C code nesting: -flto or not -flto

One of my recent programs is very dependent on how to make some hot features for performance. These hot functions are part of an external file .c

that I would rather not modify.

Unfortunately, while Visual is pretty good at this exercise, gcc and clang are not. Apparently due to the hot functions being within the other .c

, they cannot inline them.

This leaves me with 2 options:

• Alternatively, include the relevant code directly in the target file. In practice, this means #include "perf.c"
  
  instead of #include "perf.h"
  
  . A trivial change, but it looks ugly. It is clear that this works. It's a little trickier to explain the build chain what perf.c
  
  should be there but not compile or link.

• Use -flto
  
  to optimize communication times. It looks cleaner and by default Visual reaches.

  The problem is that during -flto
  
  the link step, gcc generates several warnings that seem to be internal errors (they refer to a piece of code from the standard libraries, so I have little control over them). It's awkward when you target a "zero warning" policy (even though the generated binary works fine).

  As far as clang is concerned, it just fails with -flto
  
  a packaging error (error loading plugin: LLVMgold.so), which seems to be very common on several linux distributions.

2 questions:

• Is there a way to disable these warning messages when used -flto
  
  on gcc?
• Which of the two methods above seems to be the best given pro and con?
• Optional: is there another solution?