[Inliner] Add a flag to disable manual alloca merging in the Inliner.

This is off for now while testing can take place to make sure that in
fact we do sufficient stack coloring to fully obviate the manual alloca
array merging.

Some context on why we should be using stack coloring rather than
merging allocas in this way:

LLVM relies very heavily on analyzing pointers as coming from different
allocas in order to make aliasing decisions. These are some of the most
powerful aliasing signals available in LLVM. So merging allocas is an
extremely destructive operation on the LLVM IR -- it takes away highly
valuable and hard to reconstruct information.

As a consequence, inlined functions which happen to have array allocas
that this pattern matches will fail to be properly interleaved unless
SROA manages to hoist everything to an SSA register. Instead, the
inliner will have added an unnecessary dependence that one inlined
function execute after the other because they will have been rewritten
to refer to the same memory.

All that said, folks will reasonably want some time to experiment here
and make sure there are no significant regressions. A flag should give
us an easy knob to test.

For more context, see the thread here:
http://lists.llvm.org/pipermail/llvm-dev/2016-July/103277.html
http://lists.llvm.org/pipermail/llvm-dev/2016-August/103285.html

Differential Revision: https://reviews.llvm.org/D23052

llvm-svn: 278892

llvm/lib/Transforms/IPO/Inliner.cpp

@@ -48,6 +48,17 @@ STATISTIC(NumMergedAllocas, "Number of allocas merged together");
 // if those would be more profitable and blocked inline steps.
 STATISTIC(NumCallerCallersAnalyzed, "Number of caller-callers analyzed");
 
+/// 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);
+
 namespace {
 enum class InlinerFunctionImportStatsOpts {
   No = 0,
@@ -84,55 +95,29 @@ void Inliner::getAnalysisUsage(AnalysisUsage &AU) const {
 
 typedef DenseMap<ArrayType *, std::vector<AllocaInst *>> InlinedArrayAllocasTy;
 
-/// If it is possible to inline the specified call site,
-/// do so and update the CallGraph for this operation.
+/// 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.
 ///
-/// 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.
-static bool InlineCallIfPossible(
-    CallSite CS, InlineFunctionInfo &IFI,
-    InlinedArrayAllocasTy &InlinedArrayAllocas, int InlineHistory,
-    bool InsertLifetime, function_ref<AAResults &(Function &)> AARGetter,
-    ImportedFunctionsInliningStatistics &ImportedFunctionsStats) {
-  Function *Callee = CS.getCalledFunction();
-  Function *Caller = CS.getCaller();
-
-  AAResults &AAR = AARGetter(*Callee);
-
-  // Try to inline the function.  Get the list of static allocas that were
-  // inlined.
-  if (!InlineFunction(CS, IFI, &AAR, InsertLifetime))
-    return false;
-
-  if (InlinerFunctionImportStats != InlinerFunctionImportStatsOpts::No)
-    ImportedFunctionsStats.recordInline(*Caller, *Callee);
-
-  AttributeFuncs::mergeAttributesForInlining(*Caller, *Callee);
-
-  // 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.
-  //
+/// 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 CS was inlined from some other
@@ -148,7 +133,7 @@ static bool InlineCallIfPossible(
   // 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 true;
+    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.
@@ -234,6 +219,38 @@ static bool InlineCallIfPossible(
     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.
+static bool InlineCallIfPossible(
+    CallSite CS, InlineFunctionInfo &IFI,
+    InlinedArrayAllocasTy &InlinedArrayAllocas, int InlineHistory,
+    bool InsertLifetime, function_ref<AAResults &(Function &)> &AARGetter,
+    ImportedFunctionsInliningStatistics &ImportedFunctionsStats) {
+  Function *Callee = CS.getCalledFunction();
+  Function *Caller = CS.getCaller();
+
+  AAResults &AAR = AARGetter(*Callee);
+
+  // Try to inline the function.  Get the list of static allocas that were
+  // inlined.
+  if (!InlineFunction(CS, IFI, &AAR, InsertLifetime))
+    return false;
+
+  if (InlinerFunctionImportStats != InlinerFunctionImportStatsOpts::No)
+    ImportedFunctionsStats.recordInline(*Caller, *Callee);
+
+  AttributeFuncs::mergeAttributesForInlining(*Caller, *Callee);
+
+  if (!DisableInlinedAllocaMerging)
+    mergeInlinedArrayAllocas(Caller, IFI, InlinedArrayAllocas, InlineHistory);
 
   return true;
 }