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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
llvm-svn: 226600
This commit is contained in:
Daniel Jasper 2015-01-20 19:43:33 +00:00
parent be10ece5ed
commit 6b77455f81
2 changed files with 68 additions and 8 deletions
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18
llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
View File

@ -2420,6 +2420,7 @@ bool SelectionDAGBuilder::handleBTSplitSwitchCase(CaseRec& CR,
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::handleBTSplitSwitchCase(CaseRec& CR,
"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::handleBTSplitSwitchCase(CaseRec& CR,
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;
}

58
llvm/test/CodeGen/X86/switch-bt.ll
View File

@ -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
}