[llvm-dev] devirtualization with new-PM pipeline

[Easwaran Raman via llvm-dev]

Yes, this looks broken in the new PM. The DevirtSCCRepeatedPass::run method
first scans the functions in the SCC to collect value handles for indirect
calls, runs the CGSCC pass pipeline and then checks if any of the call
value handles now point to  a direct call, in which case it runs the
pipeline again (which should inline the devirtualized call) . The problem
is scanning the initial SCC for indirect calls is not enough since inlining
can produce indirect calls (which could then get devirtualized by later
passes). One (ugly) fix is to pass the value handles to the inliner
through  CGSCCUpdateResult and let the inliner augment it as the caller
gets indirect calls due to inlining.

On Thu, Dec 7, 2017 at 12:20 PM, Fedor Sergeev via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On 12/07/2017 11:09 PM, Fedor Sergeev via llvm-dev wrote:
> > Chandler et al,
> >
> > I have been playing with the new PM pipeline, being particularly
> interested in how it can handle devirtualization.
> > Now, I discovered what I believe is a "regression" vs old PM on a rather
> simple one-translation-unit testcase.
> >
> > clang is able to devirtualize it with -O3 and fails to do so with
> -fexperimental-new-pass-manager added.
>
> I hate to correct myself ;) yet it is able to devirtualize (that is GVN
> does its job) though it does fail to perform
> a rather expected inline afterwards.
>
> regards,
>   Fedor.
>
>
>
> >
> > It looks like a pipeline issue, though I did not dig deeper as I'm not
> sure if this kind of behavior
> > is expected at current stage of New PM life or not?
> >
> > If it is not expected then I'ld be happy to file a bug and do a bit
> deeper look for the cause.
> >
> > regards,
> >   Fedor.
> > -------------------------------
> > ] cat devirt.cpp
> > struct A {
> >   virtual int virt1();
> > };
> > struct B : A {
> >   int virt1() override {
> >     return 20;
> >   }
> > };
> > static int redirect(A* a) {
> >   return a->virt1();
> > }
> > int calc() {
> >   B b;
> >   return redirect(&b);
> > }
> > ] clang++ -O3 devirt.cpp -std=c++11 -S -emit-llvm -o -
> > ...
> > define i32 @_Z4calcv() local_unnamed_addr #0 {
> > entry:
> >   ret i32 20    <--- nicely devirtualized (&b)->virt1() call
> > }
> > ...
> > ] clang++ -O3 -fexperimental-new-pass-manager -std=c++11 -S -emit-llvm
> -o -
> > ...
> > define i32 @_Z4calcv() local_unnamed_addr #0 {
> > entry:
> >   %b = alloca %struct.B, align 8
> >   %0 = bitcast %struct.B* %b to i8*
> >   call void @llvm.lifetime.start.p0i8(i64 8, i8* nonnull %0) #3
> >   %1 = getelementptr inbounds %struct.B, %struct.B* %b, i64 0, i32 0,
> i32 0
> >   store i32 (...)** bitcast (i8** getelementptr inbounds ({ [3 x i8*] },
> { [3 x i8*] }* @_ZTV1B, i64 0, inrange i32 0, i64 2) to i32 (...)**), i32
> (...)*** %1, align 8, !tbaa !2
> >   %call.i = call i32 @_ZN1B5virt1Ev(%struct.B* nonnull %b)
> >   call void @llvm.lifetime.end.p0i8(i64 8, i8* nonnull %0) #3
> >   ret i32 %call.i
> > }
> > ...
> > ] clang++ --version
> > clang version 6.0.0 (trunk 319748) (llvm/trunk 319768)
> > Target: x86_64-unknown-linux-gnu
> > Thread model: posix
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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