# [llvm] r261981 - [JumpThreading] Simplify Instructions first in ComputeValueKnownInPredecessors()

Haicheng Wu via llvm-commits llvm-commits at lists.llvm.org
Thu Feb 25 22:06:04 PST 2016

```Author: haicheng
Date: Fri Feb 26 00:06:04 2016
New Revision: 261981

URL: http://llvm.org/viewvc/llvm-project?rev=261981&view=rev
Log:
[JumpThreading] Simplify Instructions first in ComputeValueKnownInPredecessors()

This change tries to find more opportunities to thread over basic blocks.

Modified:

==============================================================================
+++ llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp Fri Feb 26 00:06:04 2016
@@ -152,6 +152,7 @@ namespace {
bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
PredValueInfo &Result,
ConstantPreference Preference,
+                                         bool &Changed,
Instruction *CxtI = nullptr);
ConstantPreference Preference,
@@ -395,10 +396,9 @@ static Constant *getKnownConstant(Value
///
/// This returns true if there were any known values.
///
-ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,
-                                ConstantPreference Preference,
-                                Instruction *CxtI) {
+    Value *V, BasicBlock *BB, PredValueInfo &Result,
+    ConstantPreference Preference, bool &Changed, Instruction *CxtI) {
// This method walks up use-def chains recursively.  Because of this, we could
// get into an infinite loop going around loops in the use-def chain.  To
// prevent this, keep track of what (value, block) pairs we've already visited
@@ -410,6 +410,16 @@ ComputeValueKnownInPredecessors(Value *V
// stack pops back out again.
RecursionSetRemover remover(RecursionSet, std::make_pair(V, BB));

+  // Simplify the instruction before inferring the value.
+  Instruction *I = dyn_cast<Instruction>(V);
+  if (I && !isa<PHINode>(I))
+    if (auto *NewV = SimplifyInstruction(I, BB->getModule()->getDataLayout())) {
+      I->replaceAllUsesWith(NewV);
+      I->eraseFromParent();
+      V = NewV;
+      Changed = true;
+    }
+
// If V is a constant, then it is known in all predecessors.
if (Constant *KC = getKnownConstant(V, Preference)) {
for (BasicBlock *Pred : predecessors(BB))
@@ -420,7 +430,7 @@ ComputeValueKnownInPredecessors(Value *V

// If V is a non-instruction value, or an instruction in a different block,
// then it can't be derived from a PHI.
-  Instruction *I = dyn_cast<Instruction>(V);
+  I = dyn_cast<Instruction>(V);
if (!I || I->getParent() != BB) {

// Okay, if this is a live-in value, see if it has a known value at the end
@@ -475,9 +485,9 @@ ComputeValueKnownInPredecessors(Value *V
if (I->getOpcode() == Instruction::Or ||
I->getOpcode() == Instruction::And) {
ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals,
-                                      WantInteger, CxtI);
+                                      WantInteger, Changed, CxtI);
ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals,
-                                      WantInteger, CxtI);
+                                      WantInteger, Changed, CxtI);

if (LHSVals.empty() && RHSVals.empty())
return false;
@@ -512,8 +522,8 @@ ComputeValueKnownInPredecessors(Value *V
if (I->getOpcode() == Instruction::Xor &&
isa<ConstantInt>(I->getOperand(1)) &&
cast<ConstantInt>(I->getOperand(1))->isOne()) {
-      ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result,
-                                      WantInteger, CxtI);
+      ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result, WantInteger,
+                                      Changed, CxtI);
if (Result.empty())
return false;

@@ -531,7 +541,7 @@ ComputeValueKnownInPredecessors(Value *V
if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1))) {
PredValueInfoTy LHSVals;
ComputeValueKnownInPredecessors(BO->getOperand(0), BB, LHSVals,
-                                      WantInteger, CxtI);
+                                      WantInteger, Changed, CxtI);

// Try to use constant folding to simplify the binary operator.
