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Teach simplifycfg to recompute branch weights when merging some branches, and

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to discard weights when appropriate. Still more to do (and a new TODO), but
it's a start!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147286 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Nick Lewycky 2011-12-27 04:31:52 +00:00
parent da32cc6176
commit 06cc66f65a
2 changed files with 131 additions and 2 deletions
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67
lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -18,6 +18,8 @@
#include "llvm/GlobalVariable.h"
#include "llvm/Instructions.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/LLVMContext.h"
#include "llvm/Metadata.h"
#include "llvm/Type.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/ValueTracking.h"
@ -1462,6 +1464,26 @@ static bool SimplifyCondBranchToTwoReturns(BranchInst *BI,
return true;
}
/// ExtractBranchMetadata - Given a conditional BranchInstruction, retrieve the
/// probabilities of the branch taking each edge. Fills in the two APInt
/// parameters and return true, or returns false if no or invalid metadata was
/// found.
static bool ExtractBranchMetadata(BranchInst *BI,
APInt &ProbTrue, APInt &ProbFalse) {
assert(BI->isConditional() &&
"Looking for probabilities on unconditional branch?");
MDNode *ProfileData = BI->getMetadata(LLVMContext::MD_prof);
if (!ProfileData || ProfileData->getNumOperands() != 3) return 0;
ConstantInt *CITrue = dyn_cast<ConstantInt>(ProfileData->getOperand(1));
ConstantInt *CIFalse = dyn_cast<ConstantInt>(ProfileData->getOperand(2));
if (!CITrue || !CIFalse) return 0;
ProbTrue = CITrue->getValue();
ProbFalse = CIFalse->getValue();
assert(ProbTrue.getBitWidth() == 32 && ProbFalse.getBitWidth() == 32 &&
"Branch probability metadata must be 32-bit integers");
return true;
}
/// 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.
@ -1636,6 +1658,51 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI) {
PBI->setSuccessor(1, FalseDest);
}
// 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 (ExtractBranchMetadata(PBI, C, D) && ExtractBranchMetadata(BI, A, B)) {
// bbA: br bbB (a% probability), bbC (b% prob.)
// bbB: br bbD (c% probability), bbC (d% prob.)
// --> bbA: br bbD ((a*c)% prob.), bbC ((b+a*d)% prob.)
//
// Probabilities aren't stored as ratios directly. Converting to
// probability-numerator form, 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);
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);
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);
}
// 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))

66
test/Transforms/SimplifyCFG/preserve-branchweights.ll
View File

@ -8,6 +8,45 @@ entry:
br i1 %a, label %Y, label %X, !prof !0
; CHECK: br i1 %or.cond, label %Z, label %Y, !prof !0
X:
%c = or i1 %b, false
br i1 %c, label %Z, label %Y, !prof !1
Y:
call void @helper(i32 0)
ret void
Z:
call void @helper(i32 1)
ret void
}
define void @test2(i1 %a, i1 %b) {
; CHECK: @test2
entry:
br i1 %a, label %X, label %Y, !prof !1
; CHECK: br i1 %or.cond, label %Z, label %Y, !prof !1
; CHECK-NOT: !prof
X:
%c = or i1 %b, false
br i1 %c, label %Z, label %Y, !prof !2
Y:
call void @helper(i32 0)
ret void
Z:
call void @helper(i32 1)
ret void
}
define void @test3(i1 %a, i1 %b) {
; CHECK: @test3
; CHECK-NOT: !prof
entry:
br i1 %a, label %X, label %Y, !prof !1
X:
%c = or i1 %b, false
br i1 %c, label %Z, label %Y
@ -21,6 +60,29 @@ Z:
ret void
}
!0 = metadata !{metadata !"branch_weights", i32 1, i32 2}
define void @test4(i1 %a, i1 %b) {
; CHECK: @test4
; CHECK-NOT: !prof
entry:
br i1 %a, label %X, label %Y
; CHECK: !0 = metadata !{metadata !"branch_weights", i32 2, i32 1}
X:
%c = or i1 %b, false
br i1 %c, label %Z, label %Y, !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}
; CHECK: !0 = metadata !{metadata !"branch_weights", i32 5, i32 11}
; CHECK: !1 = metadata !{metadata !"branch_weights", i32 1, i32 5}
; CHECK-NOT: !2