[BDCE][DemandedBits] Detect dead uses of undead instructions

This (mostly) fixes https://bugs.llvm.org/show_bug.cgi?id=39771.

BDCE currently detects instructions that don't have any demanded bits
and replaces their uses with zero. However, if an instruction has
multiple uses, then some of the uses may be dead (have no demanded bits)
even though the instruction itself is still live. This patch extends
DemandedBits/BDCE to detect such uses and replace them with zero.
While this will not immediately render any instructions dead, it may
lead to simplifications (in the motivating case, by converting a rotate
into a simple shift), break dependencies, etc.

The implementation tries to strike a balance between analysis power and
complexity/memory usage. Originally I wanted to track demanded bits on
a per-use level, but ultimately we're only really interested in whether
a use is entirely dead or not. I'm using an extra set to track which uses
are dead. However, as initially all uses are dead, I'm not storing uses
those user is also dead. This case is checked separately instead.

The test case has a couple of cases that are not simplified yet. In
particular, we're only looking at uses of instructions right now. I think
it would make sense to also extend this to arguments. Furthermore
DemandedBits doesn't yet know some of the tricks that InstCombine does
for the demanded bits or bitwise or/and/xor in combination with known
bits information.

Differential Revision: https://reviews.llvm.org/D55563

llvm-svn: 349674

llvm/include/llvm/Analysis/DemandedBits.h

@@ -57,6 +57,9 @@ class DemandedBits {
   /// Return true if, during analysis, I could not be reached.
   bool isInstructionDead(Instruction *I);
 
+  /// Return whether this use is dead by means of not having any demanded bits.
+  bool isUseDead(Use *U);
+
   void print(raw_ostream &OS);
 
 private:
@@ -75,6 +78,9 @@ class DemandedBits {
   // The set of visited instructions (non-integer-typed only).
   SmallPtrSet<Instruction*, 32> Visited;
   DenseMap<Instruction *, APInt> AliveBits;
+  // Uses with no demanded bits. If the user also has no demanded bits, the use
+  // might not be stored explicitly in this map, to save memory during analysis.
+  SmallPtrSet<Use *, 16> DeadUses;
 };
 
 class DemandedBitsWrapperPass : public FunctionPass {

llvm/lib/Analysis/DemandedBits.cpp

@@ -314,6 +314,7 @@ void DemandedBits::performAnalysis() {
 
   Visited.clear();
   AliveBits.clear();
+  DeadUses.clear();
 
   SmallVector<Instruction*, 128> Worklist;
 
@@ -374,26 +375,35 @@ void DemandedBits::performAnalysis() {
         Type *T = I->getType();
         if (T->isIntOrIntVectorTy()) {
           unsigned BitWidth = T->getScalarSizeInBits();
+
+          // Previous demanded bits information for this use.
+          APInt ABPrev(BitWidth, 0);
+          auto ABI = AliveBits.find(I);
+          if (ABI != AliveBits.end())
+            ABPrev = ABI->second;
+
           APInt AB = APInt::getAllOnesValue(BitWidth);
           if (UserI->getType()->isIntOrIntVectorTy() && !AOut &&
               !isAlwaysLive(UserI)) {
+            // If all bits of the output are dead, then all bits of the input
+            // are also dead.
             AB = APInt(BitWidth, 0);
           } else {
-            // If all bits of the output are dead, then all bits of the input
             // Bits of each operand that are used to compute alive bits of the
             // output are alive, all others are dead.
             determineLiveOperandBits(UserI, I, OI.getOperandNo(), AOut, AB,
                                      Known, Known2);
+
+            // Keep track of uses which have no demanded bits.
+            if (AB.isNullValue())
+              DeadUses.insert(&OI);
+            else if (ABPrev.isNullValue())
+              DeadUses.erase(&OI);
           }
 
           // If we've added to the set of alive bits (or the operand has not
           // been previously visited), then re-queue the operand to be visited
           // again.
-          APInt ABPrev(BitWidth, 0);
-          auto ABI = AliveBits.find(I);
-          if (ABI != AliveBits.end())
-            ABPrev = ABI->second;
-
           APInt ABNew = AB | ABPrev;
           if (ABNew != ABPrev || ABI == AliveBits.end()) {
             AliveBits[I] = std::move(ABNew);
@@ -426,6 +436,31 @@ bool DemandedBits::isInstructionDead(Instruction *I) {
     !isAlwaysLive(I);
 }
 
