compiler: calculate max number of entries in slice at compile time

This avoids difficult multiply-with-overflow code and avoids a multiply
at runtime.

compiler/compiler.go

@@ -1736,9 +1736,10 @@ func (c *Compiler) parseExpr(frame *Frame, expr ssa.Value) (llvm.Value, error) {
 		elemSize := c.targetData.TypeAllocSize(llvmElemType)
 		elemSizeValue := llvm.ConstInt(c.uintptrType, elemSize, false)
 
-		// Calculate ^uintptr(0)
-		maxSize := llvm.ConstNot(llvm.ConstInt(c.uintptrType, 0, false)).ZExtValue()
-		if elemSize > maxSize {
+		// Calculate (^uintptr(0)) >> 1, which is the max value that fits in
+		// uintptr if uintptr were signed.
+		maxSize := llvm.ConstLShr(llvm.ConstNot(llvm.ConstInt(c.uintptrType, 0, false)), llvm.ConstInt(c.uintptrType, 1, false))
+		if elemSize > maxSize.ZExtValue() {
 			// This seems to be checked by the typechecker already, but let's
 			// check it again just to be sure.
 			return llvm.Value{}, c.makeError(expr.Pos(), fmt.Sprintf("slice element type is too big (%v bytes)", elemSize))
@@ -1770,10 +1771,11 @@ func (c *Compiler) parseExpr(frame *Frame, expr ssa.Value) (llvm.Value, error) {
 					sliceCap = c.builder.CreateSExt(sliceCap, biggestInt, "")
 				}
 			}
-			// Note: the max element size needs to be doubled to make sure it
-			// fits in an int for, for example, len().
-			elemSizeDoubled := c.builder.CreateMul(elemSizeValue, llvm.ConstInt(c.uintptrType, 2, false), "")
-			c.createRuntimeCall(checkFunc, []llvm.Value{sliceLen, sliceCap, elemSizeDoubled}, "")
+			maxSliceSize := maxSize
+			if elemSize != 0 { // avoid divide by zero
+				maxSliceSize = llvm.ConstSDiv(maxSize, llvm.ConstInt(c.uintptrType, elemSize, false))
+			}
+			c.createRuntimeCall(checkFunc, []llvm.Value{sliceLen, sliceCap, maxSliceSize}, "")
 		}
 
 		// Allocate the backing array.

src/runtime/panic.go

@@ -57,21 +57,15 @@ func sliceBoundsCheck64(capacity uintptr, low, high uint64) {
 }
 
 // Check for bounds in *ssa.MakeSlice.
-func sliceBoundsCheckMake(length, capacity uintptr, elementSizeDoubled uintptr) {
-	overflow := uint64(capacity*elementSizeDoubled) != uint64(capacity)*uint64(elementSizeDoubled)
-	if length > capacity || overflow {
+func sliceBoundsCheckMake(length, capacity uintptr, max uintptr) {
+	if length > capacity || capacity > max {
 		runtimePanic("slice size out of range")
 	}
 }
 
 // Check for bounds in *ssa.MakeSlice. Supports 64-bit indexes.
-func sliceBoundsCheckMake64(length, capacity uint64, elementSizeDoubled uintptr) {
-	// This function is only ever called on systems where uintptr is smaller
-	// than uint64 (thus must be 32-bit or less). So multiplying as uint64 will
-	// never overflow if we know that capacity fits in uintptr.
-	// That elementSizeDoubled fits in uintptr is checked by the compiler.
-	overflow := capacity != uint64(uintptr(capacity)) || capacity != uint64(uintptr(capacity*uint64(elementSizeDoubled)))
-	if length > capacity || overflow {
+func sliceBoundsCheckMake64(length, capacity uint64, max uintptr) {
+	if length > capacity || capacity > uint64(max) {
 		runtimePanic("slice size out of range")
 	}
 }

testdata/slice.go

@@ -13,17 +13,17 @@ func main() {
 	println("sum foo:", sum(foo))
 
 	// creating a slice with uncommon len, cap types
-	assert(len(make([]int, int(2), int(3))) == 2)
-	assert(len(make([]int, int8(2), int8(3))) == 2)
-	assert(len(make([]int, int16(2), int16(3))) == 2)
-	assert(len(make([]int, int32(2), int32(3))) == 2)
-	assert(len(make([]int, int64(2), int64(3))) == 2)
-	assert(len(make([]int, uint(2), uint(3))) == 2)
-	assert(len(make([]int, uint8(2), uint8(3))) == 2)
-	assert(len(make([]int, uint16(2), uint16(3))) == 2)
-	assert(len(make([]int, uint32(2), uint32(3))) == 2)
-	assert(len(make([]int, uint64(2), uint64(3))) == 2)
-	assert(len(make([]int, uintptr(2), uintptr(3))) == 2)
+	assert(len(make([]int, makeInt(2), makeInt(3))) == 2)
+	assert(len(make([]int, makeInt8(2), makeInt8(3))) == 2)
+	assert(len(make([]int, makeInt16(2), makeInt16(3))) == 2)
+	assert(len(make([]int, makeInt32(2), makeInt32(3))) == 2)
+	assert(len(make([]int, makeInt64(2), makeInt64(3))) == 2)
+	assert(len(make([]int, makeUint(2), makeUint(3))) == 2)
+	assert(len(make([]int, makeUint8(2), makeUint8(3))) == 2)
+	assert(len(make([]int, makeUint16(2), makeUint16(3))) == 2)
+	assert(len(make([]int, makeUint32(2), makeUint32(3))) == 2)
+	assert(len(make([]int, makeUint64(2), makeUint64(3))) == 2)
+	assert(len(make([]int, makeUintptr(2), makeUintptr(3))) == 2)
 
 	// indexing into a slice with uncommon index types
 	assert(foo[int(2)] == 4)
@@ -120,3 +120,18 @@ func assert(ok bool) {
 		panic("assert failed")
 	}
 }
+
+// Helper functions used to hide const values from the compiler during IR
+// construction.
+
+func makeInt(x int) int             { return x }
+func makeInt8(x int8) int8          { return x }
+func makeInt16(x int16) int16       { return x }
+func makeInt32(x int32) int32       { return x }
+func makeInt64(x int64) int64       { return x }
+func makeUint(x uint) uint          { return x }
+func makeUint8(x uint8) uint8       { return x }
+func makeUint16(x uint16) uint16    { return x }
+func makeUint32(x uint32) uint32    { return x }
+func makeUint64(x uint64) uint64    { return x }
+func makeUintptr(x uintptr) uintptr { return x }