dblaikie added inline comments.
================
Comment at: clang/lib/Sema/SemaDecl.cpp:14254
int SectionFlags = ASTContext::PSF_Read;
- if (var->getType().isConstQualified()) {
- if (HasConstInit)
----------------
rnk wrote:
> efriedma wrote:
> > dblaikie wrote:
> > > efriedma wrote:
> > > > dblaikie wrote:
> > > > > rnk wrote:
> > > > > > rsmith wrote:
> > > > > > > efriedma wrote:
> > > > > > > > rnk wrote:
> > > > > > > > > I think this is not compatible with MSVC. MSVC uses simple
> > > > > > > > > logic, it doesn't look for mutable:
> > > > > > > > > https://gcc.godbolt.org/z/sj6d4saxx
> > > > > > > > >
> > > > > > > > > The const mutable struct appears in the myrdata section in
> > > > > > > > > that example.
> > > > > > > > >
> > > > > > > > > I think the solution is to separate the flag logic from the
> > > > > > > > > pragma stack selection logic, which has to remain
> > > > > > > > > MSVC-compatible.
> > > > > > > > MSVC apparently looks at whether the variable is marked
> > > > > > > > "const", and nothing else; it doesn't look at mutable, it
> > > > > > > > doesn't look at whether the variable has a constant
> > > > > > > > initializer. So the current code isn't right either; if we're
> > > > > > > > trying to implement MSVC-compatible logic, we shouldn't check
> > > > > > > > HasConstInit.
> > > > > > > >
> > > > > > > > That said, I'm not sure how precisely/in what modes we want to
> > > > > > > > precisely emulate MSVC. Probably anything we do here will be
> > > > > > > > confusing.
> > > > > > > We should at least issue a warning if the sensible logic and the
> > > > > > > MSVC-compatible calculation differ. @rnk, do you know how
> > > > > > > important it is to follow the MSVC semantics in this regard?
> > > > > > I think it depends on whether you think that users are primarily
> > > > > > using these pragmas to override the default rdata/bss/data sections
> > > > > > without any care about precisely what goes where, or if they are
> > > > > > using them to do something finer grained.
> > > > > >
> > > > > > If I had to guess, I'd say it's more likely the former, given that
> > > > > > `__declspec(allocate)` and `#pragma(section)` exist to handle cases
> > > > > > where users are putting specific globals into specific sections.
> > > > > >
> > > > > > Which, if we follow Richard's suggestion, would mean warning when
> > > > > > we put a global marked `const` into a writable section when
> > > > > > `ConstSegStack` is non-empty. That seems reasonable.
> > > > > > `-Wmicrosoft-const-seg` for the new warning group?
> > > > > Does the MSVC situation only apply to custom sections? (presumably
> > > > > when not customizing the section, MSVC gets it right and can support
> > > > > a const global with a runtime initializer, mutable member, or
> > > > > mutating dtor?)
> > > > >
> > > > > I think this code still needs to be modified, since this is the code
> > > > > that drives the error about incompatible sections. So it'll need to
> > > > > behave differently depending on the target platform?
> > > > Yes, the MSVC situation is specifically if you specify `#pragma
> > > > const_seg`; without the pragma, it does what you'd expect.
> > > Went with the "let's do the thing that the user probably wants, but isn't
> > > what MSVC does, and warn when that difference comes up" - if that's OK
> > > with everyone.
> > >
> > > (always open to wordsmithing the warning - and if we want to, can go to
> > > the extra layer and specifically diagnose which reason (mutable members,
> > > non-const init) - and I can't quite figure out the best phrasing to say
> > > "we're putting it in section X insetad of section Y, because Z, but
> > > Microsoft would use X because A" or something... it's all a bit of a
> > > mouthful)
> > Describing which reason actually applies would make the warning a lot
> > easier to read.
> That is true, but I think very few people will see this diagnostic. I'm not
> sure it's worth the added code complexity to implement that improvement.
Updated with a specific diagnosis. Phrasing still feels a bit awkward/I'm open
to wordsmithing suggestions.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D156726/new/
https://reviews.llvm.org/D156726
_______________________________________________
cfe-commits mailing list
cfe-commits@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
