[Inliner] Remove alloca merging

[aeubanks]

Summary

See https://discourse.llvm.org/t/rfc-we-should-stop-merging-allocas-in-the-inliner/41975 for the RFC from 2016 around this.

Backend stack coloring takes care of what this initially set out to help with.

This only affects the legacy inliner, which for the most part doesn't matter anymore. The new PM inliner never looked at this flag.

There are some backends running the legacy always-inline pass, but that's unlikely to matter.

[asbirlea]

Adding a line in release notes might be useful; next to a note with the removal of legacy PM pieces (e.g. PassManagerBuilder).

[aeubanks]

In D146091#4197717, @asbirlea wrote:

Adding a line in release notes might be useful; next to a note with the removal of legacy PM pieces (e.g. PassManagerBuilder).

https://reviews.llvm.org/D146243

llvm/lib/Transforms/IPO/Inliner.cpp

 
 using namespace llvm;
 
 #define DEBUG_TYPE "inline"
 
 STATISTIC(NumInlined, "Number of functions inlined");
 STATISTIC(NumCallsDeleted, "Number of call sites deleted, not inlined");
 STATISTIC(NumDeleted, "Number of functions deleted because all callers found");
-STATISTIC(NumMergedAllocas, "Number of allocas merged together");
-
-/// Flag to disable manual alloca merging.
-///
-/// Merging of allocas was originally done as a stack-size saving technique
-/// prior to LLVM's code generator having support for stack coloring based on
-/// lifetime markers. It is now in the process of being removed. To experiment
-/// with disabling it and relying fully on lifetime marker based stack
-/// coloring, you can pass this flag to LLVM.
-static cl::opt<bool>
-    DisableInlinedAllocaMerging("disable-inlined-alloca-merging",
-                                cl::init(false), cl::Hidden);
 
 static cl::opt<int> IntraSCCCostMultiplier(
     "intra-scc-cost-multiplier", cl::init(2), cl::Hidden,
     cl::desc(
         "Cost multiplier to multiply onto inlined call sites where the "
         "new call was previously an intra-SCC call (not relevant when the "
         "original call was already intra-SCC). This can accumulate over "
         "multiple inlinings (e.g. if a call site already had a cost "
void LegacyInlinerBase::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.addRequired<ProfileSummaryInfoWrapperPass>();
   AU.addRequired<TargetLibraryInfoWrapperPass>();
   getAAResultsAnalysisUsage(AU);
   CallGraphSCCPass::getAnalysisUsage(AU);
 }
 
 using InlinedArrayAllocasTy = DenseMap<ArrayType *, std::vector<AllocaInst *>>;
 
