[llvm-commits] [llvm] r163954 - in /llvm/trunk: lib/Transforms/Utils/SimplifyCFG.cpp test/Transforms/SimplifyCFG/preserve-branchweights.ll
Manman Ren
mren at apple.com
Fri Sep 14 17:39:57 PDT 2012
Author: mren
Date: Fri Sep 14 19:39:57 2012
New Revision: 163954
URL: http://llvm.org/viewvc/llvm-project?rev=163954&view=rev
Log:
PGO: preserve branch-weight metadata when simplifying two branches with a common
destination.
Updated previous implementation to fix a case not covered:
// PBI: br i1 %x, TrueDest, BB
// BI: br i1 %y, TrueDest, FalseDest
The other case was handled correctly.
// PBI: br i1 %x, BB, FalseDest
// BI: br i1 %y, TrueDest, FalseDest
Also tried to use 64-bit arithmetic instead of APInt with scale to simplify the
computation. Let me know if you have other opinions about this.
Modified:
llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
llvm/trunk/test/Transforms/SimplifyCFG/preserve-branchweights.ll
Modified: llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp?rev=163954&r1=163953&r2=163954&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp Fri Sep 14 19:39:57 2012
@@ -1658,7 +1658,7 @@
/// parameters and return true, or returns false if no or invalid metadata was
/// found.
static bool ExtractBranchMetadata(BranchInst *BI,
- APInt &ProbTrue, APInt &ProbFalse) {
+ uint64_t &ProbTrue, uint64_t &ProbFalse) {
assert(BI->isConditional() &&
"Looking for probabilities on unconditional branch?");
MDNode *ProfileData = BI->getMetadata(LLVMContext::MD_prof);
@@ -1666,35 +1666,11 @@
ConstantInt *CITrue = dyn_cast<ConstantInt>(ProfileData->getOperand(1));
ConstantInt *CIFalse = dyn_cast<ConstantInt>(ProfileData->getOperand(2));
if (!CITrue || !CIFalse) return false;
- ProbTrue = CITrue->getValue();
- ProbFalse = CIFalse->getValue();
- assert(ProbTrue.getBitWidth() == 32 && ProbFalse.getBitWidth() == 32 &&
- "Branch probability metadata must be 32-bit integers");
+ ProbTrue = CITrue->getValue().getZExtValue();
+ ProbFalse = CIFalse->getValue().getZExtValue();
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;
-}
-
/// checkCSEInPredecessor - Return true if the given instruction is available
/// in its predecessor block. If yes, the instruction will be removed.
///
@@ -1919,14 +1895,53 @@
New, "or.cond"));
PBI->setCondition(NewCond);
+ uint64_t PredTrueWeight, PredFalseWeight, SuccTrueWeight, SuccFalseWeight;
+ bool PredHasWeights = ExtractBranchMetadata(PBI, PredTrueWeight,
+ PredFalseWeight);
+ bool SuccHasWeights = ExtractBranchMetadata(BI, SuccTrueWeight,
+ SuccFalseWeight);
+ SmallVector<uint64_t, 8> NewWeights;
+
if (PBI->getSuccessor(0) == BB) {
+ if (PredHasWeights && SuccHasWeights) {
+ // PBI: br i1 %x, BB, FalseDest
+ // BI: br i1 %y, TrueDest, FalseDest
+ //TrueWeight is TrueWeight for PBI * TrueWeight for BI.
+ NewWeights.push_back(PredTrueWeight * SuccTrueWeight);
+ //FalseWeight is FalseWeight for PBI * TotalWeight for BI +
+ // TrueWeight for PBI * FalseWeight for BI.
+ // We assume that total weights of a BranchInst can fit into 32 bits.
+ // Therefore, we will not have overflow using 64-bit arithmetic.
+ NewWeights.push_back(PredFalseWeight * (SuccFalseWeight +
+ SuccTrueWeight) + PredTrueWeight * SuccFalseWeight);
+ }
AddPredecessorToBlock(TrueDest, PredBlock, BB);
PBI->setSuccessor(0, TrueDest);
}
if (PBI->getSuccessor(1) == BB) {
+ if (PredHasWeights && SuccHasWeights) {
+ // PBI: br i1 %x, TrueDest, BB
+ // BI: br i1 %y, TrueDest, FalseDest
+ //TrueWeight is TrueWeight for PBI * TotalWeight for BI +
+ // FalseWeight for PBI * TrueWeight for BI.
+ NewWeights.push_back(PredTrueWeight * (SuccFalseWeight +
+ SuccTrueWeight) + PredFalseWeight * SuccTrueWeight);
+ //FalseWeight is FalseWeight for PBI * FalseWeight for BI.
