efriedma added inline comments.
================
Comment at: clang/lib/Sema/SemaDecl.cpp:14254
int SectionFlags = ASTContext::PSF_Read;
- if (var->getType().isConstQualified()) {
- if (HasConstInit)
----------------
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.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D156726/new/
https://reviews.llvm.org/D156726
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