[LLVMdev] Runtime Array-Length

[Jeremy Lakeman]

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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