[PATCH] D32782: Add pthread_self function prototype and make it speculatable.

[Xin Tong via llvm-commits]

Let me revert that.  thanks for raising the issue. I will take a closer look.

Thanks
-Xni

On Sun, May 21, 2017 at 8:21 AM, Davide Italiano <dccitaliano at gmail.com> wrote:
> On Sat, May 20, 2017 at 3:51 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:
>> Couldn't pthread_self() return a different value if fork() is called in the
>> body of the loop?
>>
>
> I was worried about a case where this could break but I wasn't able to
> find a counterexample.
> I guess yours holds. FWIW, I'm not sure what's the interaction between
> fork() and pthread_self() but if fork()'ing can actually change the
> value, GCC miscompiles the following code. Xin, I recommend to revert
> this one until we get a better understanding.
>
> #include <pthread.h>
> #include <unistd.h>
>
> extern void x(pthread_t);
>
> int foo(int n) {
>    for (int i = 0; i < n; i++) {
>     auto id = pthread_self();
>     x(id);
>     fork();
>   }
> }
> foo(int):
>         test    edi, edi
>         jle     .L6
>         push    r12
>         push    rbp
>         mov     ebp, edi
>         push    rbx
>         xor     ebx, ebx
>         call    pthread_self
>         mov     r12, rax
> .L3:
>         mov     rdi, r12
>         add     ebx, 1
>         call    x(unsigned long)
>         call    fork
>         cmp     ebp, ebx
>         jne     .L3
>         pop     rbx
>         pop     rbp
>         pop     r12
>         ret
> .L6:
>         rep ret



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Software Engineer - Compiler Toolchain
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