[llvm-dev] llvm emits unoptimized code

[David Blaikie via llvm-dev]

On Thu, Oct 31, 2019 at 11:17 AM Jorg Brown via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On Thu, Oct 31, 2019 at 8:50 AM kamlesh kumar via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi Devs,
>> Consider testcase here
>> https://godbolt.org/z/qHZzqw
>> When optimization is O1 or above it produces unoptimized code
>> because it calls __tls_get_address in loops.
>> While with optimization disabled
>> It produce single call to __tls_get_address outside of loop.
>> is this a missed optimization by llvm?
>>
>
> It's interesting to me that there's a big difference in -fpie and -fpic.
>
> https://godbolt.org/z/klX3q3
>
> In particular, with -fpie, no call to __tls_get_addr is needed, so the
> underlying considerations for optimization change.  This feels like the
> optimizer isn't taking in to account the overhead of -fpic, when
> determining whether to hoist the address calculation out of the loop.
>
> On Thu, Oct 31, 2019 at 10:36 AM David Blaikie via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Looks pretty similar to the GCC generated code
>
>
> Challenge accepted => https://godbolt.org/z/8PX2La
>

Which challenge? Sorry, could've linked to the godbolt I was looking at
when I said that: https://godbolt.org/z/_07tOk - comparing GCC and Clang
trunk on the code linked in the original post. Looked/looks fairly similar
to me. But yeah, I don't know much beyond that.


>
> -- Jorg
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20191031/143aa026/attachment.html>