[llvm] r251557 - [JumpThreading] Use dominating conditions to prove implications
Sanjoy Das via llvm-commits
llvm-commits at lists.llvm.org
Wed Oct 28 14:27:08 PDT 2015
Author: sanjoy
Date: Wed Oct 28 16:27:08 2015
New Revision: 251557
URL: http://llvm.org/viewvc/llvm-project?rev=251557&view=rev
Log:
[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
Added:
llvm/trunk/test/Transforms/JumpThreading/implied-cond.ll
Modified:
llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp
Modified: llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp?rev=251557&r1=251556&r2=251557&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp Wed Oct 28 16:27:08 2015
@@ -29,6 +29,7 @@
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/LLVMContext.h"
@@ -57,6 +58,13 @@ BBDuplicateThreshold("jump-threading-thr
cl::desc("Max block size to duplicate for jump threading"),
cl::init(6), cl::Hidden);
+static cl::opt<unsigned>
+ImplicationSearchThreshold(
+ "jump-threading-implication-search-threshold",
+ cl::desc("The number of predecessors to search for a stronger "
+ "condition to use to thread over a weaker condition"),
+ cl::init(3), cl::Hidden);
+
namespace {
// These are at global scope so static functions can use them too.
typedef SmallVectorImpl<std::pair<Constant*, BasicBlock*> > PredValueInfo;
@@ -151,6 +159,7 @@ namespace {
bool ProcessBranchOnPHI(PHINode *PN);
bool ProcessBranchOnXOR(BinaryOperator *BO);
+ bool ProcessImpliedCondition(BasicBlock *BB);
bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
bool TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB);
@@ -868,9 +877,38 @@ bool JumpThreading::ProcessBlock(BasicBl
CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst));
+ // Search for a stronger dominating condition that can be used to simplify a
+ // conditional branch leaving BB.
+ if (ProcessImpliedCondition(BB))
+ return true;
+
+ return false;
+}
- // TODO: If we have: "br (X > 0)" and we have a predecessor where we know
- // "(X == 4)", thread through this block.
+bool JumpThreading::ProcessImpliedCondition(BasicBlock *BB) {
+ auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
+ if (!BI || !BI->isConditional())
+ return false;
+
+ Value *Cond = BI->getCondition();
+ BasicBlock *CurrentBB = BB;
+ BasicBlock *CurrentPred = BB->getSinglePredecessor();
+ unsigned Iter = 0;
+
+ while (CurrentPred && Iter++ < ImplicationSearchThreshold) {
+ auto *PBI = dyn_cast<BranchInst>(CurrentPred->getTerminator());
+ if (!PBI || !PBI->isConditional() || PBI->getSuccessor(0) != CurrentBB)
+ return false;
+
+ if (isImpliedCondition(PBI->getCondition(), Cond)) {
+ BI->getSuccessor(1)->removePredecessor(BB);
+ BranchInst::Create(BI->getSuccessor(0), BI);
+ BI->eraseFromParent();
+ return true;
+ }
+ CurrentBB = CurrentPred;
+ CurrentPred = CurrentBB->getSinglePredecessor();
+ }
return false;
}
Added: llvm/trunk/test/Transforms/JumpThreading/implied-cond.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/JumpThreading/implied-cond.ll?rev=251557&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/JumpThreading/implied-cond.ll (added)
+++ llvm/trunk/test/Transforms/JumpThreading/implied-cond.ll Wed Oct 28 16:27:08 2015
@@ -0,0 +1,98 @@
+; RUN: opt -jump-threading -S < %s | FileCheck %s
+
+declare void @side_effect(i32)
+
+define void @test0(i32 %i, i32 %len) {
+; CHECK-LABEL: @test0(
+ entry:
+ call void @side_effect(i32 0)
+ %i.inc = add nuw i32 %i, 1
+ %c0 = icmp ult i32 %i.inc, %len
+ br i1 %c0, label %left, label %right
+
+ left:
+; CHECK: entry:
+; CHECK: br i1 %c0, label %left0, label %right
+
+; CHECK: left0:
+; CHECK: call void @side_effect
+; CHECK-NOT: br i1 %c1
+; CHECK: call void @side_effect
+ call void @side_effect(i32 0)
+ %c1 = icmp ult i32 %i, %len
+ br i1 %c1, label %left0, label %right
+
+ left0:
+ call void @side_effect(i32 0)
+ ret void
+
+ right:
+ %t = phi i32 [ 1, %left ], [ 2, %entry ]
+ call void @side_effect(i32 %t)
+ ret void
+}
+
+define void @test1(i32 %i, i32 %len) {
+; CHECK-LABEL: @test1(
+ entry:
+ call void @side_effect(i32 0)
+ %i.inc = add nsw i32 %i, 1
+ %c0 = icmp slt i32 %i.inc, %len
+ br i1 %c0, label %left, label %right
+
+ left:
+; CHECK: entry:
+; CHECK: br i1 %c0, label %left0, label %right
+
+; CHECK: left0:
+; CHECK: call void @side_effect
+; CHECK-NOT: br i1 %c1
+; CHECK: call void @side_effect
+ call void @side_effect(i32 0)
+ %c1 = icmp slt i32 %i, %len
+ br i1 %c1, label %left0, label %right
+
+ left0:
+ call void @side_effect(i32 0)
+ ret void
+
+ right:
+ %t = phi i32 [ 1, %left ], [ 2, %entry ]
+ call void @side_effect(i32 %t)
+ ret void
+}
+
+define void @test2(i32 %i, i32 %len, i1* %c.ptr) {
+; CHECK-LABEL: @test2(
+
+; CHECK: entry:
+; CHECK: br i1 %c0, label %cont, label %right
+; CHECK: cont:
+; CHECK: br i1 %c, label %left0, label %right
+; CHECK: left0:
+; CHECK: call void @side_effect(i32 0)
+; CHECK: call void @side_effect(i32 0)
+ entry:
+ call void @side_effect(i32 0)
+ %i.inc = add nsw i32 %i, 1
+ %c0 = icmp slt i32 %i.inc, %len
+ br i1 %c0, label %cont, label %right
+
+ cont:
+ %c = load i1, i1* %c.ptr
+ br i1 %c, label %left, label %right
+
+ left:
+ call void @side_effect(i32 0)
+ %c1 = icmp slt i32 %i, %len
+ br i1 %c1, label %left0, label %right
+
+ left0:
+ call void @side_effect(i32 0)
+ ret void
+
+ right:
+ %t = phi i32 [ 1, %left ], [ 2, %entry ], [ 3, %cont ]
+ call void @side_effect(i32 %t)
+ ret void
+}
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