Re: [edk2-devel] [PATCH] BaseTools/tools_def: Disable unneeded-internal-declaration warning in CLANGPDB

["Ard Biesheuvel" <ardb@kernel.org>]

On Tue, 12 Dec 2023 at 08:17, Mike Beaton <mjsbeaton@gmail.com> wrote:
>
> > > A completely different approach, which allows clang to spot that the
> > > usage has been 'optimised away' and so to not complain (and therefore
> > > allows us to re-enable the warning in CLANGDWARF as well), is the
> > > following:
> > >
> > > --- a/MdePkg/Include/Library/DebugLib.h
> > > +++ b/MdePkg/Include/Library/DebugLib.h
> > > @@ -426,7 +426,10 @@ UnitTestDebugAssert (
> > >        }                            \
> > >      } while (FALSE)
> > >  #else
> > > -#define DEBUG(Expression)
> > > +#define DEBUG(Expression)        \
> > > +    if (FALSE) {                   \
> > > +      _DEBUG (Expression);         \
> > > +    }
> > >  #endif
> > >
> > >  /**
> > >
> >
> > But will this not litter the object files with a bunch of format strings
> > etc?
>
> Yes. Which would be optimized away at link time. (Or rather, I believe
> so, would need further tests to confirm exactly what is optimized away
> when.)
>

Link time optimization does not usually optimize away assets at this
granularity. Even if --gc-sections is set, the only thing it can
optimize away is an entire input section.

However, the compiler should be smart enough not to emit any
references to format strings etc in the first place, as it knows the
condition can never become true (but NOOPT builds should retain them).

> > It feels like, for CLANGPDB (and maybe CLANGDWARF), the RELEASE target
> > should not define MDEPKG_NDEBUG, but make sure (instead) that
> > DebugPrintEnabled() evals to FALSE. If PcdDebugPropertyMask is
> > fixed-at-build, then DebugPrintEnabled() should be possible to evaluate
> > at compile time; is that right? (At least for clang?)
>
> Yes, that is my understanding of how compile-time Pcds work too - but
> wouldn't the net result be the same as what I suggested?

It depends on the kind of access. For PCDs that are fixed-at-build
only, the FixedPcdGet###() accessors will evaluate to constant
preprocessor expressions, allowing the compiler to do optimizations.
The ordinary PcdGet###() routines will not produce compile time
constant expressions in the same way, so there, all the logic is
retained (again, modulo LTO)


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