-/// Look at all of the allocas that we inlined through this call site.  If we
-/// have already inlined other allocas through other calls into this function,
-/// then we know that they have disjoint lifetimes and that we can merge them.
-///
-/// There are many heuristics possible for merging these allocas, and the
-/// different options have different tradeoffs.  One thing that we *really*
-/// don't want to hurt is SRoA: once inlining happens, often allocas are no
-/// longer address taken and so they can be promoted.
-///
-/// Our "solution" for that is to only merge allocas whose outermost type is an
-/// array type.  These are usually not promoted because someone is using a
-/// variable index into them.  These are also often the most important ones to
-/// merge.
-///
-/// A better solution would be to have real memory lifetime markers in the IR
-/// and not have the inliner do any merging of allocas at all.  This would
-/// allow the backend to do proper stack slot coloring of all allocas that
-/// *actually make it to the backend*, which is really what we want.
-///
-/// Because we don't have this information, we do this simple and useful hack.
-static void mergeInlinedArrayAllocas(Function *Caller, InlineFunctionInfo &IFI,
-                                     InlinedArrayAllocasTy &InlinedArrayAllocas,
-                                     int InlineHistory) {
-  SmallPtrSet<AllocaInst *, 16> UsedAllocas;
-
-  // When processing our SCC, check to see if the call site was inlined from
-  // some other call site.  For example, if we're processing "A" in this code:
-  //   A() { B() }
-  //   B() { x = alloca ... C() }
-  //   C() { y = alloca ... }
-  // Assume that C was not inlined into B initially, and so we're processing A
-  // and decide to inline B into A.  Doing this makes an alloca available for
-  // reuse and makes a callsite (C) available for inlining.  When we process
-  // the C call site we don't want to do any alloca merging between X and Y
-  // because their scopes are not disjoint.  We could make this smarter by
-  // keeping track of the inline history for each alloca in the
-  // InlinedArrayAllocas but this isn't likely to be a significant win.
-  if (InlineHistory != -1) // Only do merging for top-level call sites in SCC.
-    return;
-
-  // Loop over all the allocas we have so far and see if they can be merged with
-  // a previously inlined alloca.  If not, remember that we had it.
-  for (unsigned AllocaNo = 0, E = IFI.StaticAllocas.size(); AllocaNo != E;
-       ++AllocaNo) {
-    AllocaInst *AI = IFI.StaticAllocas[AllocaNo];
-
-    // Don't bother trying to merge array allocations (they will usually be
-    // canonicalized to be an allocation *of* an array), or allocations whose
-    // type is not itself an array (because we're afraid of pessimizing SRoA).
-    ArrayType *ATy = dyn_cast<ArrayType>(AI->getAllocatedType());
-    if (!ATy || AI->isArrayAllocation())
-      continue;
-
-    // Get the list of all available allocas for this array type.
-    std::vector<AllocaInst *> &AllocasForType = InlinedArrayAllocas[ATy];
-
-    // Loop over the allocas in AllocasForType to see if we can reuse one.  Note
-    // that we have to be careful not to reuse the same "available" alloca for
-    // multiple different allocas that we just inlined, we use the 'UsedAllocas'
-    // set to keep track of which "available" allocas are being used by this
-    // function.  Also, AllocasForType can be empty of course!
-    bool MergedAwayAlloca = false;
-    for (AllocaInst *AvailableAlloca : AllocasForType) {
-      Align Align1 = AI->getAlign();
-      Align Align2 = AvailableAlloca->getAlign();
-
-      // The available alloca has to be in the right function, not in some other
-      // function in this SCC.
-      if (AvailableAlloca->getParent() != AI->getParent())
-        continue;
-
-      // If the inlined function already uses this alloca then we can't reuse
-      // it.
-      if (!UsedAllocas.insert(AvailableAlloca).second)
-        continue;
-
-      // Otherwise, we *can* reuse it, RAUW AI into AvailableAlloca and declare
-      // success!
-      LLVM_DEBUG(dbgs() << "    ***MERGED ALLOCA: " << *AI
-                        << "\n\t\tINTO: " << *AvailableAlloca << '\n');
-
-      // Move affected dbg.declare calls immediately after the new alloca to
-      // avoid the situation when a dbg.declare precedes its alloca.
-      if (auto *L = LocalAsMetadata::getIfExists(AI))
-        if (auto *MDV = MetadataAsValue::getIfExists(AI->getContext(), L))
-          for (User *U : MDV->users())
-            if (DbgDeclareInst *DDI = dyn_cast<DbgDeclareInst>(U))
-              DDI->moveBefore(AvailableAlloca->getNextNode());
-
-      AI->replaceAllUsesWith(AvailableAlloca);
-
-      if (Align1 > Align2)
-        AvailableAlloca->setAlignment(AI->getAlign());
-
-      AI->eraseFromParent();
-      MergedAwayAlloca = true;
-      ++NumMergedAllocas;
-      IFI.StaticAllocas[AllocaNo] = nullptr;
-      break;
-    }
-
-    // If we already nuked the alloca, we're done with it.
-    if (MergedAwayAlloca)
-      continue;
-
-    // If we were unable to merge away the alloca either because there are no
-    // allocas of the right type available or because we reused them all
-    // already, remember that this alloca came from an inlined function and mark
-    // it used so we don't reuse it for other allocas from this inline
-    // operation.
-    AllocasForType.push_back(AI);
-    UsedAllocas.insert(AI);
-  }
-}
-
 /// If it is possible to inline the specified call site,
 /// do so and update the CallGraph for this operation.
 ///
 /// This function also does some basic book-keeping to update the IR.  The
 /// InlinedArrayAllocas map keeps track of any allocas that are already
 /// available from other functions inlined into the caller.  If we are able to
 /// inline this call site we attempt to reuse already available allocas or add
 /// any new allocas to the set if not possible.
InlineResult IR =
       InlineFunction(CB, IFI,
                      /*MergeAttributes=*/true, &AAR, InsertLifetime);
   if (!IR.isSuccess())
     return IR;
 
   if (InlinerFunctionImportStats != InlinerFunctionImportStatsOpts::No)
     ImportedFunctionsStats.recordInline(*Caller, *Callee);
 
-  if (!DisableInlinedAllocaMerging)
-    mergeInlinedArrayAllocas(Caller, IFI, InlinedArrayAllocas, InlineHistory);
-
   return IR; // success
 }
 
 /// Return true if the specified inline history ID
 /// indicates an inline history that includes the specified function.
 static bool inlineHistoryIncludes(
     Function *F, int InlineHistoryID,
     const SmallVectorImpl<std::pair<Function *, int>> &InlineHistory) {