Teach the BranchProbabilityInfo analysis pass to read any metadata
encoding of probabilities. In the absense of metadata, it continues to
fall back on static heuristics.
This allows __builtin_expect, after lowering through llvm.expect
a branch instruction's metadata, to actually enter the branch
probability model. This is one component of resolving PR2577.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142492 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Analysis/BranchProbabilityInfo.cpp b/lib/Analysis/BranchProbabilityInfo.cpp
index c37987e..70de3d1 100644
--- a/lib/Analysis/BranchProbabilityInfo.cpp
+++ b/lib/Analysis/BranchProbabilityInfo.cpp
@@ -13,6 +13,8 @@
#include "llvm/Constants.h"
#include "llvm/Instructions.h"
+#include "llvm/LLVMContext.h"
+#include "llvm/Metadata.h"
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Support/Debug.h"
@@ -117,6 +119,9 @@
: BP(BP), LI(LI) {
}
+ // Metadata Weights
+ bool calcMetadataWeights(BasicBlock *BB);
+
// Return Heuristics
bool calcReturnHeuristics(BasicBlock *BB);
@@ -133,6 +138,36 @@
};
} // end anonymous namespace
+// Propagate existing explicit probabilities from either profile data or
+// 'expect' intrinsic processing.
+// FIXME: This doesn't correctly extract probabilities for switches.
+bool BranchProbabilityAnalysis::calcMetadataWeights(BasicBlock *BB) {
+ BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
+ if (!BI || !BI->isConditional())
+ return false;
+
+ MDNode *WeightsNode = BI->getMetadata(LLVMContext::MD_prof);
+ if (!WeightsNode || WeightsNode->getNumOperands() < 3)
+ return false;
+
+ // Pull the weights out of the metadata. Note that the zero operand is the
+ // name.
+ ConstantInt *Weights[] = {
+ dyn_cast<ConstantInt>(WeightsNode->getOperand(1)),
+ dyn_cast<ConstantInt>(WeightsNode->getOperand(2))
+ };
+ if (!Weights[0] || !Weights[1])
+ return false;
+
+ uint32_t WeightLimit = getMaxWeightFor(BB);
+ BP->setEdgeWeight(BB, BI->getSuccessor(0),
+ Weights[0]->getLimitedValue(WeightLimit));
+ BP->setEdgeWeight(BB, BI->getSuccessor(1),
+ Weights[1]->getLimitedValue(WeightLimit));
+
+ return true;
+}
+
// Calculate Edge Weights using "Return Heuristics". Predict a successor which
// leads directly to Return Instruction will not be taken.
bool BranchProbabilityAnalysis::calcReturnHeuristics(BasicBlock *BB){
@@ -341,6 +376,9 @@
for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
BasicBlock *BB = I++;
+ if (calcMetadataWeights(BB))
+ continue;
+
if (calcLoopBranchHeuristics(BB))
continue;
diff --git a/test/Analysis/BlockFrequencyInfo/basic.ll b/test/Analysis/BlockFrequencyInfo/basic.ll
index c09e3ff..f043dc0 100644
--- a/test/Analysis/BlockFrequencyInfo/basic.ll
+++ b/test/Analysis/BlockFrequencyInfo/basic.ll
@@ -22,3 +22,28 @@
exit:
ret i32 %sum
}
+
+define i32 @test2(i32 %i, i32 %a, i32 %b) {
+; CHECK: Printing analysis {{.*}} for function 'test2'
+; CHECK: entry = 1024
+entry:
+ %cond = icmp ult i32 %i, 42
+ br i1 %cond, label %then, label %else, !prof !0
+
+; The 'then' branch is predicted more likely via branch weight metadata.
+; CHECK: then = 963
+then:
+ br label %exit
+
+; CHECK: else = 60
+else:
+ br label %exit
+
+; FIXME: It may be a bug that we don't sum back to 1024.
+; CHECK: exit = 1023
+exit:
+ %result = phi i32 [ %a, %then ], [ %b, %else ]
+ ret i32 %result
+}
+
+!0 = metadata !{metadata !"branch_weights", i32 64, i32 4}
