RE: [PATCH v15 1/3] eal: add diagnostics macros to make code portable

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Andre Muezerie [mailto:andremue@linux.microsoft.com]
> Sent: Tuesday, 21 January 2025 15.28
> 
> On Tue, Jan 21, 2025 at 10:53:14AM +0100, Morten Brørup wrote:
> > > From: Andre Muezerie [mailto:andremue@linux.microsoft.com]
> > > Sent: Saturday, 18 January 2025 22.55
> > >
> > > It was a common pattern to have "GCC diagnostic ignored" pragmas
> > > sprinkled over the code and only activate these pragmas for certain
> > > compilers (gcc and clang). Clang supports GCC's pragma for
> > > compatibility with existing source code, so #pragma GCC diagnostic
> > > and #pragma clang diagnostic are synonyms for Clang
> > > (https://clang.llvm.org/docs/UsersManual.html).
> > >
> > > Now that effort is being made to make the code compatible with MSVC
> > > these expressions would become more complex. It makes sense to hide
> > > this complexity behind macros. This makes maintenance easier as
> these
> > > macros are defined in a single place. As a plus the code becomes
> > > more readable as well.
> > >
> > > Signed-off-by: Andre Muezerie <andremue@linux.microsoft.com>
> > > ---
> > >  lib/eal/include/rte_common.h | 46
> ++++++++++++++++++++++++++++++++++++
> > >  1 file changed, 46 insertions(+)
> > >
> > > diff --git a/lib/eal/include/rte_common.h
> > > b/lib/eal/include/rte_common.h
> > > index 40592f71b1..4b87a0a352 100644
> > > --- a/lib/eal/include/rte_common.h
> > > +++ b/lib/eal/include/rte_common.h
> > > @@ -156,6 +156,52 @@ typedef uint16_t unaligned_uint16_t;
> > >  #define RTE_DEPRECATED(x)
> > >  #endif
> > >
> > > +/**
> > > + * Macros to cause the compiler to remember the state of the
> diagnostics as of
> > > + * each push, and restore to that point at each pop.
> > > + */
> > > +#if !defined(__INTEL_COMPILER) && !defined(RTE_TOOLCHAIN_MSVC)
> > > +#define __rte_diagnostic_push _Pragma("GCC diagnostic push")
> > > +#define __rte_diagnostic_pop  _Pragma("GCC diagnostic pop")
> > > +#else
> > > +#define __rte_diagnostic_push
> > > +#define __rte_diagnostic_pop
> > > +#endif
> > > +
> > > +/**
> > > + * Macro to disable compiler warnings about removing a type
> > > + * qualifier from the target type.
> > > + */
> > > +#if !defined(__INTEL_COMPILER) && !defined(RTE_TOOLCHAIN_MSVC)
> > > +#define __rte_diagnostic_ignored_wcast_qual \
> > > +		_Pragma("GCC diagnostic ignored \"-Wcast-qual\"")
> > > +#else
> > > +#define __rte_diagnostic_ignored_wcast_qual
> > > +#endif
> > > +
> > > +/**
> > > + * Workaround to discard qualifiers (such as const, volatile,
> restrict) from a pointer,
> > > + * without the compiler emitting a warning.
> > > + */
> > > +#define RTE_PTR_UNQUAL(X) ((void *)(uintptr_t)(X))
> >
> > It seems the C23 typeof_unqual and the built-in pre-C23
> __typeof_unqual__ couldn't be used.
> > Was it a generic issue, or only when operating on (the return value
> of) functions?
> 
> I experimented with C23 typeof_unqual. It indeed works on gcc, clang
> and MSVC, but there are some details:
> 
> a) With gcc the project needs to be compiled with -std=c2x. Many other
> warnings show up, unrelated to the scope of this patchset. Some look
> suspicious and should be looked at. An error also showed up, for which
> I sent out a small patch.
> 
> b) When using typeof_unqual and passing "-Wcast-qual" to the compiler,
> a warning about the qualifier being dropped is emitted. The project
> currently uses "-Wcast-qual". Perhaps it shouldn't?

The compiler is our friend; when more warnings enabled, the code quality requirements are higher.
Although this statement may not be universally true, I think it is for "-Wcast-qual".

> 
> Due to (a) I decided to not use typeof_unqual for now, but it would be
> trivial to change the macro to do so in the future.

How about __typeof_unqual__ (with double underscores prefix and postfix)?
It seems to be available in both GCC [1] and MSVC [2] without requiring C23.

[1]: https://gcc.gnu.org/onlinedocs/gcc/Typeof.html
[2]: https://learn.microsoft.com/en-us/cpp/c-language/typeof-unqual-c?view=msvc-170

> 
> >
> > > +
> > > +/**
> > > + * Workaround to discard qualifiers (such as const, volatile,
> restrict) from a pointer
> > > + * and cast it to a specific type, without the compiler emitting a
> warning.
> >
> > Propose new description with emphasis on casting rather than
> discarding qualifiers:
> >
> > Workaround to cast a pointer to a specific type,
> > without the compiler emitting a warning about discarding qualifiers.
> >
> 
> I'll update this.
> 
> > > + *
> > > + * @warning
> > > + * Although this macro can be abused for casting a pointer to
> point to a different type,
> > > + * alignment may be incorrect when casting to point to a larger
> type. E.g.:
> >
> > This macro is now meant for abuse, so propose softening the warning:
> >
> > When casting a pointer to point to a larger type,
> > the resulting pointer may be misaligned,
> > which causes undefined behavior.
> 
> I'll update this.
> 
> > E.g.:
> >
> > > + *   struct s {
> > > + *       uint16_t a;
> > > + *       uint8_t  b;
> > > + *       uint8_t  c;
> > > + *       uint8_t  d;
> > > + *   } v;
> > > + *   uint16_t * p = RTE_CAST_PTR(uint16_t *, &v.c); // "p" is not
> 16 bit aligned!
> > > + */
> > > +#define RTE_CAST_PTR(type, ptr) ((type)(uintptr_t)(ptr))
> >
> > I am somewhat concerned about these macros...
> >
> > There's a good reason why MSVC doesn't allow casting to discard
> qualifiers or changing the type like this.
> >
> > If in doubt, read this:
> > https://www.trust-in-soft.com/resources/blogs/2020-04-06-gcc-always-
> assumes-aligned-pointer-accesses
> >
> > We need these workarounds because DPDK currently contains code with
> formally "undefined behavior".
> > And instead of fixing the root causes, we choose the pragmatic
> solution and introduce workarounds to allow it.
> >
> > Would it be possible for the RTE_CAST_PTR macro to check if the
> casted-to pointer changes from a smaller type to a larger type, and
> warn/fail if it does?
> 
> I'll think about it.
> >
> > Should the use of these workaround macros be disallowed in new code?
> > I.e. should checkpatches check for them?
> 
> We can certainly add a check to checkpatches.