[analyzer] Ignore flex generated files

Some projects [1,2,3] have flex-generated files besides bison-generated
ones.
Unfortunately, the comment `"/* A lexical scanner generated by flex */"`
generated by the tools is not necessarily at the beginning of the file,
thus we need to quickly skim through the file for this needle string.

Luckily, StringRef can do this operation in an efficient way.

That being said, now the bison comment is not required to be at the very
beginning of the file. This allows us to detect a couple more cases
[4,5,6].

Alternatively, we could say that we only allow whitespace characters
before matching the bison/flex header comment. That would prevent the
(probably) unnecessary string search in the buffer. However, I could not
verify that these tools would actually respect this assumption.

Additionally to this, e.g. the Twin project [1] has other non-whitespace
characters (some preprocessor directives) before the flex-generated
header comment. So the heuristic in the previous paragraph won't work
with that.
Thus, I would advocate the current implementation.

According to my measurement, this patch won't introduce measurable
performance degradation, even though we will do 2 linear scans.

I introduce the ignore-bison-generated-files and
ignore-flex-generated-files to disable skipping these files.
Both of these options are true by default.

[1]: https://github.com/cosmos72/twin/blob/master/server/rcparse_lex.cpp#L7
[2]: https://github.com/marcauresoar/make-examples/blob/22362cdcf9dd7c597b5049ce7f176621e2e9ac7a/sandbox/count-words/lexer.c#L6
[3]: https://github.com/vladcmanea/2nd-faculty-year-Formal-Languages---Automata-assignments/blob/11abdf64629d9eb741438ba69f04636769d5a374/lab1/lex.yy.c#L6

[4]: https://github.com/KritikaChoudhary/System-Software-Lab/blob/47f5b2cfe2a2738fd54eae9f8439817f6a22034e/B_yacc/1/y1.tab.h#L2
[5]: https://github.com/VirtualMonitor/VirtualMonitor/blob/71d1bf9b1e7b392a7bd0c73dc217138dc5865651/src/VBox/Additions/x11/x11include/xorg-server-1.8.0/parser.h#L2
[6]: https://github.com/bspaulding/DrawTest/blob/3f773ceb13de14275429036b9cbc5aa19e29bab9/Framework/OpenEars.framework/Versions/A/Headers/jsgf_parser.h#L2

Reviewed By: xazax.hun

Differential Revision: https://reviews.llvm.org/D114510

Author: Balazs Benics

clang/include/clang/StaticAnalyzer/Core/AnalyzerOptions.def

@@ -336,6 +336,18 @@ ANALYZER_OPTION(
     "might be modeled by the analyzer to never return NULL.",
     false)
 
+ANALYZER_OPTION(
+    bool, ShouldIgnoreBisonGeneratedFiles, "ignore-bison-generated-files",
+    "If enabled, any files containing the \"/* A Bison parser, made by\" "
+    "won't be analyzed.",
+    true)
+
+ANALYZER_OPTION(
+    bool, ShouldIgnoreFlexGeneratedFiles, "ignore-flex-generated-files",
+    "If enabled, any files containing the \"/* A lexical scanner generated by "
+    "flex\" won't be analyzed.",
+    true)
+
 //===----------------------------------------------------------------------===//
 // Unsigned analyzer options.
 //===----------------------------------------------------------------------===//

clang/lib/StaticAnalyzer/Frontend/AnalysisConsumer.cpp

@@ -35,6 +35,7 @@
 #include "clang/StaticAnalyzer/Core/PathSensitive/AnalysisManager.h"
 #include "clang/StaticAnalyzer/Core/PathSensitive/ExprEngine.h"
 #include "llvm/ADT/PostOrderIterator.h"
+#include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
@@ -493,13 +494,11 @@ void AnalysisConsumer::HandleDeclsCallGraph(const unsigned LocalTUDeclsSize) {
   }
 }
 
-static bool isBisonFile(ASTContext &C) {
+static bool fileContainsString(StringRef Substring, ASTContext &C) {
   const SourceManager &SM = C.getSourceManager();
   FileID FID = SM.getMainFileID();
   StringRef Buffer = SM.getBufferOrFake(FID).getBuffer();
-  if (Buffer.startswith("/* A Bison parser, made by"))
-    return true;
-  return false;
+  return Buffer.contains(Substring);
 }
 
 void AnalysisConsumer::runAnalysisOnTranslationUnit(ASTContext &C) {
@@ -546,38 +545,48 @@ void AnalysisConsumer::reportAnalyzerProgress(StringRef S) {
 }
 
 void AnalysisConsumer::HandleTranslationUnit(ASTContext &C) {
-
   // Don't run the actions if an error has occurred with parsing the file.
   DiagnosticsEngine &Diags = PP.getDiagnostics();
   if (Diags.hasErrorOccurred() || Diags.hasFatalErrorOccurred())
     return;
 
