[Attributor][NFC] Introduce a flag to skip liveness checks

While we can disallow AAs, liveness checks are everywhere and if the
user doesn't want them it is costly to go through just to find out
everything is assumed live.

Author: jdoerfert

llvm/include/llvm/Transforms/IPO/Attributor.h

@@ -1453,6 +1453,9 @@ struct AttributorConfig {
   /// function marked live. See also: InitializationCallback>
   bool DefaultInitializeLiveInternals = true;
 
+  /// Flag to determine if we should skip all liveness checks early on.
+  bool UseLiveness = true;
+
   /// Callback function to be invoked on internal functions marked live.
   std::function<void(Attributor &A, const Function &F)> InitializationCallback =
       nullptr;

llvm/lib/Transforms/IPO/Attributor.cpp

@@ -1407,6 +1407,8 @@ bool Attributor::isAssumedDead(const AbstractAttribute &AA,
                                const AAIsDead *FnLivenessAA,
                                bool &UsedAssumedInformation,
                                bool CheckBBLivenessOnly, DepClassTy DepClass) {
+  if (!Configuration.UseLiveness)
+    return false;
   const IRPosition &IRP = AA.getIRPosition();
   if (!Functions.count(IRP.getAnchorScope()))
     return false;
@@ -1419,6 +1421,8 @@ bool Attributor::isAssumedDead(const Use &U,
                                const AAIsDead *FnLivenessAA,
                                bool &UsedAssumedInformation,
                                bool CheckBBLivenessOnly, DepClassTy DepClass) {
+  if (!Configuration.UseLiveness)
+    return false;
   Instruction *UserI = dyn_cast<Instruction>(U.getUser());
   if (!UserI)
     return isAssumedDead(IRPosition::value(*U.get()), QueryingAA, FnLivenessAA,
@@ -1467,6 +1471,8 @@ bool Attributor::isAssumedDead(const Instruction &I,
                                bool &UsedAssumedInformation,
                                bool CheckBBLivenessOnly, DepClassTy DepClass,
                                bool CheckForDeadStore) {
+  if (!Configuration.UseLiveness)
+    return false;
   const IRPosition::CallBaseContext *CBCtx =
       QueryingAA ? QueryingAA->getCallBaseContext() : nullptr;
 
@@ -1527,6 +1533,8 @@ bool Attributor::isAssumedDead(const IRPosition &IRP,
                                const AAIsDead *FnLivenessAA,
                                bool &UsedAssumedInformation,
                                bool CheckBBLivenessOnly, DepClassTy DepClass) {
+  if (!Configuration.UseLiveness)
+    return false;
   // Don't check liveness for constants, e.g. functions, used as (floating)
   // values since the context instruction and such is here meaningless.
   if (IRP.getPositionKind() == IRPosition::IRP_FLOAT &&
@@ -1572,6 +1580,8 @@ bool Attributor::isAssumedDead(const BasicBlock &BB,
                                const AbstractAttribute *QueryingAA,
                                const AAIsDead *FnLivenessAA,
                                DepClassTy DepClass) {
+  if (!Configuration.UseLiveness)
+    return false;
   const Function &F = *BB.getParent();
   if (!FnLivenessAA || FnLivenessAA->getAnchorScope() != &F)
     FnLivenessAA = getOrCreateAAFor<AAIsDead>(IRPosition::function(F),