[llvm] r294632 - [LoadCombine] Fix combining of loads which span an aliasing store.

[Michael Spencer via llvm-commits]

On Thu, Feb 9, 2017 at 2:11 PM, Davide Italiano <davide at freebsd.org> wrote:

> On Thu, Feb 9, 2017 at 2:09 PM, Michael Spencer <bigcheesegs at gmail.com>
> wrote:
> > On Thu, Feb 9, 2017 at 2:01 PM, Davide Italiano <davide at freebsd.org>
> wrote:
> >>
> >> On Thu, Feb 9, 2017 at 1:46 PM, Michael J. Spencer via llvm-commits
> >> <llvm-commits at lists.llvm.org> wrote:
> >> > Author: mspencer
> >> > Date: Thu Feb  9 15:46:49 2017
> >> > New Revision: 294632
> >> >
> >> > URL: http://llvm.org/viewvc/llvm-project?rev=294632&view=rev
> >> > Log:
> >> > [LoadCombine] Fix combining of loads which span an aliasing store.
> >> >
> >> > Fixes PR31517
> >> >
> >> > Differential Revision: https://reviews.llvm.org/D28922
> >> >
> >> > Modified:
> >> >     llvm/trunk/lib/Transforms/Scalar/LoadCombine.cpp
> >> >     llvm/trunk/test/Transforms/LoadCombine/load-combine-aa.ll
> >> >
> >> > Modified: llvm/trunk/lib/Transforms/Scalar/LoadCombine.cpp
> >> > URL:
> >> > http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/
> Transforms/Scalar/LoadCombine.cpp?rev=294632&r1=294631&r2=294632&view=diff
> >> >
> >> > ============================================================
> ==================
> >> > --- llvm/trunk/lib/Transforms/Scalar/LoadCombine.cpp (original)
> >> > +++ llvm/trunk/lib/Transforms/Scalar/LoadCombine.cpp Thu Feb  9
> 15:46:49
> >> > 2017
> >> > @@ -245,13 +245,17 @@ bool LoadCombine::runOnBasicBlock(BasicB
> >> >    bool Combined = false;
> >> >    unsigned Index = 0;
> >> >    for (auto &I : BB) {
> >> > -    if (I.mayThrow() || (I.mayWriteToMemory() &&
> >> > AST.containsUnknown(&I))) {
> >> > +    if (I.mayThrow() || AST.containsUnknown(&I)) {
> >> >        if (combineLoads(LoadMap))
> >> >          Combined = true;
> >> >        LoadMap.clear();
> >> >        AST.clear();
> >> >        continue;
> >> >      }
> >> > +    if (I.mayWriteToMemory()) {
> >> > +      AST.add(&I);
> >> > +      continue;
> >> > +    }
> >> >      LoadInst *LI = dyn_cast<LoadInst>(&I);
> >> >      if (!LI)
> >> >        continue;
> >> >
> >> > Modified: llvm/trunk/test/Transforms/LoadCombine/load-combine-aa.ll
> >> > URL:
> >> > http://llvm.org/viewvc/llvm-project/llvm/trunk/test/
> Transforms/LoadCombine/load-combine-aa.ll?rev=294632&r1=
> 294631&r2=294632&view=diff
> >> >
> >> > ============================================================
> ==================
> >> > --- llvm/trunk/test/Transforms/LoadCombine/load-combine-aa.ll
> (original)
> >> > +++ llvm/trunk/test/Transforms/LoadCombine/load-combine-aa.ll Thu
> Feb  9
> >> > 15:46:49 2017
> >> > @@ -37,3 +37,24 @@ define i64 @test2(i32* nocapture readonl
> >> >    ret i64 %add
> >> >  }
> >> >
> >> > +%rec11 = type { i16, i16, i16 }
> >> > + at str = global %rec11 { i16 1, i16 2, i16 3 }
> >> > +
> >> > +; PR31517 - Check that loads which span an aliasing store are not
> >> > combined.
> >> > +define i16 @test3() {
> >> > +; CHECK-LABEL: @test3
> >> > +
> >> > +; CHECK: load i16, i16*
> >> > +; CHECK: store i16
> >> > +; CHECK: ret i16
> >> > +
> >> > +  %_tmp9 = getelementptr %rec11, %rec11* @str, i16 0, i32 1
> >> > +  %_tmp10 = load i16, i16* %_tmp9
> >> > +  %_tmp12 = getelementptr %rec11, %rec11* @str, i16 0, i32 0
> >> > +  store i16 %_tmp10, i16* %_tmp12
> >> > +  %_tmp13 = getelementptr %rec11, %rec11* @str, i16 0, i32 0
> >> > +  %_tmp14 = load i16, i16* %_tmp13
> >> > +  %_tmp15 = icmp eq i16 %_tmp14, 3
> >> > +  %_tmp16 = select i1 %_tmp15, i16 1, i16 0
> >> > +  ret i16 %_tmp16
> >> > +}
> >> >
> >> >
> >>
> >> I think it's a good idea to change this tests to not rely on
> >> instcombine anymore.
> >
> >
> > The IR gets pretty ugly if you do that and makes the test harder to
> > understand.
> >
>
> Hmm, but that also makes the test more fragile as we rely on
> instcombine doing some cleanups before loadcombine. I don't think we
> want that dependency. What do you think?
>
>
It's been in tree for 2 years with no changes due to changes in
instcombine. So I think it's fine.

- Michael Spencer
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