hgreving added a comment. In D74183#1942262 <https://reviews.llvm.org/D74183#1942262>, @nikic wrote:
> In D74183#1941741 <https://reviews.llvm.org/D74183#1941741>, @efriedma wrote: > > > If it's just tramp3d-v4, I'm not that concerned... but that's a weird > > result. On x86 in particular, alignment markings have almost no effect on > > optimization, generally. > > > I've just looked at the IR diff for tramp3d-v4 and it turns out that the root > cause is an old friend of mine: The insertion of alignment assumptions during > inlining > (https://github.com/llvm/llvm-project/blob/b58902bc72c2b479b5ed27ec0d3422ba9782edbb/llvm/lib/Transforms/Utils/InlineFunction.cpp#L1139-L1173). > That is, the IR now contains many instances of this sequence: > > %ptrint = ptrtoint %class.GuardLayers* %guards_m to i64 > %maskedptr = and i64 %ptrint, 3 > %maskcond = icmp eq i64 %maskedptr, 0 > tail call void @llvm.assume(i1 %maskcond) > > > to preserve the alignment information. From a cursory look I cannot tell > whether these additional assumes also regress optimization (due to > multi-use), but given the size increase on the final binary it seems pretty > likely that this is the case. > > This preservation of alignment during inlining is the reason why we used to > not emit alignment information for pointer arguments in Rust for a long time: > It caused serious regressions in optimization and increased compile-time. > Nowadays we do emit alignment information, but set > `-preserve-alignment-assumptions-during-inlining=false` to prevent this > inlining behavior. > > I think for the purposes of this revision, this means that we should probably > either a) default `preserve-alignment-assumptions-during-inlining` to false > (I would prefer this) or b) not emit the alignment unless it is smaller than > the ABI alignment (I guess this was what this patch originally did?) We are having a problem with this very issue on a target not supporting a stack, with sroa bailing due to above, in our case causing a crash. Our only workaround for this is currently `preserve-alignment-assumptions-during-inlining` to false. We were actually wondering if this is causing performance issues on targets that due support a stack. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D74183/new/ https://reviews.llvm.org/D74183 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
