[LLVMdev] LLVM and Interrupt Service Routines.

[Alireza.Moshtaghi at microchip.com]

> If you have to generate two copies of the function with different
> entrypoints, the *front-end* should handle the duplication.  This is
> just like C++ constructors.
> 
> One really old patch that apple guys experimented in the past was a
> "slow and fast call" attribute, which you could stick on function
> declarations.  If you added it to a function, the frontend would
> generate an entry point with a standard calling convention as well as
> one with a faster in-register ABI.  Direct calls would use the fast
> entry point, but if you took the address, you'd get the address of the
> normal one.
> 
> All of this was handled by the front-end, and works fine.  I think the
> patch eventually got ripped out of the compiler for other reasons
> though.


I see 2 problems with this approach:
1 - Front end does not know which functions need to be cloned. The
cloned functions are the ones that are called both from ISR thread and
main thread; and this information is not available until all compilation
units are merged. We collect this information in a pass at the end of
llvm-ld.
2- Based on what you propose, for example the ISR thread would need to
make indirect call to cloned function; and main thread make direct call
to original function. Apart from the performance issue on PIC16 in
relation to indirect function calls, if the main thread already makes
indirect call to the function, then the same function would be called
from both threads, and the original problem is still there

A.

>