[PATCH] D124242: [Frontend] shrink in-memory PCH buffers to fit

Sam McCall via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Fri Apr 22 03:02:55 PDT 2022

sammccall created this revision.
sammccall added a reviewer: kadircet.
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sammccall requested review of this revision.
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After building a PCH, the vector capacity is on average ~1/3 unused.
If we're going to keep it in memory for a while, reallocate to the right size.
Take care to do this once clang is destroyed so that we can reuse its
memory rather than requesting more.

  rG LLVM Github Monorepo



Index: clang/lib/Frontend/PrecompiledPreamble.cpp
--- clang/lib/Frontend/PrecompiledPreamble.cpp
+++ clang/lib/Frontend/PrecompiledPreamble.cpp
@@ -374,6 +374,15 @@
     return StringRef(Memory->Data.data(), Memory->Data.size());
+  // Shrink in-memory buffers to fit.
+  // This incurs a copy, but preambles tend to be long-lived.
+  // Only safe to call once nothing can alias the buffer.
+  void shrink() {
+    if (!Memory)
+      return;
+    Memory->Data = decltype(Memory->Data)(Memory->Data);
+  }
   PCHStorage() = default;
   PCHStorage(const PCHStorage &) = delete;
@@ -520,7 +529,7 @@
   if (!Act->hasEmittedPreamblePCH())
     return BuildPreambleError::CouldntEmitPCH;
-  Act.reset(); // Frees the PCH buffer frees, unless Storage keeps it in memory.
+  Act.reset(); // Frees the PCH buffer, unless Storage keeps it in memory.
   // Keep track of all of the files that the source manager knows about,
   // so we can verify whether they have changed or not.
@@ -545,6 +554,11 @@
+  // Shrinking the storage requires extra temporary memory.
+  // Destroying clang first reduces peak memory usage.
+  CICleanup.unregister();
+  Clang.reset();
+  Storage->shrink();
   return PrecompiledPreamble(
       std::move(Storage), std::move(PreambleBytes), PreambleEndsAtStartOfLine,
       std::move(FilesInPreamble), std::move(MissingFiles));

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