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? ./Kamlesh
Looks pretty similar to the GCC generated code - have you benchmarked this code compared to something else hand-crafted to see if the tls_get_address is measurably slow? (& I guess there's the possibility that the loop runs zero times - in which case doing it outside the loop would be a pessimization, potentially) On Thu, Oct 31, 2019 at 8:49 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? > > ./Kamlesh > _______________________________________________ > 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/f60c279a/attachment.html>
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 codeChallenge accepted => https://godbolt.org/z/8PX2La -- Jorg -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20191031/d1b8fb17/attachment.html>
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>