expression statements, volatiles, and C vs. C++

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expression statements, volatiles, and C vs. C++

John Regehr
[This is a re-send of a message I sent to the main LLVM list last night.
  Resending since it's really a front-end issue.]

The question is, what should C and C++ compilers do with this code?

   volatile int x;

   void foo (void) {
     x;
   }

This question is not totally stupid: embedded systems use code like this
when reading a hardware register has a useful side effect (usually
clearing the register).

It is reasonably clear that a C compiler should load from x and throw
away the value.  clang and llvm-gcc do this, as do most other good C
compilers.

However, clang++ and llvm-g++ also load from x and this does not appear
to be supported by the 1998 C++ standard.  In 6.2, it is explicitly
stated that for an expression statement, no conversion from lvalue to
rvalue occurs.  If there's no rvalue, there should not be a load from x.

Anyway, I'm curious: is the C-like interpretation of a volatile
expression statement considered to be a feature by the LLVM maintainers?
  If so, what is the rationale?

I haven't done extensive testing, but there do exist compiler families
(such as those from IAR and Intel) where the C compiler loads from x and
the C++ compiler does not.

[I just sent a very message very much like this one to the GCC mailing
list.]

Thanks,

John Regehr
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