+bool DemandedBits::isUseDead(Use *U) {
+  // We only track integer uses, everything else is assumed live.
+  if (!(*U)->getType()->isIntOrIntVectorTy())
+    return false;
+
+  // Uses by always-live instructions are never dead.
+  Instruction *UserI = cast<Instruction>(U->getUser());
+  if (isAlwaysLive(UserI))
+    return false;
+
+  performAnalysis();
+  if (DeadUses.count(U))
+    return true;
+
+  // If no output bits are demanded, no input bits are demanded and the use
+  // is dead. These uses might not be explicitly present in the DeadUses map.
+  if (UserI->getType()->isIntOrIntVectorTy()) {
+    auto Found = AliveBits.find(UserI);
+    if (Found != AliveBits.end() && Found->second.isNullValue())
+      return true;
+  }
+
+  return false;
+}
+
 void DemandedBits::print(raw_ostream &OS) {
   performAnalysis();
   for (auto &KV : AliveBits) {

llvm/lib/Transforms/Scalar/BDCE.cpp

@@ -96,30 +96,38 @@ static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
     if (I.mayHaveSideEffects() && I.use_empty())
       continue;
 
-    if (I.getType()->isIntOrIntVectorTy() &&
-        !DB.getDemandedBits(&I).getBoolValue()) {
-      // For live instructions that have all dead bits, first make them dead by
-      // replacing all uses with something else. Then, if they don't need to
-      // remain live (because they have side effects, etc.) we can remove them.
-      LLVM_DEBUG(dbgs() << "BDCE: Trivializing: " << I << " (all bits dead)\n");
+    // Remove instructions not reached during analysis.
+    if (DB.isInstructionDead(&I)) {
+      salvageDebugInfo(I);
+      Worklist.push_back(&I);
+      I.dropAllReferences();
+      Changed = true;
+      continue;
+    }
+
+    for (Use &U : I.operands()) {
+      // DemandedBits only detects dead integer uses.
+      if (!U->getType()->isIntOrIntVectorTy())
+        continue;
+
+      // TODO: We could also find dead non-instruction uses, e.g. arguments.
+      if (!isa<Instruction>(U))
+        continue;
+
+      if (!DB.isUseDead(&U))
+        continue;
+
+      LLVM_DEBUG(dbgs() << "BDCE: Trivializing: " << U << " (all bits dead)\n");
 
       clearAssumptionsOfUsers(&I, DB);
 
       // FIXME: In theory we could substitute undef here instead of zero.
       // This should be reconsidered once we settle on the semantics of
       // undef, poison, etc.
-      Value *Zero = ConstantInt::get(I.getType(), 0);
+      U.set(ConstantInt::get(U->getType(), 0));
       ++NumSimplified;
-      I.replaceNonMetadataUsesWith(Zero);
       Changed = true;
     }
-    if (!DB.isInstructionDead(&I))
-      continue;
-
-    salvageDebugInfo(I);
-    Worklist.push_back(&I);
-    I.dropAllReferences();
-    Changed = true;
   }
 
   for (Instruction *&I : Worklist) {

llvm/test/Transforms/BDCE/dead-uses.ll

@@ -10,7 +10,7 @@ declare <2 x i32> @llvm.fshr.v2i32(<2 x i32>, <2 x i32>, <2 x i32>)
 define i32 @pr39771_fshr_multi_use_instr(i32 %a) {
 ; CHECK-LABEL: @pr39771_fshr_multi_use_instr(
 ; CHECK-NEXT:    [[X:%.*]] = or i32 [[A:%.*]], 0
-; CHECK-NEXT:    [[B:%.*]] = tail call i32 @llvm.fshr.i32(i32 [[X]], i32 [[X]], i32 1)
+; CHECK-NEXT:    [[B:%.*]] = tail call i32 @llvm.fshr.i32(i32 0, i32 [[X]], i32 1)
 ; CHECK-NEXT:    [[C:%.*]] = lshr i32 [[B]], 23
 ; CHECK-NEXT:    [[D:%.*]] = xor i32 [[C]], [[B]]
 ; CHECK-NEXT:    [[E:%.*]] = and i32 [[D]], 31
@@ -28,7 +28,7 @@ define i32 @pr39771_fshr_multi_use_instr(i32 %a) {
 define <2 x i32> @pr39771_fshr_multi_use_instr_vec(<2 x i32> %a) {
 ; CHECK-LABEL: @pr39771_fshr_multi_use_instr_vec(
 ; CHECK-NEXT:    [[X:%.*]] = or <2 x i32> [[A:%.*]], zeroinitializer
-; CHECK-NEXT:    [[B:%.*]] = tail call <2 x i32> @llvm.fshr.v2i32(<2 x i32> [[X]], <2 x i32> [[X]], <2 x i32> <i32 1, i32 1>)
+; CHECK-NEXT:    [[B:%.*]] = tail call <2 x i32> @llvm.fshr.v2i32(<2 x i32> zeroinitializer, <2 x i32> [[X]], <2 x i32> <i32 1, i32 1>)
 ; CHECK-NEXT:    [[C:%.*]] = lshr <2 x i32> [[B]], <i32 23, i32 23>
 ; CHECK-NEXT:    [[D:%.*]] = xor <2 x i32> [[C]], [[B]]
 ; CHECK-NEXT:    [[E:%.*]] = and <2 x i32> [[D]], <i32 31, i32 31>