[llvm-dev] llvm emits unoptimized code

Jorg Brown via llvm-dev llvm-dev at lists.llvm.org
Thu Oct 31 11:16:46 PDT 2019

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.


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

-- Jorg
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