On Wednesday 25 June 2025 19:45:56 LIU Hao wrote:
> 在 2025-6-25 16:08, Martin Storsjö 写道:
> > The null pointer access in itself is UB, and Clang would previously
> > optimize based on that being UB, assuming that that code is
> > unreachable, so it would assume that the printf function never
> > returns and not generate any code after that call.
> >
> > Change the pointer into a volatile char, which makes Clang not
> > infer things about it in the same way, and actually retain the
> > null pointer write.
> >
> > This fixes hangs in this testcase on armv7 with msvcrt.
>
>
> In order to inhibit this optimization, I think this might be better?
>
> char *p = NULL;
> __asm__ ("" : "+m"(p)); // pretend `p` is being modified
Maybe this could be another trick to prevent compilers doing some
optimizations.
>
> If we insist on use of `volatile` I'd expect the pointer itself is volatile:
>
> char * volatile p = NULL;
>
We are dereferencing the pointer, so I think that the pointer value has
to be marked as volatile, as Martin did it.
But whether the variable (on stack) has to be volatile too, I'm not sure.
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