[InterfaceFile] Improve flag extraction from interface file

Improve the version/flags extract from interface file by moving away
from using Regex and limiting the search to the beginning of the file.

Switch away from Regex will give 5-10% improvement in time and
instruction counts, and limiting the search lines can save a lot of time
if the swiftinterface is large. For example, the extract time for Swift
stdlib is 10x faster after the patch.

Current strategey for limiting the line to search is by only parsing the
first comment block.

Author: cachemeifyoucan

lib/Serialization/SerializedModuleLoader.cpp

@@ -37,6 +37,7 @@
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/CommandLine.h"
+#include <optional>
 #include <system_error>
 
 using namespace swift;
@@ -1259,16 +1260,32 @@ void swift::serialization::diagnoseSerializedASTLoadFailureTransitive(
   }
 }
 
+static std::optional<StringRef> getFlagsFromInterfaceFile(StringRef &file,
+                                                          StringRef prefix) {
+  StringRef line, buffer = file;
+  while (!buffer.empty()) {
+    std::tie(line, buffer) = buffer.split('\n');
+    // If the line is no longer comments, return not found.
+    if (!line.consume_front("// "))
+      return std::nullopt;
+
+    if (line.consume_front(prefix) && line.consume_front(":")) {
+      file = buffer;
+      return line;
+    }
+  }
+
+  return std::nullopt;
+}
+
 bool swift::extractCompilerFlagsFromInterface(
     StringRef interfacePath, StringRef buffer, llvm::StringSaver &ArgSaver,
     SmallVectorImpl<const char *> &SubArgs,
     std::optional<llvm::Triple> PreferredTarget) {
-  SmallVector<StringRef, 1> FlagMatches;
-  auto FlagRe = llvm::Regex("^// swift-module-flags:(.*)$", llvm::Regex::Newline);
-  if (!FlagRe.match(buffer, &FlagMatches))
+  auto FlagMatch = getFlagsFromInterfaceFile(buffer, SWIFT_MODULE_FLAGS_KEY);
+  if (!FlagMatch)
     return true;
-  assert(FlagMatches.size() == 2);
-  llvm::cl::TokenizeGNUCommandLine(FlagMatches[1], ArgSaver, SubArgs);
+  llvm::cl::TokenizeGNUCommandLine(*FlagMatch, ArgSaver, SubArgs);
 
   // If the target triple parsed from the Swift interface file differs
   // only in subarchitecture from the compatible target triple, then
@@ -1289,28 +1306,22 @@ bool swift::extractCompilerFlagsFromInterface(
     SubArgs[I] = ArgSaver.save(target.str()).data();
   }
 
-  SmallVector<StringRef, 1> IgnFlagMatches;
-  // Cherry-pick supported options from the ignorable list.
-  auto IgnFlagRe = llvm::Regex("^// swift-module-flags-ignorable:(.*)$",
-                               llvm::Regex::Newline);
-  auto hasIgnorableFlags = IgnFlagRe.match(buffer, &IgnFlagMatches);
-
-  // Check for ignorable-private flags
-  SmallVector<StringRef, 1> IgnPrivateFlagMatches;
-  auto IgnPrivateFlagRe = llvm::Regex("^// swift-module-flags-ignorable-private:(.*)$",
-                               llvm::Regex::Newline);
-  auto hasIgnorablePrivateFlags = IgnPrivateFlagRe.match(buffer, &IgnPrivateFlagMatches);
+  auto IgnFlagMatch =
+      getFlagsFromInterfaceFile(buffer, SWIFT_MODULE_FLAGS_IGNORABLE_KEY);
+  auto IgnPrivateFlagMatch = getFlagsFromInterfaceFile(
+      buffer, SWIFT_MODULE_FLAGS_IGNORABLE_PRIVATE_KEY);
 
   // It's OK the interface doesn't have the ignorable list (private or not), we just
   // ignore them all.
-  if (!hasIgnorableFlags && !hasIgnorablePrivateFlags)
+  if (!IgnFlagMatch && !IgnPrivateFlagMatch)
     return false;
 
   SmallVector<const char *, 8> IgnSubArgs;
-  if (hasIgnorableFlags)
-    llvm::cl::TokenizeGNUCommandLine(IgnFlagMatches[1], ArgSaver, IgnSubArgs);
-  if (hasIgnorablePrivateFlags)
-    llvm::cl::TokenizeGNUCommandLine(IgnPrivateFlagMatches[1], ArgSaver, IgnSubArgs);
+  if (IgnFlagMatch)
+    llvm::cl::TokenizeGNUCommandLine(*IgnFlagMatch, ArgSaver, IgnSubArgs);
+  if (IgnPrivateFlagMatch)
+    llvm::cl::TokenizeGNUCommandLine(*IgnPrivateFlagMatch, ArgSaver,
+                                     IgnSubArgs);
 
   std::unique_ptr<llvm::opt::OptTable> table = swift::createSwiftOptTable();
   unsigned missingArgIdx = 0;

test/ModuleInterface/SmokeTest.swiftinterface

@@ -1,7 +1,6 @@
 // The "flags" line in this test deliberately has no flags and no space after
 // the colon, just to make sure that works (even though it'll never be printed
 // that way).
-
 // swift-module-flags:
 // swift-interface-format-version: 1.0
 

test/SPI/spi_only_import_flag_check.swift

@@ -31,7 +31,6 @@
 @_spiOnly import Empty // expected-error {{'@_spiOnly' requires setting the frontend flag '-experimental-spi-only-imports'}}
 
 //--- Client.private.swiftinterface
-
 // swift-interface-format-version: 1.0
 // swift-compiler-version: Swift version 5.8-dev effective-4.1.50
 // swift-module-flags: -swift-version 4 -module-name Client