[HotColdSplitting] Use TTI to inform outlining threshold

Using TargetTransformInfo allows the splitting pass to factor in the
code size cost of instructions as it decides whether or not outlining is
profitable.

This did not regress the overall amount of outlining seen on the handful
of internal frameworks I tested.

Thanks to Jun Bum Lim for suggesting this!

Differential Revision: https://reviews.llvm.org/D53835

llvm-svn: 346108

1 files changed, 26 insertions, 18 deletions

diff --git a/llvm/lib/Transforms/IPO/HotColdSplitting.cpp b/llvm/lib/Transforms/IPO/HotColdSplitting.cpp
index ce8a5060a3a..621ac7dc8ab 100644
--- a/llvm/lib/Transforms/IPO/HotColdSplitting.cpp
+++ b/llvm/lib/Transforms/IPO/HotColdSplitting.cpp
@@ -66,10 +66,10 @@ using namespace llvm;
 static cl::opt<bool> EnableStaticAnalyis("hot-cold-static-analysis",
                               cl::init(true), cl::Hidden);
 
-static cl::opt<unsigned> MinOutliningInstCount(
-    "min-outlining-inst-count", cl::init(3), cl::Hidden,
-    cl::desc("Minimum number of instructions needed for a single-block region "
-             "to be an outlining candidate"));
+static cl::opt<int>
+    MinOutliningThreshold("min-outlining-thresh", cl::init(3), cl::Hidden,
+                          cl::desc("Code size threshold for outlining within a "
+                                   "single BB (as a multiple of TCC_Basic)"));
 
 namespace {
 
@@ -135,14 +135,18 @@ static bool mayExtractBlock(const BasicBlock &BB) {
   return !BB.hasAddressTaken();
 }
 
-/// Check whether \p BB has at least \p Min non-debug, non-terminator
-/// instructions.
-static bool hasMinimumInstCount(const BasicBlock &BB, unsigned Min) {
-  unsigned Count = 0;
+/// Check whether \p BB is profitable to outline (i.e. its code size cost meets
+/// the threshold set in \p MinOutliningThreshold).
+static bool isProfitableToOutline(const BasicBlock &BB,
+                                  TargetTransformInfo &TTI) {
+  int Cost = 0;
   for (const Instruction &I : BB) {
     if (isa<DbgInfoIntrinsic>(&I) || &I == BB.getTerminator())
       continue;
-    if (++Count >= Min)
+
+    Cost += TTI.getInstructionCost(&I, TargetTransformInfo::TCK_CodeSize);
+
+    if (Cost >= (MinOutliningThreshold * TargetTransformInfo::TCC_Basic))
       return true;
   }
   return false;
@@ -156,8 +160,10 @@ static bool hasMinimumInstCount(const BasicBlock &BB, unsigned Min) {
 ///
 /// Return an empty sequence if the cold region is too small to outline, or if
 /// the cold region has no warm predecessors.
-static BlockSequence
-findMaximalColdRegion(BasicBlock &SinkBB, DominatorTree &DT, PostDomTree &PDT) {
+static BlockSequence findMaximalColdRegion(BasicBlock &SinkBB,
+                                           TargetTransformInfo &TTI,
+                                           DominatorTree &DT,
+                                           PostDomTree &PDT) {
   // The maximal cold region.
   BlockSequence ColdRegion = {};
 
@@ -241,8 +247,7 @@ findMaximalColdRegion(BasicBlock &SinkBB, DominatorTree &DT, PostDomTree &PDT) {
     ++SuccIt;
   }
 
-  if (ColdRegion.size() == 1 &&
-      !hasMinimumInstCount(*ColdRegion[0], MinOutliningInstCount))
+  if (ColdRegion.size() == 1 && !isProfitableToOutline(*ColdRegion[0], TTI))
     return {};
 
   return ColdRegion;
@@ -251,6 +256,7 @@ findMaximalColdRegion(BasicBlock &SinkBB, DominatorTree &DT, PostDomTree &PDT) {
 /// Get the largest cold region in \p F.
 static BlockSequence getLargestColdRegion(Function &F, ProfileSummaryInfo &PSI,
                                           BlockFrequencyInfo *BFI,
+                                          TargetTransformInfo &TTI,
                                           DominatorTree &DT, PostDomTree &PDT) {
   // Keep track of the largest cold region.
   BlockSequence LargestColdRegion = {};
@@ -270,7 +276,7 @@ static BlockSequence getLargestColdRegion(Function &F, ProfileSummaryInfo &PSI,
     });
 
     // Find a maximal cold region we can outline.
-    BlockSequence ColdRegion = findMaximalColdRegion(BB, DT, PDT);
+    BlockSequence ColdRegion = findMaximalColdRegion(BB, TTI, DT, PDT);
     if (ColdRegion.empty()) {
       LLVM_DEBUG(dbgs() << "  Skipping (block not profitable to extract)\n");
       continue;
@@ -305,7 +311,7 @@ public:
 private:
   bool shouldOutlineFrom(const Function &F) const;
   Function *extractColdRegion(const BlockSequence &Region, DominatorTree &DT,
-                              BlockFrequencyInfo *BFI,
+                              BlockFrequencyInfo *BFI, TargetTransformInfo &TTI,
                               OptimizationRemarkEmitter &ORE, unsigned Count);
   SmallPtrSet<const Function *, 2> OutlinedFunctions;
   ProfileSummaryInfo *PSI;
@@ -365,6 +371,7 @@ bool HotColdSplitting::shouldOutlineFrom(const Function &F) const {
 Function *HotColdSplitting::extractColdRegion(const BlockSequence &Region,
                                               DominatorTree &DT,
                                               BlockFrequencyInfo *BFI,
+                                              TargetTransformInfo &TTI,
                                               OptimizationRemarkEmitter &ORE,
                                               unsigned Count) {
   assert(!Region.empty());
@@ -393,7 +400,7 @@ Function *HotColdSplitting::extractColdRegion(const BlockSequence &Region,
     CallInst *CI = cast<CallInst>(U);
     CallSite CS(CI);
     NumColdRegionsOutlined++;
-    if (GetTTI(*OutF).useColdCCForColdCall(*OutF)) {
+    if (TTI.useColdCCForColdCall(*OutF)) {
       OutF->setCallingConv(CallingConv::Cold);
       CS.setCallingConv(CallingConv::Cold);
     }
@@ -437,14 +444,15 @@ bool HotColdSplitting::run(Module &M) {
     PostDomTree PDT(F);
     PDT.recalculate(F);
     BlockFrequencyInfo *BFI = GetBFI(F);
+    TargetTransformInfo &TTI = GetTTI(F);
 
-    BlockSequence ColdRegion = getLargestColdRegion(F, *PSI, BFI, DT, PDT);
+    BlockSequence ColdRegion = getLargestColdRegion(F, *PSI, BFI, TTI, DT, PDT);
     if (ColdRegion.empty())
       continue;
 
     OptimizationRemarkEmitter &ORE = (*GetORE)(F);
     Function *Outlined =
-        extractColdRegion(ColdRegion, DT, BFI, ORE, /*Count=*/1);
+        extractColdRegion(ColdRegion, DT, BFI, TTI, ORE, /*Count=*/1);
     if (Outlined) {
       OutlinedFunctions.insert(Outlined);
       Changed = true;