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

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

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")
 	}
 }