[cfe-dev] Zero length function pointer equality

[David Blaikie via cfe-dev]

Looks perfect to me!

well, a couple of questions: Why a noop, rather than int3/ud2/etc?
Might be worth using the existing code that places such an instruction
when building at -O0?
& you mention that this causes problems on Windows - but ICF done by
the Windows linker does not cause such problems? (I'd have thought
they'd result in the same situation - two functions described as being
at the same address?) is there a quick summary of why those two cases
turn out differently?

On Fri, Jul 24, 2020 at 6:17 AM Hans Wennborg <hans at chromium.org> wrote:
>
> Maybe we can just expand this to always apply: https://reviews.llvm.org/D32330
>
> On Fri, Jul 24, 2020 at 2:46 AM David Blaikie via cfe-dev
> <cfe-dev at lists.llvm.org> 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)
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