Wrong code gen / missing warning / my mistake?

[Steven Schveighoffer]

On Mon, 10 Sep 2012 13:08:29 -0400, Benjamin Thaut  
<code at benjamin-thaut.de> wrote:

> The following code
>
>
> bool endsWith(string str, string end)
> {
> 	size_t to = str.length - end.length;
> 	for(sizediff_t i = str.length - 1; i >= to; --i)
> 	{
> 		if(str[i] != end[i-to])
> 			return false;
> 	}
> 	return true;
> }
>
> int main(string[] args)
> {
> 	return cast(int)endsWith("blub", "blub");
> }
>
> compiled with dmd 2.060 gives me a range violation. (with i = -1)  
> although it shouldn't. If I change to from size_t "to" sizediff_t  
> everything is fine. The comparison between the unsigned "to" and the  
> signed "i" is not done correctly.
>
> Is this a code gen bug? Or is it missing a compiler warning / error? Or  
> is this entierly my fault?

It is behaving as expected.

through integer promotion rules, i >= to is converted to unsigned  
comparison.  So if i becomes -1, then it really becomes a comparison  
between 2^32 - 1 and 0.

In other words, if to is 0, then the loop will never terminate (because  
unsigned can never be less than 0).

The solution is to re-design your loop.  I try to avoid conditions that  
could be done with negative values for the above reason.

sizediff_t offset = str.length - end.length;
for(sizediff_t i = 0; i < end.length; ++i)
{
    if(str[offset + i] != end[i])
        return false;
}
return true;

Or if you want to skip all this, you can just use the wonderful D slice  
syntax :)

if(str.length > end.length)
    return str[$-end.length..$] == end;
else
    return false;

-Steve