[cfe-dev] Why compiler can not recognize what variables are volatile?

[Eli Friedman via cfe-dev]

The question of explicitly modeling setjmp control flow has come up before; see http://lists.llvm.org/pipermail/llvm-dev/2017-November/118820.html .  It's probably feasible, but I don't think anyone has actually attempted to implement it.

-Eli

> -----Original Message-----
> From: cfe-dev <cfe-dev-bounces at lists.llvm.org> On Behalf Of Peng Yu via cfe-
> dev
> Sent: Wednesday, February 6, 2019 4:43 PM
> To: cfe-dev <cfe-dev at lists.llvm.org>
> Subject: [EXT] [cfe-dev] Why compiler can not recognize what variables are
> volatile?
> 
> Hi,
> 
> If I compile the following program without -O*, it will print this.
> 
> $ ./main.exe
> 2
> 20
> 
> If I compile it with -O1 or any other number > 1, it will print this.
> 
> $ ./main.exe
> 2
> 10
> 
> The optimization clearly changes the semantics of the program. Why the
> compiler can not figure out local_var2 is volatile on its own to
> reduce the burdens of the programmers in having to figure out what
> variables should be specified as volatile?
> 
> Thanks.
> 
> #include <stdio.h>
> #include <setjmp.h>
> 
> static jmp_buf buf;
> 
> int main() {
>     volatile int local_var = 1;
>     int local_var2 = 10;
>     if(!setjmp(buf)) {
>         local_var = 2;
>         local_var2 = 20;
>         longjmp(buf, 1);
>     } else {
>         printf("%d\n", local_var);
>         printf("%d\n", local_var2);
>     }
> 
>     return 0;
> }
> 
> --
> Regards,
> Peng
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