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In https://reviews.llvm.org/D68103 the InstCombine learned that shift-by-sext is simply a shift-by-zext. But the transform is limited to single-use sext. We can quite trivially get a case where there are two shifts by the same sext: https://godbolt.org/z/j6mO3t <- We should handle those cases. In https://reviews.llvm.org/D68103...

Thanks for taking a look! On Tue, Oct 1, 2019 at 9:09 PM Philip Reames <listmail at philipreames.com> wrote: > On 9/27/19 1:40 PM, Roman Lebedev via llvm-dev wrote: > > In https://reviews.llvm.org/D68103 the InstCombine learned that shift-by-sext > > is simply a shift-by-zext. > > Just to make sure I'm following, the reasoning here is that the shift > amount must be positive or the shift would produce poison? And thus, > it's safe to assume that the sext == zext because we...

...-dev at lists.llvm.org> wrote: >> >> Thanks for taking a look! >> >> On Tue, Oct 1, 2019 at 9:09 PM Philip Reames <listmail at philipreames.com> wrote: >> > On 9/27/19 1:40 PM, Roman Lebedev via llvm-dev wrote: >> > > In https://reviews.llvm.org/D68103 the InstCombine learned that shift-by-sext >> > > is simply a shift-by-zext. >> > >> > Just to make sure I'm following, the reasoning here is that the shift >> > amount must be positive or the shift would produce poison? And thus, >> > it's sa...

...t; >> Thanks for taking a look! > > >> > > >> On Tue, Oct 1, 2019 at 9:09 PM Philip Reames <listmail at philipreames.com> wrote: > > >> > On 9/27/19 1:40 PM, Roman Lebedev via llvm-dev wrote: > > >> > > In https://reviews.llvm.org/D68103 the InstCombine learned that shift-by-sext > > >> > > is simply a shift-by-zext. > > >> > > > >> > Just to make sure I'm following, the reasoning here is that the shift > > >> > amount must be positive or the shift would produce poi...