[NFC] Hash LLVM options more robustly

Replaces getLLVMCodeGenOptionsHash(), which combined a bunch of individual bits into a string, with writeLLVMCodeGenOptionsTo(), which writes each one separately into a raw_ostream.

This is intended to be make the hashing more future-proof. The current design needs to know exactly how many bits each of the values needs; if any of the values grew and you forgot to update this function, its bits would interfere with those of an earlier value in the hash. This new design is expected to be slightly slower, but more robust to future change in the compiler.

Author: beccadax

include/swift/AST/IRGenOptions.h

@@ -26,6 +26,7 @@
 // FIXME: This include is just for llvm::SanitizerCoverageOptions. We should
 // split the header upstream so we don't include so much.
 #include "llvm/Transforms/Instrumentation.h"
+#include "llvm/Support/raw_ostream.h"
 #include "llvm/Support/VersionTuple.h"
 #include <string>
 #include <vector>
@@ -253,15 +254,18 @@ class IRGenOptions {
         SanitizeCoverage(llvm::SanitizerCoverageOptions()),
         TypeInfoFilter(TypeInfoDumpFilter::All) {}
 
-  // Get a hash of all options which influence the llvm compilation but are not
-  // reflected in the llvm module itself.
-  unsigned getLLVMCodeGenOptionsHash() {
-    unsigned Hash = (unsigned)OptMode;
-    Hash = (Hash << 1) | DisableLLVMOptzns;
-    Hash = (Hash << 1) | DisableSwiftSpecificLLVMOptzns;
-    Hash = (Hash << 1) | GenerateProfile;
-    Hash = (Hash << 1) | Sanitizers.contains(SanitizerKind::Fuzzer);
-    return Hash;
+  /// Appends to \p os an arbitrary string representing all options which
+  /// influence the llvm compilation but are not reflected in the llvm module
+  /// itself.
+  void writeLLVMCodeGenOptionsTo(llvm::raw_ostream &os) {
+    // We put a letter between each value simply to keep them from running into
+    // one another. There might be a vague correspondence between meaning and
+    // letter, but don't sweat it.
+    os << 'O' << (unsigned)OptMode
+       << 'd' << DisableLLVMOptzns
+       << 'D' << DisableSwiftSpecificLLVMOptzns
+       << 'p' << GenerateProfile
+       << 's' << Sanitizers.contains(SanitizerKind::Fuzzer);
   }
 
   /// Should LLVM IR value names be emitted and preserved?

lib/IRGen/IRGen.cpp

@@ -349,7 +349,7 @@ static void getHashOfModule(MD5::MD5Result &Result, IRGenOptions &Opts,
 
   // Add all options which influence the llvm compilation but are not yet
   // reflected in the llvm module itself.
-  HashStream << Opts.getLLVMCodeGenOptionsHash();
+  Opts.writeLLVMCodeGenOptionsTo(HashStream);
 
   HashStream.final(Result);
 }
@@ -614,7 +614,7 @@ bool swift::performLLVM(IRGenOptions &Opts, DiagnosticEngine *Diags,
       );
       // Note for future debuggers: If you see this error, either you changed
       // LLVM and need to clean your build folder to rebuild everything with it,
-      // or IRGenOptions::getLLVMCodeGenOptionsHash() doesn't account for a flag
+      // or IRGenOptions::writeLLVMCodeGenOptionsTo() doesn't account for a flag
       // that changed LLVM's output between this compile and the previous one.
 
     case FileDifference::SameContents: