[clang] 00f0c80 - [Frontend] shrink in-memory PCH buffers to fit

[Sam McCall via cfe-commits]

Author: Sam McCall
Date: 2022-04-25T14:31:14+02:00
New Revision: 00f0c805ff7c0b5780d651d273899abe977fdca7

URL: https://github.com/llvm/llvm-project/commit/00f0c805ff7c0b5780d651d273899abe977fdca7
DIFF: https://github.com/llvm/llvm-project/commit/00f0c805ff7c0b5780d651d273899abe977fdca7.diff

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

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.

Differential Revision: https://reviews.llvm.org/D124242

Added: 
    

Modified: 
    clang/lib/Frontend/PrecompiledPreamble.cpp

Removed: 
    


################################################################################
diff  --git a/clang/lib/Frontend/PrecompiledPreamble.cpp b/clang/lib/Frontend/PrecompiledPreamble.cpp
index e8128e3828edd..341ea6121637d 100644
--- a/clang/lib/Frontend/PrecompiledPreamble.cpp
+++ b/clang/lib/Frontend/PrecompiledPreamble.cpp
@@ -374,6 +374,15 @@ class PrecompiledPreamble::PCHStorage {
     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);
+  }
+
 private:
   PCHStorage() = default;
   PCHStorage(const PCHStorage &) = delete;
@@ -520,7 +529,7 @@ llvm::ErrorOr<PrecompiledPreamble> PrecompiledPreamble::Build(
 
   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 @@ llvm::ErrorOr<PrecompiledPreamble> PrecompiledPreamble::Build(
     }
   }
 
+  // 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));