[ThinLTO] Add an import cutoff for debugging/triaging

[tejohnson]

Summary

Adds -import-cutoff=N which will stop importing during the thin link
after N imports. Default is -1 (no limit).

[wmi]

The patch was very helpful for my last triaging of a runtime issue only exposed in thinlto mode. Thanks for contributing it to upstream!

lib/Transforms/IPO/FunctionImport.cpp

 STATISTIC(NumDeadSymbols, "Number of dead stripped symbols in index");
 STATISTIC(NumLiveSymbols, "Number of live symbols in index");
 
 /// Limit on instruction count of imported functions.
 static cl::opt<unsigned> ImportInstrLimit(
     "import-instr-limit", cl::init(100), cl::Hidden, cl::value_desc("N"),
     cl::desc("Only import functions with less than N instructions"));
 
+static cl::opt<int> ImportCutoff(
+    "import-cutoff", cl::init(-1), cl::Hidden, cl::value_desc("N"),
+    cl::desc("Only import first N functions if N>=0 (default -1)"));
+
 static cl::opt<float>
     ImportInstrFactor("import-instr-evolution-factor", cl::init(0.7),
                       cl::Hidden, cl::value_desc("x"),
                       cl::desc("As we import functions, multiply the "
                                "`import-instr-limit` threshold by this factor "
                                "before processing newly imported functions"));
 
 static cl::opt<float> ImportHotInstrFactor(

 static void computeImportForFunction(
     const FunctionSummary &Summary, const ModuleSummaryIndex &Index,
     const unsigned Threshold, const GVSummaryMapTy &DefinedGVSummaries,
     SmallVectorImpl<EdgeInfo> &Worklist,
     FunctionImporter::ImportMapTy &ImportList,
     StringMap<FunctionImporter::ExportSetTy> *ExportLists = nullptr) {
   computeImportForReferencedGlobals(Summary, DefinedGVSummaries, ImportList,
                                     ExportLists);
+  static int ImportCount = 0;
   for (auto &Edge : Summary.calls()) {
     ValueInfo VI = Edge.first;
     DEBUG(dbgs() << " edge -> " << VI.getGUID() << " Threshold:" << Threshold
                  << "\n");
 
+    if (ImportCutoff >= 0 && ImportCount >= ImportCutoff) {
+      DEBUG(dbgs() << "ignored! import-cutoff value of " << ImportCutoff
+                   << " reached.\n");
+      continue;
+    }
+
     VI = updateValueInfoForIndirectCalls(Index, VI);
     if (!VI)
       continue;
 
     if (DefinedGVSummaries.count(VI.getGUID())) {
       DEBUG(dbgs() << "ignored! Target already in destination module.\n");
       continue;
     }
if (ProcessedThreshold && ProcessedThreshold >= AdjThreshold) {
       DEBUG(dbgs() << "ignored! Target was already seen with Threshold "
                    << ProcessedThreshold << "\n");
       continue;
     }
     bool PreviouslyImported = ProcessedThreshold != 0;
     // Mark this function as imported in this module, with the current Threshold
     ProcessedThreshold = AdjThreshold;
 
+    ImportCount++;
+
     // Make exports in the source module.
     if (ExportLists) {
       auto &ExportList = (*ExportLists)[ExportModulePath];
       ExportList.insert(VI.getGUID());
       if (!PreviouslyImported) {
         // This is the first time this function was exported from its source
         // module, so mark all functions and globals it references as exported
         // to the outside if they are defined in the same source module.

test/Transforms/FunctionImport/Inputs/funcimport_cutoff.ll

+define void @foo() {
+entry:
+  ret void
+}
+
+define void @bar() {
+entry:
+  ret void
+}

test/Transforms/FunctionImport/funcimport_cutoff.ll

+; Test to ensure that thin linking with -import-cutoff stops importing when
+; expected.
+
+; "-stats" and "-debug-only" require +Asserts.
+; REQUIRES: asserts
+
+; RUN: opt -module-summary %s -o %t.bc
+; RUN: opt -module-summary %p/Inputs/funcimport_cutoff.ll -o %t2.bc
+; RUN: llvm-lto -thinlto -o %t3 %t.bc %t2.bc
+
+; First do with default options, which should import both foo and bar
+; RUN: opt -function-import -stats -print-imports -summary-file %t3.thinlto.bc %t.bc -S 2>&1 | FileCheck %s --check-prefix=IMPORT
+
+; Next try to restrict to 1 import. This should import just foo.
+; RUN: opt -import-cutoff=1 -function-import -stats -print-imports -summary-file %t3.thinlto.bc %t.bc -S 2>&1 | FileCheck %s --check-prefix=IMPORT1
+
+; Next try to restrict to 0 imports. This should not import.
+; RUN: opt -import-cutoff=0 -function-import -stats -print-imports -summary-file %t3.thinlto.bc %t.bc -S 2>&1 | FileCheck %s --check-prefix=NOIMPORT
+
+define i32 @main() {
+entry:
+  call void @foo()
+  call void @bar()
+  ret i32 0
+}
+
+declare void @foo()
+declare void @bar()
+
+; Check -print-imports output
+; IMPORT: Import foo
+; IMPORT: Import bar
+; IMPORT1: Import foo
+; IMPORT1-NOT: Import bar
+; NOIMPORT-NOT: Import foo
+; NOIMPORT-NOT: Import bar
+
+; Check -S output
+; IMPORT-DAG: define available_externally void @foo()
+; IMPORT-DAG: define available_externally void @bar()
+; NOIMPORT-DAG: declare void @foo()
+; NOIMPORT-DAG: declare void @bar()
+; IMPORT1-DAG: define available_externally void @foo()
+; IMPORT1-DAG: declare void @bar()
+
+; Check -stats output
+; IMPORT: 2 function-import - Number of functions imported
+; IMPORT1: 1 function-import - Number of functions imported
+; NOIMPORT-NOT: function-import - Number of functions imported