ctfe bug?

[Johannes Pfau]

Jacob Carlborg wrote:

> On 2011-12-22 14:39, Timon Gehr wrote:
>> On 12/22/2011 10:28 AM, Jacob Carlborg wrote:
>>> On 2011-12-22 08:47, Johannes Pfau wrote:
>>>> Hi,
>>>> the following code is reduced from a parser generated with Ragel
>>>> (http://www.complang.org/ragel/). That's also the reason why it's
>>>> using pointers instead of array access, but Ragel guarantees that there
>>>> won't be any out-of-bound reads.
>>>>
>>>> AFAIK pointers are supported in CTFE now as long as they're pointing
>>>> to an
>>>> array and there are no out-of-bounds reads. Still, the following code
>>>> fails:
>>>>
>>>> --------------------
>>>> ubyte[4] testCTFE()
>>>> {
>>>> ubyte[4] data;
>>>> string input = "8ab3060e2cba4f23b74cb52db3bdfb46";
>>>> auto p = input.ptr;
>>>> p++; p++;
>>>> data[0] = parse!ubyte((p-2)[0 .. 2], 16);
>>>> p++; p++;
>>>> data[1] = parse!ubyte((p-2)[0 .. 2], 16);
>>>> p++; p++;
>>>> data[2] = parse!ubyte((p-2)[0 .. 2], 16);
>>>> p++; p++;
>>>> data[3] = parse!ubyte((p-2)[0 .. 2], 16);
>>>> p++; p++;
>>>> return data;
>>>> }
>>>> enum ctfe = testCTFE();
>>>>
>>>> void main()
>>>> {
>>>> import std.stdio;
>>>> writeln(testCTFE()); //[138, 179, 6, 14]
>>>> writeln(ctfe); //[138, 138, 138, 138]
>>>> }
>>>> --------------------
>>>>
>>>> Has this bug already been filed? I could possibly circumvent it by
>>>> making
>>>> ragel use array indexing instead of pointers, but that'd be a
>>>> performance
>>>> issue for runtime code as well.
>>>
>>> Why would arrays be slower than pointers? You do know that you can turn
>>> off array bounds checking?
>>>
>>
>> Yes but the length has to be stored and updated, therefore for example
>> p++ is less machine instructions/memory accesses/register pressure than
>> arr = arr[1..$].
> 
> Ok, I see. Then this seems to be a very performance critical piece of
> code.
> 
In this special case it's not that important (simple UUID parser), but ragel 
is also used for HTTP parsers in webservers (lighttpd2), json parsers, etc 
and it's main advantage is speed.