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Gracefully degrade precision in branch probability numbers.

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llvm-svn: 148946
This commit is contained in:
Nick Lewycky 2012-01-25 09:43:14 +00:00
parent 7693b940b6
commit 670c3c4fb5
1 changed files with 72 additions and 17 deletions
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89
lib/Transforms/Utils/SimplifyCFG.cpp
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@ -1466,6 +1466,29 @@ static bool ExtractBranchMetadata(BranchInst *BI,
return true;
}
/// MultiplyAndLosePrecision - Multiplies A and B, then returns the result. In
/// the event of overflow, logically-shifts all four inputs right until the
/// multiply fits.
static APInt MultiplyAndLosePrecision(APInt &A, APInt &B, APInt &C, APInt &D,
unsigned &BitsLost) {
BitsLost = 0;
bool Overflow = false;
APInt Result = A.umul_ov(B, Overflow);
if (Overflow) {
APInt MaxB = APInt::getMaxValue(A.getBitWidth()).udiv(A);
do {
B = B.lshr(1);
++BitsLost;
} while (B.ugt(MaxB));
A = A.lshr(BitsLost);
C = C.lshr(BitsLost);
D = D.lshr(BitsLost);
Result = A * B;
}
return Result;
}
/// FoldBranchToCommonDest - If this basic block is simple enough, and if a
/// predecessor branches to us and one of our successors, fold the block into
/// the predecessor and use logical operations to pick the right destination.
@ -1665,32 +1688,64 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI) {
// we get:
// (a*c)% = A*C, (b+(a*d))% = A*D+B*C+B*D.
bool Overflow1 = false, Overflow2 = false, Overflow3 = false;
bool Overflow4 = false, Overflow5 = false, Overflow6 = false;
APInt ProbTrue = A.umul_ov(C, Overflow1);
// In the event of overflow, we want to drop the LSB of the input
// probabilities.
unsigned BitsLost;
APInt Tmp1 = A.umul_ov(D, Overflow2);
APInt Tmp2 = B.umul_ov(C, Overflow3);
APInt Tmp3 = B.umul_ov(D, Overflow4);
APInt Tmp4 = Tmp1.uadd_ov(Tmp2, Overflow5);
APInt ProbFalse = Tmp4.uadd_ov(Tmp3, Overflow6);
// Ignore overflow result on ProbTrue.
APInt ProbTrue = MultiplyAndLosePrecision(A, C, B, D, BitsLost);
APInt GCD = APIntOps::GreatestCommonDivisor(ProbTrue, ProbFalse);
ProbTrue = ProbTrue.udiv(GCD);
ProbFalse = ProbFalse.udiv(GCD);
APInt Tmp1 = MultiplyAndLosePrecision(B, D, A, C, BitsLost);
if (BitsLost) {
ProbTrue = ProbTrue.lshr(BitsLost*2);
}
APInt Tmp2 = MultiplyAndLosePrecision(A, D, C, B, BitsLost);
if (BitsLost) {
ProbTrue = ProbTrue.lshr(BitsLost*2);
Tmp1 = Tmp1.lshr(BitsLost*2);
}
APInt Tmp3 = MultiplyAndLosePrecision(B, C, A, D, BitsLost);
if (BitsLost) {
ProbTrue = ProbTrue.lshr(BitsLost*2);
Tmp1 = Tmp1.lshr(BitsLost*2);
Tmp2 = Tmp2.lshr(BitsLost*2);
}
bool Overflow1 = false, Overflow2 = false;
APInt Tmp4 = Tmp2.uadd_ov(Tmp3, Overflow1);
APInt ProbFalse = Tmp4.uadd_ov(Tmp1, Overflow2);
if (Overflow1 || Overflow2) {
ProbTrue = ProbTrue.lshr(1);
Tmp1 = Tmp1.lshr(1);
Tmp2 = Tmp2.lshr(1);
Tmp3 = Tmp3.lshr(1);
Tmp4 = Tmp2 + Tmp3;
ProbFalse = Tmp4 + Tmp1;
}
// The sum of branch weights must fit in 32-bits.
if (ProbTrue.isNegative() && ProbFalse.isNegative()) {
ProbTrue = ProbTrue.lshr(1);
ProbFalse = ProbFalse.lshr(1);
}
if (ProbTrue != ProbFalse) {
// Normalize the result.
APInt GCD = APIntOps::GreatestCommonDivisor(ProbTrue, ProbFalse);
ProbTrue = ProbTrue.udiv(GCD);
ProbFalse = ProbFalse.udiv(GCD);
if (Overflow1 || Overflow2 || Overflow3 || Overflow4 || Overflow5 ||
Overflow6) {
DEBUG(dbgs() << "Overflow recomputing branch weight on: " << *PBI
<< "when merging with: " << *BI);
PBI->setMetadata(LLVMContext::MD_prof, NULL);
} else {
LLVMContext &Context = BI->getContext();
Value *Ops[3];
Ops[0] = BI->getMetadata(LLVMContext::MD_prof)->getOperand(0);
Ops[1] = ConstantInt::get(Context, ProbTrue);
Ops[2] = ConstantInt::get(Context, ProbFalse);
PBI->setMetadata(LLVMContext::MD_prof, MDNode::get(Context, Ops));
} else {
PBI->setMetadata(LLVMContext::MD_prof, NULL);
}
} else {
PBI->setMetadata(LLVMContext::MD_prof, NULL);