ctfe bug?

[Johannes Pfau]

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?
> 

Don't know, but I remember some benchmarks showed that arrays were slower, 
even with bounds-checking off. (I think that was brought up in some 
discussion about the tango xml parser).

Also the default for ragel is to use pointers, so I'd like to use that. 
Making it use arrays means extra work ;-)

And turning off bounds-checking is not a perfect solution, as it applies to 
the complete module. As I said, ragel makes sure that the pointer access is 
safe, so there's really no issue in using pointers.