+ NewWeights.push_back(PredFalseWeight * SuccFalseWeight);
+ }
AddPredecessorToBlock(FalseDest, PredBlock, BB);
PBI->setSuccessor(1, FalseDest);
}
+ if (NewWeights.size() == 2) {
+ // Halve the weights if any of them cannot fit in an uint32_t
+ FitWeights(NewWeights);
+
+ SmallVector<uint32_t, 8> MDWeights(NewWeights.begin(),NewWeights.end());
+ PBI->setMetadata(LLVMContext::MD_prof,
+ MDBuilder(BI->getContext()).
+ createBranchWeights(MDWeights));
+ } else
+ PBI->setMetadata(LLVMContext::MD_prof, NULL);
} else {
// Update PHI nodes in the common successors.
for (unsigned i = 0, e = PHIs.size(); i != e; ++i) {
@@ -1981,90 +1996,6 @@
// TODO: If BB is reachable from all paths through PredBlock, then we
// could replace PBI's branch probabilities with BI's.
- // Merge probability data into PredBlock's branch.
- APInt A, B, C, D;
- if (PBI->isConditional() && BI->isConditional() &&
- ExtractBranchMetadata(PBI, C, D) && ExtractBranchMetadata(BI, A, B)) {
- // Given IR which does:
- // bbA:
- // br i1 %x, label %bbB, label %bbC
- // bbB:
- // br i1 %y, label %bbD, label %bbC
- // Let's call the probability that we take the edge from %bbA to %bbB
- // 'a', from %bbA to %bbC, 'b', from %bbB to %bbD 'c' and from %bbB to
- // %bbC probability 'd'.
- //
- // We transform the IR into:
- // bbA:
- // br i1 %z, label %bbD, label %bbC
- // where the probability of going to %bbD is (a*c) and going to bbC is
- // (b+a*d).
- //
- // Probabilities aren't stored as ratios directly. Using branch weights,
- // we get:
- // (a*c)% = A*C, (b+(a*d))% = A*D+B*C+B*D.
-
- // In the event of overflow, we want to drop the LSB of the input
- // probabilities.
- unsigned BitsLost;
-
- // Ignore overflow result on ProbTrue.
- APInt ProbTrue = MultiplyAndLosePrecision(A, C, B, D, BitsLost);
-
- 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);
-
- MDBuilder MDB(BI->getContext());
- MDNode *N = MDB.createBranchWeights(ProbTrue.getZExtValue(),
- ProbFalse.getZExtValue());
- PBI->setMetadata(LLVMContext::MD_prof, N);
- } else {
- PBI->setMetadata(LLVMContext::MD_prof, NULL);
- }
- } else {
- PBI->setMetadata(LLVMContext::MD_prof, NULL);
- }
-
// Copy any debug value intrinsics into the end of PredBlock.
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
if (isa<DbgInfoIntrinsic>(*I))
Modified: llvm/trunk/test/Transforms/SimplifyCFG/preserve-branchweights.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SimplifyCFG/preserve-branchweights.ll?rev=163954&r1=163953&r2=163954&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/SimplifyCFG/preserve-branchweights.ll (original)
+++ llvm/trunk/test/Transforms/SimplifyCFG/preserve-branchweights.ll Fri Sep 14 19:39:57 2012
@@ -154,6 +154,26 @@
ret void
}
+;; This test is based on test1 but swapped the targets of the second branch.
+define void @test1_swap(i1 %a, i1 %b) {
+; CHECK: @test1_swap
+entry:
+ br i1 %a, label %Y, label %X, !prof !0
+; CHECK: br i1 %or.cond, label %Y, label %Z, !prof !4
+
+X:
+ %c = or i1 %b, false
+ br i1 %c, label %Y, label %Z, !prof !1
+
+Y:
+ call void @helper(i32 0)
+ ret void
+
+Z:
+ call void @helper(i32 1)
+ ret void
+}
+
!0 = metadata !{metadata !"branch_weights", i32 3, i32 5}
!1 = metadata !{metadata !"branch_weights", i32 1, i32 1}
!2 = metadata !{metadata !"branch_weights", i32 1, i32 2}
@@ -165,4 +185,5 @@
; CHECK: !1 = metadata !{metadata !"branch_weights", i32 1, i32 5}
; CHECK: !2 = metadata !{metadata !"branch_weights", i32 7, i32 1, i32 2}
; CHECK: !3 = metadata !{metadata !"branch_weights", i32 49, i32 12, i32 24, i32 35}
-; CHECK-NOT: !4
+; CHECK: !4 = metadata !{metadata !"branch_weights", i32 11, i32 5}
+; CHECK-NOT: !5
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