-  if (isBisonFile(C)) {
+  // Explicitly destroy the PathDiagnosticConsumer.  This will flush its output.
+  // FIXME: This should be replaced with something that doesn't rely on
+  // side-effects in PathDiagnosticConsumer's destructor. This is required when
+  // used with option -disable-free.
+  const auto DiagFlusherScopeExit =
+      llvm::make_scope_exit([this] { Mgr.reset(); });
+
+  if (Opts->ShouldIgnoreBisonGeneratedFiles &&
+      fileContainsString("/* A Bison parser, made by", C)) {
     reportAnalyzerProgress("Skipping bison-generated file\n");
-  } else if (Opts->DisableAllCheckers) {
+    return;
+  }
 
-    // Don't analyze if the user explicitly asked for no checks to be performed
-    // on this file.
+  if (Opts->ShouldIgnoreFlexGeneratedFiles &&
+      fileContainsString("/* A lexical scanner generated by flex", C)) {
+    reportAnalyzerProgress("Skipping flex-generated file\n");
+    return;
+  }
+
+  // Don't analyze if the user explicitly asked for no checks to be performed
+  // on this file.
+  if (Opts->DisableAllCheckers) {
     reportAnalyzerProgress("All checks are disabled using a supplied option\n");
-  } else {
-    // Otherwise, just run the analysis.
-    runAnalysisOnTranslationUnit(C);
+    return;
   }
 
+  // Otherwise, just run the analysis.
+  runAnalysisOnTranslationUnit(C);
+
   // Count how many basic blocks we have not covered.
   NumBlocksInAnalyzedFunctions = FunctionSummaries.getTotalNumBasicBlocks();
   NumVisitedBlocksInAnalyzedFunctions =
       FunctionSummaries.getTotalNumVisitedBasicBlocks();
   if (NumBlocksInAnalyzedFunctions > 0)
     PercentReachableBlocks =
-      (FunctionSummaries.getTotalNumVisitedBasicBlocks() * 100) /
+        (FunctionSummaries.getTotalNumVisitedBasicBlocks() * 100) /
         NumBlocksInAnalyzedFunctions;
-
-  // Explicitly destroy the PathDiagnosticConsumer.  This will flush its output.
-  // FIXME: This should be replaced with something that doesn't rely on
-  // side-effects in PathDiagnosticConsumer's destructor. This is required when
-  // used with option -disable-free.
-  Mgr.reset();
 }
 
 AnalysisConsumer::AnalysisMode

clang/test/Analysis/analyzer-config.c

@@ -83,6 +83,8 @@
 // CHECK-NEXT: exploration_strategy = unexplored_first_queue
 // CHECK-NEXT: faux-bodies = true
 // CHECK-NEXT: graph-trim-interval = 1000
+// CHECK-NEXT: ignore-bison-generated-files = true
+// CHECK-NEXT: ignore-flex-generated-files = true
 // CHECK-NEXT: inline-lambdas = true
 // CHECK-NEXT: ipa = dynamic-bifurcate
 // CHECK-NEXT: ipa-always-inline-size = 3

clang/test/Analysis/flexignore.c

@@ -0,0 +1,14 @@
+// RUN: %clang_analyze_cc1 -analyzer-checker=core,debug.ExprInspection -verify -DEXPECT_NO_DIAGNOSTICS %s
+// RUN: %clang_analyze_cc1 -analyzer-checker=core,debug.ExprInspection -verify=conditional %s \
+// RUN:    -analyzer-config ignore-flex-generated-files=false
+
+#ifdef EXPECT_NO_DIAGNOSTICS
+// expected-no-diagnostics
+#endif
+
+/* A lexical scanner generated by flex */
+
+void clang_analyzer_warnIfReached();
+void foo() {
+  clang_analyzer_warnIfReached(); // conditional-warning {{REACHABLE}}
+}

clang/test/Analysis/yaccignore.c

@@ -1,13 +1,14 @@
-/* A Bison parser, made by GNU Bison 1.875.  */
-
-// RUN: rm -rf %t.plist
-// RUN: %clang_analyze_cc1 -analyzer-checker=core -analyzer-output=plist -o %t.plist -verify %s
-// RUN: FileCheck --input-file=%t.plist %s
+// RUN: %clang_analyze_cc1 -analyzer-checker=core,debug.ExprInspection -verify -DEXPECT_NO_DIAGNOSTICS %s
+// RUN: %clang_analyze_cc1 -analyzer-checker=core,debug.ExprInspection -verify=conditional %s \
+// RUN:    -analyzer-config ignore-bison-generated-files=false
 
+#ifdef EXPECT_NO_DIAGNOSTICS
 // expected-no-diagnostics
-int foo() {
-  int *x = 0;
-  return *x; // no-warning
-}
+#endif
 
-// CHECK:   <key>diagnostics</key>
+/* A Bison parser, made by GNU Bison 1.875.  */
+
+void clang_analyzer_warnIfReached();
+void foo() {
+  clang_analyzer_warnIfReached(); // conditional-warning {{REACHABLE}}
+}