[cfe-dev] Zero length function pointer equality

[Richard Smith via cfe-dev]

On Thu, 23 Jul 2020 at 17:46, David Blaikie <dblaikie at gmail.com> wrote:

> 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)
>

Yes, I think at least if the optimizer turns a non-empty function into an
empty function, that's a miscompile for C and C++ source-language programs.
My (possibly flawed) understanding is that LLVM is obliged to give a
different address to distinct globals if neither of them is marked
unnamed_addr, so it seems to me that this is a backend bug. Generating a
ud2 function body in this case seems ideal to me.
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