[LowerTypeTests] Limit when icall jumptable entries are emitted

Summary:
Currently LowerTypeTests emits jumptable entries for all live external
and address-taken functions; however, we could limit the number of
functions that we emit entries for significantly.

For Cross-DSO CFI, we continue to emit jumptable entries for all
exported definitions.  In the non-Cross-DSO CFI case, we only need to
emit jumptable entries for live functions that are address-taken in live
functions. This ignores exported functions and functions that are only
address taken in dead functions. This change uses ThinLTO summary data
(now emitted for all modules during ThinLTO builds) to determine
address-taken and liveness info.

The logic for emitting jumptable entries is more conservative in the
regular LTO case because we don't have summary data in the case of
monolithic LTO builds; however, once summaries are emitted for all LTO
builds we can unify the Thin/monolithic LTO logic to only use summaries
to determine the liveness of address taking functions.

This change is a partial fix for PR37474. It reduces the build size for
nacl_helper by ~2-3%, the reduction is due to nacl_helper compiling in
lots of unused code and unused functions that are address taken in dead
functions no longer being being considered live due to emitted jumptable
references. The reduction for chromium is ~0.1-0.2%.

Reviewers: pcc, eugenis, javed.absar

Reviewed By: pcc

Subscribers: aheejin, dexonsmith, dschuff, mehdi_amini, eraman, steven_wu, llvm-commits, kcc

Differential Revision: https://reviews.llvm.org/D47652

llvm-svn: 337038

1 files changed, 43 insertions, 6 deletions

diff --git a/llvm/lib/Transforms/IPO/LowerTypeTests.cpp b/llvm/lib/Transforms/IPO/LowerTypeTests.cpp
index b6033908204..9d6442b51ee 100644
--- a/llvm/lib/Transforms/IPO/LowerTypeTests.cpp
+++ b/llvm/lib/Transforms/IPO/LowerTypeTests.cpp
@@ -1750,26 +1750,54 @@ bool LowerTypeTestsModule::lower() {
   unsigned CurUniqueId = 0;
   SmallVector<MDNode *, 2> Types;
 
+  // Cross-DSO CFI emits jumptable entries for exported functions as well as
+  // address taken functions in case they are address taken in other modules.
+  const bool CrossDsoCfi = M.getModuleFlag("Cross-DSO CFI") != nullptr;
+
   struct ExportedFunctionInfo {
     CfiFunctionLinkage Linkage;
     MDNode *FuncMD; // {name, linkage, type[, type...]}
   };
   DenseMap<StringRef, ExportedFunctionInfo> ExportedFunctions;
   if (ExportSummary) {
+    // A set of all functions that are address taken by a live global object.
+    DenseSet<GlobalValue::GUID> AddressTaken;
+    for (auto &I : *ExportSummary)
+      for (auto &GVS : I.second.SummaryList)
+        if (GVS->isLive())
+          for (auto &Ref : GVS->refs())
+            AddressTaken.insert(Ref.getGUID());
+
     NamedMDNode *CfiFunctionsMD = M.getNamedMetadata("cfi.functions");
     if (CfiFunctionsMD) {
       for (auto FuncMD : CfiFunctionsMD->operands()) {
         assert(FuncMD->getNumOperands() >= 2);
         StringRef FunctionName =
             cast<MDString>(FuncMD->getOperand(0))->getString();
-        if (!ExportSummary->isGUIDLive(GlobalValue::getGUID(
-                GlobalValue::dropLLVMManglingEscape(FunctionName))))
-          continue;
         CfiFunctionLinkage Linkage = static_cast<CfiFunctionLinkage>(
             cast<ConstantAsMetadata>(FuncMD->getOperand(1))
                 ->getValue()
                 ->getUniqueInteger()
                 .getZExtValue());
+        const GlobalValue::GUID GUID = GlobalValue::getGUID(
+                GlobalValue::dropLLVMManglingEscape(FunctionName));
+        // Do not emit jumptable entries for functions that are not-live and
+        // have no live references (and are not exported with cross-DSO CFI.)
+        if (!ExportSummary->isGUIDLive(GUID))
+          continue;
+        if (!AddressTaken.count(GUID)) {
+          if (!CrossDsoCfi || Linkage != CFL_Definition)
+            continue;
+
+          bool Exported = false;
+          if (auto VI = ExportSummary->getValueInfo(GUID))
+            for (auto &GVS : VI.getSummaryList())
+              if (GVS->isLive() && !GlobalValue::isLocalLinkage(GVS->linkage()))
+                Exported = true;
+
+          if (!Exported)
+            continue;
+        }
         auto P = ExportedFunctions.insert({FunctionName, {Linkage, FuncMD}});
         if (!P.second && P.first->second.Linkage != CFL_Definition)
           P.first->second = {Linkage, FuncMD};
@@ -1829,9 +1857,18 @@ bool LowerTypeTestsModule::lower() {
 
     bool IsDefinition = !GO.isDeclarationForLinker();
     bool IsExported = false;
-    if (isa<Function>(GO) && ExportedFunctions.count(GO.getName())) {
-      IsDefinition |= ExportedFunctions[GO.getName()].Linkage == CFL_Definition;
-      IsExported = true;
+    if (Function *F = dyn_cast<Function>(&GO)) {
+      if (ExportedFunctions.count(F->getName())) {
+        IsDefinition |= ExportedFunctions[F->getName()].Linkage == CFL_Definition;
+        IsExported = true;
+      // TODO: The logic here checks only that the function is address taken,
+      // not that the address takers are live. This can be updated to check
+      // their liveness and emit fewer jumptable entries once monolithic LTO
+      // builds also emit summaries.
+      } else if (!F->hasAddressTaken()) {
+        if (!CrossDsoCfi || !IsDefinition || F->hasLocalLinkage())
+          continue;
+      }
     }
 
     auto *GTM =