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

[Teresa Johnson via llvm-commits]

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@@ -77,47 +86,106 @@ class BitstreamWriter {
   void WriteWord(unsigned Value) {
     Value =
         support::endian::byte_swap<uint32_t, llvm::endianness::little>(Value);
-    Out.append(reinterpret_cast<const char *>(&Value),
-               reinterpret_cast<const char *>(&Value + 1));
+    Buffer.append(reinterpret_cast<const char *>(&Value),
+                  reinterpret_cast<const char *>(&Value + 1));
   }
 
-  uint64_t GetNumOfFlushedBytes() const { return FS ? FS->tell() : 0; }
+  uint64_t GetNumOfFlushedBytes() const {
+    return fdStream() ? fdStream()->tell() : 0;
+  }
 
-  size_t GetBufferOffset() const { return Out.size() + GetNumOfFlushedBytes(); }
+  size_t GetBufferOffset() const {
+    return Buffer.size() + GetNumOfFlushedBytes();
+  }
 
   size_t GetWordIndex() const {
     size_t Offset = GetBufferOffset();
     assert((Offset & 3) == 0 && "Not 32-bit aligned");
     return Offset / 4;
   }
 
-  /// If the related file stream supports reading, seeking and writing, flush
-  /// the buffer if its size is above a threshold.
-  void FlushToFile() {
-    if (!FS)
+  void flushAndClear() {
+    assert(FS);
+    assert(!Buffer.empty());
+    assert(!BlockFlushingStartPos &&
+           "a call to markAndBlockFlushing should have been paired with a "
+           "call to getMarkedBufferAndResumeFlushing");
+    FS->write(Buffer.data(), Buffer.size());
+    Buffer.clear();
+  }
+
+  /// If the related file stream is a raw_fd_stream, flush the buffer if its
+  /// size is above a threshold. If \p OnClosing is true, flushing happens
+  /// regardless of thresholds.
+  void FlushToFile(bool OnClosing = false) {
+    if (!FS || Buffer.empty())
       return;
-    if (Out.size() < FlushThreshold)
+    if (OnClosing)
+      return flushAndClear();
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teresajohnson wrote:

What prevents this from being called when FS==nullptr, which will cause an assert in flushAndClear()?

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