[llvm-dev] Compiling for more than one target at once

Tim Northover via llvm-dev llvm-dev at lists.llvm.org
Mon Jun 6 16:46:23 PDT 2016

On 6 June 2016 at 15:06, Lorenzo Laneve via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Does the target-independency really exist or there are little things in the code that may change depending on the target? (I'm not talking about C macros).

Lots of little things can vary and make them incompatible. Off the top
of my head:
  + Clang knows the size and alignment of types and inserts it
directly in many places (sizeof, for example).
  + Clang emits custom (target-dependent) code for function argument
and return types so that it can follow the C ABI.
  + va_arg is similar on many targets.
  + The underlying type (int, long, ...) gets mangled into C++ names,
so variation in just what int32_t and so on is can break linking.
  + You say you're not talking about C macros, but there are obviously
countless ways they can completely mess up target independence without
even trying.

Obviously these apply mostly to C and C++, if you're designing a
language from scratch and have no need to follow any platform's
specific ABI it can be made to work (you essentially define the ABI by
whatever code gets generated).



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