for (const auto &LHSVal : LHSVals) {
@@ -608,7 +618,7 @@ ComputeValueKnownInPredecessors(Value *V
if (Constant *CmpConst = dyn_cast<Constant>(Cmp->getOperand(1))) {
PredValueInfoTy LHSVals;
ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals,
-                                        WantInteger, CxtI);
+                                        WantInteger, Changed, CxtI);

for (const auto &LHSVal : LHSVals) {
Constant *V = LHSVal.first;
@@ -631,7 +641,7 @@ ComputeValueKnownInPredecessors(Value *V
PredValueInfoTy Conds;
if ((TrueVal || FalseVal) &&
ComputeValueKnownInPredecessors(SI->getCondition(), BB, Conds,
-                                        WantInteger, CxtI)) {
+                                        WantInteger, Changed, CxtI)) {
for (auto &C : Conds) {
Constant *Cond = C.first;

return false;

PredValueInfoTy PredValues;
-  if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference, CxtI))
-    return false;
+  bool Changed = false;
+  if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference,
+                                       Changed, CxtI))
+    return Changed;

assert(!PredValues.empty() &&
"ComputeValueKnownInPredecessors returned true with no values");

// If all edges were unthreadable, we fail.
if (PredToDestList.empty())
-    return false;
+    return Changed;

// Determine which is the most common successor.  If we have many inputs and
// this block is a switch, we want to start by threading the batch that goes
getSuccessor(GetBestDestForJumpOnUndef(BB));

// Ok, try to thread it!
+  Changed |= ThreadEdge(BB, PredsToFactor, MostPopularDest);
+  return Changed;
}

/// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on
@@ -1343,12 +1356,13 @@ bool JumpThreading::ProcessBranchOnXOR(B

PredValueInfoTy XorOpValues;
bool isLHS = true;
+  bool Changed = false;
if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues,
-                                       WantInteger, BO)) {
+                                       WantInteger, Changed, BO)) {
assert(XorOpValues.empty());
if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues,
-                                         WantInteger, BO))
-      return false;
+                                         WantInteger, Changed, BO))
+      return Changed;
isLHS = false;
}

@@ -1406,7 +1420,8 @@ bool JumpThreading::ProcessBranchOnXOR(B
}

// Try to duplicate BB into PredBB.
-  return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
+  Changed |= DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
+  return Changed;
}

==============================================================================
+++ llvm/trunk/test/Transforms/JumpThreading/basic.ll Fri Feb 26 00:06:04 2016
@@ -476,6 +476,56 @@ exit1:
; CHECK: }
}

+
+;;; Verify that we can handle constraint propagation through cast.
+define i32 @test16(i1 %cond) {
+Entry:
+; CHECK-LABEL: @test16(
+	br i1 %cond, label %Merge, label %F1
+
+; CHECK: Entry:
+; CHECK-NEXT:  br i1 %cond, label %F2, label %Merge
+
+F1:
+	%v1 = call i32 @f1()
+	br label %Merge
+
+Merge:
+	%B = phi i32 [1, %Entry], [%v1, %F1]
+	%M = icmp eq i32 %B, 0
+	%M1 = zext i1 %M to i32
+	%N = icmp eq i32 %M1, 1
+	br i1 %N, label %T2, label %F2
+
+; CHECK: Merge:
+; CHECK-NOT: phi
+; CHECK-NEXT:   %v1 = call i32 @f1()
+
+T2:
+	%Q = zext i1 %M to i32
+	ret i32 %Q
+
+F2:
+	ret i32 %B
+; CHECK: F2:
+; CHECK-NEXT: phi i32
+}
+
+;;; Just check that ComputeValueKnownInPredecessors() does not return true with
+;;; no values and triggers the assert in ProcessThreadableEdges().
+define i32 @test17() {
+entry:
+	%A = add i32 0, 1
+	%B = icmp eq i32 %A, 0
+	br i1 %B, label %T, label %F
+T:
+	%v1 = call i32 @f1()
+	ret i32 %v1
+F:
+	%v2 = call i32 @f2()
+	ret i32 %v2
+}
+
; In this test we check that block duplication is inhibited by the presence
; of a function with the 'noduplicate' attribute.

```