[pdb] Change /DEBUG:GHASH to emit 8 byte hashes.

Previously we emitted 20-byte SHA1 hashes.  This is overkill
for identifying debug info records, and has the negative side
effect of making object files bigger and links slower.  By
using only the last 8 bytes of a SHA1, we get smaller object
files and ~10% faster links.

This modifies the format of the .debug$H section by adding a new
value for the hash algorithm field, so that the linker will still
work when its object files have an old format.

Differential Revision: https://reviews.llvm.org/D46855

llvm-svn: 332669

1 files changed, 4 insertions, 5 deletions

diff --git a/llvm/lib/DebugInfo/CodeView/TypeHashing.cpp b/llvm/lib/DebugInfo/CodeView/TypeHashing.cpp
index c2f8cd5466a..a51770959ac 100644
--- a/llvm/lib/DebugInfo/CodeView/TypeHashing.cpp
+++ b/llvm/lib/DebugInfo/CodeView/TypeHashing.cpp
@@ -18,10 +18,9 @@ using namespace llvm::codeview;
 LocallyHashedType DenseMapInfo<LocallyHashedType>::Empty{0, {}};
 LocallyHashedType DenseMapInfo<LocallyHashedType>::Tombstone{hash_code(-1), {}};
 
-static std::array<uint8_t, 20> EmptyHash;
-static std::array<uint8_t, 20> TombstoneHash = {
-    {0xFF, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-     0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00}};
+static std::array<uint8_t, 8> EmptyHash = { {0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00} };
+static std::array<uint8_t, 8> TombstoneHash = {
+  {0xFF, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00} };
 
 GloballyHashedType DenseMapInfo<GloballyHashedType>::Empty{EmptyHash};
 GloballyHashedType DenseMapInfo<GloballyHashedType>::Tombstone{TombstoneHash};
@@ -71,5 +70,5 @@ GloballyHashedType::hashType(ArrayRef<uint8_t> RecordData,
   auto TrailingBytes = RecordData.drop_front(Off);
   S.update(TrailingBytes);
 
-  return {S.final()};
+  return {S.final().take_back(8)};
 }