Link-time optimization (lto) for mixed C++/C programs

Question

I wonder if it would be possible to effectively use link-time optimization (lto) for programms consisting of a TU with C++ code calling some function in a TU with C code. So, is there a way to get the C function inlined into the resulting program?

The real use case is a microcontroller firmware written in C++. But there are pieces of code that have to written in C because of C semantics (e.g. access to the non-active union member).

(This has nothing to do with manual marking functions as inline)

Possible duplicate of [Link-time optimization and inline](https://stackoverflow.com/questions/7046547/link-time-optimization-and-inline) — Henri Menke, Nov 14 '17 at 07:07
I wonder what modes of union access are well-defined in C but impossible or undefined in C++. — n. m. could be an AI, Nov 14 '17 at 07:18
Type-punning via accessing the non-active member ist explicitly supported in C but UB in C++. — wimalopaan, Nov 14 '17 at 07:23
Nevermind, i think I know what you mean, not the common initial sequence. — n. m. could be an AI, Nov 14 '17 at 07:39

score 4 · Accepted Answer · answered Nov 14 '17 at 07:21

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This should not be a problem at all. In both GCC and Clang link-time optimization operates on intermediate representation of the code. That is, by using -flto you create object files with additional LTO information (gcc) or LLVM bytecode (Clang), at which point the source language stops to matter.

Some go as far as to mix even less related C++ and D yet still use LTO: http://johanengelen.github.io/ldc/2016/11/10/Link-Time-Optimization-LDC.html

answered Nov 14 '17 at 07:21

lomereiter

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Just verified that using an avr backend. The only thing one is missing here ist constexpr. – wimalopaan Nov 14 '17 at 07:24

Link-time optimization (lto) for mixed C++/C programs

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