[R-pkg-devel] Altrep header, MSVC, and STRUCT_SUBTYPES macro

[Ivan Krylov]

В Wed, 15 May 2024 18:54:37 +0200
David Cortes <david.cortes.rivera using gmail.com> пишет:

> The code compiles without errors under MSVC, but executing code that
> involves returning Altrep objects leads to segfaults and memory
> corruptions, even though it works fine under other compilers.
> 
> I see the R Altrep header has this section:
> #define STRUCT_SUBTYPES
> #ifdef STRUCT_SUBTYPES
> # define R_SEXP(x) (x).ptr
> # define R_SUBTYPE_INIT(x) { x }
>   typedef struct { SEXP ptr; } R_altrep_class_t;
> #else
> # define R_SEXP(x) ((SEXP) (x))
> # define R_SUBTYPE_INIT(x) (void *) (x)
>   typedef struct R_altcls *R_altrep_class_t;
> #endif

Interesting ABI incompatibility you've found. Can you show a minimal
example? I've tried playing with https://godbolt.org/ and passing
around values of type R_altrep_class_t between functions, but couldn't
convince "x64 msvc v19.latest" to generate different assembly no matter
whether R_altrep_class_t was a pointer or a struct containing a SEXP.

> If I manually edit the R header to remove the definition of
> 'STRUCT_SUBTYPES', leading to the second definition of
> 'R_altrep_class_t' being used, then things work as expected when the
> package is compiled with MSVC (no segfaults and no memory
> corruptions).

While it's hard to argue with results (I don't think it'll ever be
broken on x86_64 Windows), this workaround relies on undefined
behaviour and will only work as long as the ABI as understood by GCC
passes a structure with a pointer inside exactly the same way as the
ABI as understood by MSVC passes a bare pointer.

Isolating the MSVC-specific code as suggested by Vladimir should be
safer, but it's also important to find out where exactly the
incompatibility arises from. The GCC and MSVC parts still have to use a
common ABI to talk to each other.

-- 
Best regards,
Ivan