[cfe-dev] Zero length function pointer equality

[David Blaikie via cfe-dev]

LLVM can produce zero length functions from cases like this (when
optimizations are enabled):

void f1() { __builtin_unreachable(); }
int f2() { /* missing return statement */ }

This code is valid, so long as the functions are never called.

I believe C++ requires that all functions have a distinct address (ie:
&f1 != &f2) and LLVM optimizes code on this basis (assert(f1 == f2)
gets optimized into an unconditional assertion failure)

But these zero length functions can end up with identical addresses.

I'm unaware of anything in the C++ spec (or the LLVM langref) that
would indicate that would allow distinct functions to have identical
addresses - so should we do something about this in the LLVM backend?
add a little padding? a nop instruction? (if we're adding an
instruction anyway, perhaps we might as well make it an int3?)

(I came across this due to DWARF issues with zero length functions &
thinking about if/how this should be supported)