[PATCH] Add basic support for removal of load that are fed by a store of an aggregate

[Chandler Carruth]

Ok, how do i reproduce the problem you're seeing? For me:

% ./bin/opt -S -O2 < test-fca.ll
; ModuleID = '<stdin>'
target datalayout = "e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

declare void @foo()

define void @test({ i8, i8 }* nocapture) {
brancher.exit:
  %v.sroa.0.0.dst.sroa_idx = getelementptr inbounds { i8, i8 }* %0, i64 0,
i32 0
  store i8 1, i8* %v.sroa.0.0.dst.sroa_idx, align 1
  %v.sroa.2.0.dst.sroa_idx = getelementptr inbounds { i8, i8 }* %0, i64 0,
i32 1
  store i8 2, i8* %v.sroa.2.0.dst.sroa_idx, align 1
  tail call void @foo()
  ret void
}

On Fri, Jan 2, 2015 at 3:40 PM, Björn Steinbrink <bsteinbr at gmail.com> wrote:

> On 2015.01.02 23:29:00 +0100, Björn Steinbrink wrote:
> > [Added llvm-commit back to Cc, sorry, removing it wasn't intentional]
> >
> > On 2015.01.02 14:05:28 -0800, Chandler Carruth wrote:
> > > On Fri, Jan 2, 2015 at 2:02 PM, Björn Steinbrink <bsteinbr at gmail.com>
> wrote:
> > >
> > > > 2015-01-02 22:36 GMT+01:00 Chandler Carruth <chandlerc at google.com>:
> > > > >
> > > > > On Fri, Jan 2, 2015 at 1:27 PM, Björn Steinbrink <
> bsteinbr at gmail.com> wrote:
> > > > >>
> > > > >> If we have a simple load through a GEP that is fed by a store of
> an
> > > > >> aggregate, we can use the GEP indices to walk the stored
> aggregate and
> > > > >> extract the appropriate value to replace the load.
> > > > >
> > > > >
> > > > > What's the motivation for this change?
> > > >
> > > > The rust compiler hit a case where not having this optimization
> caused
> > > > a branch not to be removed. The corresponding issue for rustc is
> > > > https://github.com/rust-lang/rust/issues/20149
> > > >
> > > > > Note that SROA replaces all stores of aggregates with scalar
> stores of
> > > > > the components specifically so that neither it nor GVN needs to
> > > > > cope with aggregate loads or stores.
> > > >
> > > > SROA did the opposite thing in this case, replacing individual stores
> > > > insertvalues and a store of an FCA. I'm attaching the full output of
> > > > opt -print-after-all -O2 -S for the failing test case given in the
> > > > rust issue. The first SROA creates the FCA store, and later when the
> > > > call to `unwrap` is inlined the existing optimizations can't
> eliminate
> > > > the branch that comes with it, because of the FCA store.
> > >
> > >
> > > SROA doesn't create FCA stores in any cases I'm aware of... Can you
> provide
> > > a somewhat reduced test case rather than a massive log?
> >
> > I got myself confused because it introduced the insertvalue
> > instructions. The FCA store was already present and not constructed by
> > SROA. I've attach a small test case this time.
>
> And... I forgot it again :-/ I'm really sorry, apparently this is not my
> day.
>
> Björn
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20150102/13688edd/attachment.html>