Failure to optimize?

Alexander Monakov amonakov@ispras.ru
Tue Jan 12 14:15:59 GMT 2021


On Tue, 12 Jan 2021, ☂Josh Chia (謝任中) via Gcc-help wrote:

> I didn't mention it earlier, but I'd like to clarify that this is for "g++
> -O3 -std=c++17" on GCC 10.2.
> 
> If I use "static thread_local int cursor = 0;" then cursor is thread-local
> and such a race should be impossible, but the generated code for bar1()
> still has a branch.

Whether 'cursor' is thread-local or not does not matter, it's possible to take
the address of a thread-local variable and pass it to another thread.

> The branch persists also if I do this instead for bar1():
> char const* bar1() {
>     char const* result = foo();
>     if (result)
>         cursor += 1;
>     else
>         cursor += 0;
>     return result;
> }

This is hard to optimize as 'cursor += 0' is optimized out quite early, and
it's hard to keep track that 'cursor' is unconditionally written in the
abstract machine.

Alexander


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