[llvm] Unittests and usability for BitstreamWriter incremental flushing (PR #92983)

[Mircea Trofin via llvm-commits]

================
@@ -5047,27 +5050,28 @@ void llvm::WriteBitcodeToFile(const Module &M, raw_ostream &Out,
                               bool ShouldPreserveUseListOrder,
                               const ModuleSummaryIndex *Index,
                               bool GenerateHash, ModuleHash *ModHash) {
-  SmallVector<char, 0> Buffer;
-  Buffer.reserve(256*1024);
-
-  // If this is darwin or another generic macho target, reserve space for the
-  // header.
+  auto Write = [&](BitcodeWriter &Writer) {
+    Writer.writeModule(M, ShouldPreserveUseListOrder, Index, GenerateHash,
+                       ModHash);
+    Writer.writeSymtab();
+    Writer.writeStrtab();
+  };
   Triple TT(M.getTargetTriple());
-  if (TT.isOSDarwin() || TT.isOSBinFormatMachO())
+  if (TT.isOSDarwin() || TT.isOSBinFormatMachO()) {
+    // If this is darwin or another generic macho target, reserve space for the
+    // header. Note that the header is computed *after* the output is known, so
+    // we currently expliclty use a buffer, write to it, and then subsequently
+    // flush to Out.
+    SmallVector<char, 256 * 1024> Buffer;
     Buffer.insert(Buffer.begin(), BWH_HeaderSize, 0);
-
-  BitcodeWriter Writer(Buffer, dyn_cast<raw_fd_stream>(&Out));
-  Writer.writeModule(M, ShouldPreserveUseListOrder, Index, GenerateHash,
-                     ModHash);
-  Writer.writeSymtab();
-  Writer.writeStrtab();
-
-  if (TT.isOSDarwin() || TT.isOSBinFormatMachO())
+    BitcodeWriter Writer(Buffer);
+    Write(Writer);
     emitDarwinBCHeaderAndTrailer(Buffer, TT);
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mtrofin wrote:

it won't be - that's the thing, in this case, we use our own Buffer which won't get flushed - there's nowhere to flush (insofar as the bitcode writer is concerned). See the comment above the declaration of `Buffer`, too.

https://github.com/llvm/llvm-project/pull/92983