forked from OSchip/llvm-project
[JumpThreading] Use dominating conditions to prove implications
Summary: If P branches to Q conditional on C and Q branches to R conditional on C' and C => C' then the branch conditional on C' can be folded to an unconditional branch. Reviewers: reames Subscribers: llvm-commits Differential Revision: http://reviews.llvm.org/D13972 llvm-svn: 251557
This commit is contained in:
Sanjoy Das
parent
a83b6cc176
commit
13e63a2f21
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@ -29,6 +29,7 @@
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#include "llvm/Analysis/Loads.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Analysis/TargetLibraryInfo.h"
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#include "llvm/Analysis/ValueTracking.h"
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#include "llvm/IR/DataLayout.h"
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#include "llvm/IR/IntrinsicInst.h"
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#include "llvm/IR/LLVMContext.h"
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@ -57,6 +58,13 @@ BBDuplicateThreshold("jump-threading-threshold",
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cl::desc("Max block size to duplicate for jump threading"),
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cl::init(6), cl::Hidden);
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static cl::opt<unsigned>
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ImplicationSearchThreshold(
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"jump-threading-implication-search-threshold",
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cl::desc("The number of predecessors to search for a stronger "
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"condition to use to thread over a weaker condition"),
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cl::init(3), cl::Hidden);
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namespace {
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// These are at global scope so static functions can use them too.
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typedef SmallVectorImpl<std::pair<Constant*, BasicBlock*> > PredValueInfo;
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@ -151,6 +159,7 @@ namespace {
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bool ProcessBranchOnPHI(PHINode *PN);
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bool ProcessBranchOnXOR(BinaryOperator *BO);
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bool ProcessImpliedCondition(BasicBlock *BB);
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bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
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bool TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB);
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@ -868,9 +877,38 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) {
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CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
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return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst));
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// Search for a stronger dominating condition that can be used to simplify a
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// conditional branch leaving BB.
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if (ProcessImpliedCondition(BB))
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return true;
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// TODO: If we have: "br (X > 0)" and we have a predecessor where we know
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// "(X == 4)", thread through this block.
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return false;
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}
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bool JumpThreading::ProcessImpliedCondition(BasicBlock *BB) {
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auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
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if (!BI || !BI->isConditional())
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return false;
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Value *Cond = BI->getCondition();
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BasicBlock *CurrentBB = BB;
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BasicBlock *CurrentPred = BB->getSinglePredecessor();
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unsigned Iter = 0;
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while (CurrentPred && Iter++ < ImplicationSearchThreshold) {
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auto *PBI = dyn_cast<BranchInst>(CurrentPred->getTerminator());
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if (!PBI || !PBI->isConditional() || PBI->getSuccessor(0) != CurrentBB)
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return false;
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if (isImpliedCondition(PBI->getCondition(), Cond)) {
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BI->getSuccessor(1)->removePredecessor(BB);
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BranchInst::Create(BI->getSuccessor(0), BI);
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BI->eraseFromParent();
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return true;
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}
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CurrentBB = CurrentPred;
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CurrentPred = CurrentBB->getSinglePredecessor();
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}
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return false;
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}
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@ -0,0 +1,98 @@
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; RUN: opt -jump-threading -S < %s | FileCheck %s
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declare void @side_effect(i32)
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define void @test0(i32 %i, i32 %len) {
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; CHECK-LABEL: @test0(
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entry:
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call void @side_effect(i32 0)
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%i.inc = add nuw i32 %i, 1
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%c0 = icmp ult i32 %i.inc, %len
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br i1 %c0, label %left, label %right
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left:
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; CHECK: entry:
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; CHECK: br i1 %c0, label %left0, label %right
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; CHECK: left0:
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; CHECK: call void @side_effect
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; CHECK-NOT: br i1 %c1
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; CHECK: call void @side_effect
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call void @side_effect(i32 0)
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%c1 = icmp ult i32 %i, %len
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br i1 %c1, label %left0, label %right
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left0:
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call void @side_effect(i32 0)
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ret void
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right:
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%t = phi i32 [ 1, %left ], [ 2, %entry ]
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call void @side_effect(i32 %t)
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ret void
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}
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define void @test1(i32 %i, i32 %len) {
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; CHECK-LABEL: @test1(
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entry:
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call void @side_effect(i32 0)
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%i.inc = add nsw i32 %i, 1
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%c0 = icmp slt i32 %i.inc, %len
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br i1 %c0, label %left, label %right
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left:
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; CHECK: entry:
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; CHECK: br i1 %c0, label %left0, label %right
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; CHECK: left0:
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; CHECK: call void @side_effect
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; CHECK-NOT: br i1 %c1
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; CHECK: call void @side_effect
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call void @side_effect(i32 0)
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%c1 = icmp slt i32 %i, %len
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br i1 %c1, label %left0, label %right
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left0:
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call void @side_effect(i32 0)
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ret void
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right:
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%t = phi i32 [ 1, %left ], [ 2, %entry ]
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call void @side_effect(i32 %t)
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ret void
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}
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define void @test2(i32 %i, i32 %len, i1* %c.ptr) {
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; CHECK-LABEL: @test2(
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; CHECK: entry:
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; CHECK: br i1 %c0, label %cont, label %right
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; CHECK: cont:
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; CHECK: br i1 %c, label %left0, label %right
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; CHECK: left0:
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; CHECK: call void @side_effect(i32 0)
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; CHECK: call void @side_effect(i32 0)
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entry:
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call void @side_effect(i32 0)
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%i.inc = add nsw i32 %i, 1
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%c0 = icmp slt i32 %i.inc, %len
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br i1 %c0, label %cont, label %right
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cont:
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%c = load i1, i1* %c.ptr
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br i1 %c, label %left, label %right
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left:
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call void @side_effect(i32 0)
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%c1 = icmp slt i32 %i, %len
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br i1 %c1, label %left0, label %right
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left0:
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call void @side_effect(i32 0)
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ret void
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right:
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%t = phi i32 [ 1, %left ], [ 2, %entry ], [ 3, %cont ]
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call void @side_effect(i32 %t)
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ret void
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}
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