[Support] Don't initialize compressed buffer allocated by zlib::compress

resize() (zeroing) makes every allocated page resident. The actual size of the compressed buffer is usually much
smaller. Making every page resident is wasteful.

When linking a test binary with ~1.9GiB uncompressed debug info with LLD, this optimization decreases max RSS by ~1.5GiB.

Differential Revision: https://reviews.llvm.org/50223

llvm-svn: 338913

llvm/lib/Support/Compression.cpp

@@ -61,15 +61,15 @@ Error zlib::compress(StringRef InputBuffer,
                      SmallVectorImpl<char> &CompressedBuffer,
                      CompressionLevel Level) {
   unsigned long CompressedSize = ::compressBound(InputBuffer.size());
-  CompressedBuffer.resize(CompressedSize);
+  CompressedBuffer.reserve(CompressedSize);
   int CLevel = encodeZlibCompressionLevel(Level);
   int Res = ::compress2((Bytef *)CompressedBuffer.data(), &CompressedSize,
                         (const Bytef *)InputBuffer.data(), InputBuffer.size(),
                         CLevel);
   // Tell MemorySanitizer that zlib output buffer is fully initialized.
   // This avoids a false report when running LLVM with uninstrumented ZLib.
   __msan_unpoison(CompressedBuffer.data(), CompressedSize);
-  CompressedBuffer.resize(CompressedSize);
+  CompressedBuffer.set_size(CompressedSize);
   return Res ? createError(convertZlibCodeToString(Res)) : Error::success();
 }