[LLVMdev] Runtime Array-Length

[William Moses]

However, how would one allocate the buffer for a string if you did not know
the length of the string at compile time?

For instance, using the api how would one reproduce the code for the
following c++ function?

std::string add(std::string a, std::string b){
  return a+b;
}

When allocating the buffer required for the new string, one can determine
the length at runtime, however I do not know how one can allocate a global
array with its size determined by a Value*.


On Mon, Oct 14, 2013 at 5:46 AM, Jeremy Lakeman <Jeremy.Lakeman at gmail.com>wrote:

> The toy language I've been playing around with represents all strings as a
> struct in llvm;
>
> struct string{
>   char *ptr;
>   int str_len;
>   int buffer_len;
> }
>
> And my AST has an interface like;
>
> String_AST{
>   int measure();
>   void copy(char *dest);
>   struct string get_value();
> }
>
> A constant string can be measured at compile time, for a string variable
> measure() just extracts str_len. Strings passed in from other external
> sources are measured immediately, but llvm optimisations will eliminate the
> call if the return value isn't used.
>
> The implementation of get_value() for a concatenation AST node can
> generate code to evaluate each sub string, measure them, allocate the final
> buffer length, and only then copy each sub string directly into the final
> buffer.
>
> I also support a string append operation that will reallocate the buffer
> only if the existing one is too small.
>
> Ultimately you will need to work out if you want pascal / java style
> strings like mine, or C style NULL terminated strings. And how the memory
> for these strings will be managed.
>
>
> On Sun, Oct 13, 2013 at 4:43 AM, William Moses <
> moses.williamsteven at gmail.com> wrote:
>
>> All,
>>
>> I am building my own language with llvm as the base.
>>
>> I was working on string concatenation (where a string is just an array of
>> characters cast to a pointer to a character (i8*) ). Given two strings, it
>> is possible to determine the length of new string by summing the number of
>> characters until the null terminator and adding one.
>>
>> Unfortunately, I have no idea how to use the c-api to store this. As the
>> length of the new string is not a compile-time constant (e.g. stored in a
>> Value*), I cannot determine at compile-time what length the llvm array-type
>> will be? Therefore, I cannot create the GlobalVariable since I do not know
>> the type.
>>
>> One possible solution I thought of was linking to the malloc function and
>> calling that, but I'm sure there's a better way. If any of you have
>> implemented a similar sort of string concatenation, I would much appreciate
>> any advice that you could give.
>>
>> Thanks,
>> Billy
>>
>> _______________________________________________
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>>
>>
>
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