Re: Safe transposition of logical and operands

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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On Mon, 18 Sept 2023, 08:03 Paul Floyd via Gcc, <gcc@gcc.gnu.org> wrote:

>
>
> On 17-09-23 22:51, Jonathan Wakely wrote:
>
> >
> > Why would it be trapping? It's loading an int64_t, which might be
> > uninitialised but it can't trap.
>
> In this context I think that Valgrind is considering that any memory
> load could trap.
>

There are no padding bits in int64_t and all object representations are
valid values, so it has no trap representations.



> > *f on a std::optional is not like dereferencing a pointer, the int64_t
> > memory location is always present as part of the object.
>
> For this
>
> movq    40(%rbx), %rax
>
> unless you know that what RBX+40 is pointing to is safe to dereference
> it's not different to dereferencing a pointer.
>

If it isn't safe to load that integer then it isn't safe to call f.operator
bool() either. GCC knows they are part of the same object.


> So I think that the problem is that Valgrind is being cautious and not
> allowing any loads but GCC is accepting what it considers safe loads
> from the stack.
>

Yes, GCC assumes that the reference is bound to a valid object, because C++
requires that to be true. Of course memcheck can't assume that, because one
of its main reasons to exist is to find undefined behaviour where that
isn't true!

I think what GCC is doing is a valid transformation, in the context of a
valid C++ program. But I'm not sure that helps valgrind, which doesn't have
the liberty of assuming a valid program.