[llvm-dev] Most efficient way for a function to return two types?

David Blaikie via llvm-dev llvm-dev at lists.llvm.org
Thu Oct 17 12:49:22 PDT 2019

I'd probably do it the same way clang does for

struct str { char* ptr; int64_t length; };
str itoa(...);

But realize that LLVM IR isn't ABI neutral - Clang may generate
significantly different IR when targeting different architectures to match
their ABI. Even if you don't need to match these ABIs (you don't need your
code to be able to call into/be called from C or C++ source code compiled
with Clang or GCC, etc), just getting the machine code you desire (what
parts of a struct get passed in which registers (or memory), etc) may
require various per-architecture handling on your part/in how you generate
the IR.

- Dave

On Thu, Oct 17, 2019 at 10:51 AM Levo DeLellis via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I apologize if this is the wrong list. I'm writing a language and
> outputting llvm-ir instead of using the c api. I looked at clang's output a
> few times but I'm a little unsure what to do in this case
> As an example I have a function called itoa which takes an int and returns
> a string. Strings in my language has 2 fields, the pointer and byte length.
> On a 64bit machine it'd be returning a 64bit pointer and 64bit int value.
> My question is HOW should I do this if I want to generate fast code? And a
> follow up is should I do it that way if I want to support most
> architectures?
> Currently my function signature is `define {i8*, i64} @itoa(i64) {`.
> Should I return an i8* instead and have an int pointer? (i64*). I don't see
> an 'out' parameter attributes or anything to hint to the optimizer the
> value at the address can be uninitialized.
> https://llvm.org/docs/LangRef.html#parameter-attributes. If I pass in a
> string should I do it the same way as returning? Currently I do. I pass in
> {i8*, i64} as a parameter and been thinking I can pass in i8* and i64
> instead (note that i64 is not a pointer this time. It may be if the
> variable is mutable).
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