[llvm] r226600 - Prevent binary-tree deterioration in sparse switch statements.
Daniel Jasper
djasper at google.com
Thu Jan 22 06:37:25 PST 2015
No. How do I do that? Also, I have a better fix planned. Should get around
to implement that early next week.
On Thu, Jan 22, 2015 at 3:16 PM, Bruno Cardoso Lopes <
bruno.cardoso at gmail.com> wrote:
> Hi Daniel,
>
> I've tried the exact same approach back on Sep 2014 but I got a ~4%
> performance degradation in
> External/SPEC/CINT2006/462_libquantum/462_libquantum. Back in the day
> it would affect switch codegen in the "quantum_objcode_run" function,
> in objcode.c => jump tables and a bunch of code started to be emitted
> after the patch.
>
> Since it has been a while, much could have changed and that could no
> longer apply.
> Have you measured the impact of this on the test-suite?
>
> Thanks,
>
> On Tue, Jan 20, 2015 at 5:43 PM, Daniel Jasper <djasper at google.com> wrote:
> > Author: djasper
> > Date: Tue Jan 20 13:43:33 2015
> > New Revision: 226600
> >
> > URL: http://llvm.org/viewvc/llvm-project?rev=226600&view=rev
> > Log:
> > Prevent binary-tree deterioration in sparse switch statements.
> >
> > This addresses part of llvm.org/PR22262. Specifically, it prevents
> > considering the densities of sub-ranges that have fewer than
> > TLI.getMinimumJumpTableEntries() elements. Those densities won't help
> > jump tables.
> >
> > This is not a complete solution but works around the most pressing
> > issue.
> >
> > Review: http://reviews.llvm.org/D7070
> >
> > Modified:
> > llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
> > llvm/trunk/test/CodeGen/X86/switch-bt.ll
> >
> > Modified: llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
> > URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp?rev=226600&r1=226599&r2=226600&view=diff
> >
> ==============================================================================
> > --- llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
> (original)
> > +++ llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp Tue Jan
> 20 13:43:33 2015
> > @@ -2420,6 +2420,7 @@ bool SelectionDAGBuilder::handleBTSplitS
> > DEBUG(dbgs() << "Selecting best pivot: \n"
> > << "First: " << First << ", Last: " << Last <<'\n'
> > << "LSize: " << LSize << ", RSize: " << RSize << '\n');
> > + const TargetLowering &TLI = DAG.getTargetLoweringInfo();
> > for (CaseItr I = CR.Range.first, J=I+1, E = CR.Range.second;
> > J!=E; ++I, ++J) {
> > const APInt &LEnd = cast<ConstantInt>(I->High)->getValue();
> > @@ -2429,10 +2430,16 @@ bool SelectionDAGBuilder::handleBTSplitS
> > "Invalid case distance");
> > // Use volatile double here to avoid excess precision issues on
> some hosts,
> > // e.g. that use 80-bit X87 registers.
> > + // Only consider the density of sub-ranges that actually have
> sufficient
> > + // entries to be lowered as a jump table.
> > volatile double LDensity =
> > - LSize.roundToDouble() / (LEnd - First + 1ULL).roundToDouble();
> > + LSize.ult(TLI.getMinimumJumpTableEntries())
> > + ? 0.0
> > + : LSize.roundToDouble() / (LEnd - First +
> 1ULL).roundToDouble();
> > volatile double RDensity =
> > - RSize.roundToDouble() / (Last - RBegin + 1ULL).roundToDouble();
> > + RSize.ult(TLI.getMinimumJumpTableEntries())
> > + ? 0.0
> > + : RSize.roundToDouble() / (Last - RBegin +
> 1ULL).roundToDouble();
> > volatile double Metric = Range.logBase2() * (LDensity + RDensity);
> > // Should always split in some non-trivial place
> > DEBUG(dbgs() <<"=>Step\n"
> > @@ -2450,13 +2457,8 @@ bool SelectionDAGBuilder::handleBTSplitS
> > RSize -= J->size();
> > }
> >
> > - const TargetLowering &TLI = DAG.getTargetLoweringInfo();
> > - if (areJTsAllowed(TLI)) {
> > - // If our case is dense we *really* should handle it earlier!
> > - assert((FMetric > 0) && "Should handle dense range earlier!");
> > - } else {
> > + if (FMetric == 0 || !areJTsAllowed(TLI))
> > Pivot = CR.Range.first + Size/2;
> > - }
> > splitSwitchCase(CR, Pivot, WorkList, SV, SwitchBB);
> > return true;
> > }
> >
> > Modified: llvm/trunk/test/CodeGen/X86/switch-bt.ll
> > URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/switch-bt.ll?rev=226600&r1=226599&r2=226600&view=diff
> >
> ==============================================================================
> > --- llvm/trunk/test/CodeGen/X86/switch-bt.ll (original)
> > +++ llvm/trunk/test/CodeGen/X86/switch-bt.ll Tue Jan 20 13:43:33 2015
> > @@ -99,3 +99,61 @@ if.then:
> > if.end:
> > ret void
> > }
> > +
> > +; Ensure that optimizing for jump tables doesn't needlessly deteriorate
> the
> > +; created binary tree search. See PR22262.
> > +define void @test4(i32 %x, i32* %y) {
> > +; CHECK-LABEL: test4:
> > +
> > +entry:
> > + switch i32 %x, label %sw.default [
> > + i32 10, label %sw.bb
> > + i32 20, label %sw.bb1
> > + i32 30, label %sw.bb2
> > + i32 40, label %sw.bb3
> > + i32 50, label %sw.bb4
> > + i32 60, label %sw.bb5
> > + ]
> > +sw.bb:
> > + store i32 1, i32* %y
> > + br label %sw.epilog
> > +sw.bb1:
> > + store i32 2, i32* %y
> > + br label %sw.epilog
> > +sw.bb2:
> > + store i32 3, i32* %y
> > + br label %sw.epilog
> > +sw.bb3:
> > + store i32 4, i32* %y
> > + br label %sw.epilog
> > +sw.bb4:
> > + store i32 5, i32* %y
> > + br label %sw.epilog
> > +sw.bb5:
> > + store i32 6, i32* %y
> > + br label %sw.epilog
> > +sw.default:
> > + store i32 7, i32* %y
> > + br label %sw.epilog
> > +sw.epilog:
> > + ret void
> > +
> > +; The balanced binary switch here would start with a comparison against
> 39, but
> > +; it is currently starting with 29 because of the density-sum heuristic.
> > +; CHECK: cmpl $29
> > +; CHECK: jg
> > +; CHECK: cmpl $10
> > +; CHECK: jne
> > +; CHECK: cmpl $49
> > +; CHECK: jg
> > +; CHECK: cmpl $30
> > +; CHECK: jne
> > +; CHECK: cmpl $20
> > +; CHECK: jne
> > +; CHECK: cmpl $50
> > +; CHECK: jne
> > +; CHECK: cmpl $40
> > +; CHECK: jne
> > +; CHECK: cmpl $60
> > +; CHECK: jne
> > +}
> >
> >
> > _______________________________________________
> > llvm-commits mailing list
> > llvm-commits at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>
>
>
> --
> Bruno Cardoso Lopes
> http://www.brunocardoso.cc
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20150122/396c3127/attachment.html>
More information about the llvm-commits
mailing list