		arrayLenLLVM := llvm.ConstInt(llvm.Int32Type(), uint64(arrayLen), false)
-		lookupBoundsCheck := c.mod.NamedFunction("runtime.lookupBoundsCheck")
-		c.builder.CreateCall(lookupBoundsCheck, []llvm.Value{arrayLenLLVM, index}, "")
+		c.emitBoundsCheck(frame, arrayLenLLVM, index)
 
 		// Can't load directly from array (as index is non-constant), so have to
 		// do it using an alloca+gep+load.
@@ -1771,12 +1790,7 @@ func (c *Compiler) parseExpr(frame *Frame, expr ssa.Value) (llvm.Value, error) {
 
 		// Bounds check.
 		// LLVM optimizes this away in most cases.
-		// TODO: runtime.lookupBoundsCheck is undefined in packages imported by
-		// package runtime, so we have to remove it. This should be fixed.
-		lookupBoundsCheck := c.mod.NamedFunction("runtime.lookupBoundsCheck")
-		if !lookupBoundsCheck.IsNil() && frame.fn.llvmFn.Name() != "runtime.interfaceMethod" {
-			c.builder.CreateCall(lookupBoundsCheck, []llvm.Value{buflen, index}, "")
-		}
+		c.emitBoundsCheck(frame, buflen, index)
 
 		switch expr.X.Type().(type) {
 		case *types.Pointer:
@@ -1811,13 +1825,11 @@ func (c *Compiler) parseExpr(frame *Frame, expr ssa.Value) (llvm.Value, error) {
 
 			// Bounds check.
 			// LLVM optimizes this away in most cases.
-			if frame.fn.llvmFn.Name() != "runtime.lookupBoundsCheck" {
 			length, err := c.parseBuiltin(frame, []ssa.Value{expr.X}, "len")
 			if err != nil {
 				return llvm.Value{}, err // shouldn't happen
 			}
-				c.builder.CreateCall(c.mod.NamedFunction("runtime.lookupBoundsCheck"), []llvm.Value{length, index}, "")
-			}
+			c.emitBoundsCheck(frame, length, index)
 
 			// Lookup byte
 			buf := c.builder.CreateExtractValue(value, 1, "")
@@ -1932,8 +1944,10 @@ func (c *Compiler) parseExpr(frame *Frame, expr ssa.Value) (llvm.Value, error) {
 			}
 
 			// This check is optimized away in most cases.
+			if !frame.fn.nobounds {
 				sliceBoundsCheck := c.mod.NamedFunction("runtime.sliceBoundsCheck")
 				c.builder.CreateCall(sliceBoundsCheck, []llvm.Value{llvmLen, low, high}, "")
+			}
 
 			slice := llvm.ConstNamedStruct(sliceTyp, []llvm.Value{
 				llvm.Undef(slicePtr.Type()),

ir.go

@@ -35,8 +35,9 @@ type Program struct {
 type Function struct {
 	fn           *ssa.Function
 	llvmFn       llvm.Value
-	linkName     string
-	blocking     bool
+	linkName     string      // go:linkname pragma
+	nobounds     bool        // go:nobounds pragma
+	blocking     bool        // calculated by AnalyseBlockingRecursive
 	flag         bool        // used by dead code elimination
 	addressTaken bool        // used as function pointer, calculated by AnalyseFunctionPointers
 	parents      []*Function // calculated by AnalyseCallgraph
@@ -169,6 +170,14 @@ func (f *Function) parsePragmas() {
 				if hasUnsafeImport(f.fn.Pkg.Pkg) {
 					f.linkName = parts[2]
 				}
+			case "//go:nobounds":
+				// Skip bounds checking in this function. Useful for some
+				// runtime functions.
+				// This is somewhat dangerous and thus only imported in packages
+				// that import unsafe.
+				if hasUnsafeImport(f.fn.Pkg.Pkg) {
+					f.nobounds = true
+				}
 			}
 		}
 	}

src/runtime/hashmap.go

@@ -66,6 +66,7 @@ func hashmapMake(keySize, valueSize uint8) *hashmap {
 }
 
 // Set a specified key to a given value. Grow the map if necessary.
+//go:nobounds
 func hashmapSet(m *hashmap, key unsafe.Pointer, value unsafe.Pointer, hash uint32, keyEqual func(x, y unsafe.Pointer, n uintptr) bool) {
 	numBuckets := uintptr(1) << m.bucketBits
 	bucketNumber := (uintptr(hash) & (numBuckets - 1))
@@ -114,6 +115,7 @@ func hashmapSet(m *hashmap, key unsafe.Pointer, value unsafe.Pointer, hash uint3
 }
 
 // Get the value of a specified key, or zero the value if not found.
+//go:nobounds
 func hashmapGet(m *hashmap, key unsafe.Pointer, value unsafe.Pointer, hash uint32, keyEqual func(x, y unsafe.Pointer, n uintptr) bool) {
 	numBuckets := uintptr(1) << m.bucketBits
 	bucketNumber := (uintptr(hash) & (numBuckets - 1))

src/runtime/interface.go

@@ -44,6 +44,7 @@ var (
 
 // Get the function pointer for the method on the interface.
 // This is a compiler intrinsic.
+//go:nobounds
 func interfaceMethod(itf _interface, method uint16) *uint8 {
 	// This function doesn't do bounds checking as the supplied method must be
 	// in the list of signatures. The compiler will only emit

src/runtime/print.go

@@ -4,6 +4,7 @@ import (
 	"unsafe"
 )
 
+//go:nobounds
 func printstring(s string) {
 	for i := 0; i < len(s); i++ {
 		putchar(s[i])
@@ -42,6 +43,7 @@ func printint16(n uint16) {
 	printint32(int32(n))
 }
 
+//go:nobounds
 func printuint32(n uint32) {
 	digits := [10]byte{} // enough to hold (2^32)-1
 	// Fill in all 10 digits.

src/runtime/runtime.go

@@ -56,7 +56,7 @@ func _panic(message interface{}) {
 	abort()
 }
 
-// Check for bounds in *ssa.IndexAddr and *ssa.Lookup.
+// Check for bounds in *ssa.Index, *ssa.IndexAddr and *ssa.Lookup.
 func lookupBoundsCheck(length, index int) {
 	if index < 0 || index >= length {
 		// printstring() here is safe as this function is excluded from bounds

src/runtime/string.go

@@ -13,6 +13,7 @@ type _string struct {
 }
 
 // Return true iff the strings match.
+//go:nobounds
 func stringEqual(x, y string) bool {
 	if len(x) != len(y) {
 		return false