[X86] Introduce a large data threshold for the medium code model

Currently clang's medium code model treats all data as large, putting them in a large data section and using more expensive instruction sequences to access them.

Following gcc's -mlarge-data-threshold, which allows putting data under a certain size in a normal data section as opposed to a large data section. This allows using cheaper code sequences to access some portion of data in the binary (which will be implemented in LLVM in a future patch).

And under the medium codel mode, only put data above the large data threshold into large data sections, not all data.

Reviewed By: MaskRay, rnk

Differential Revision: https://reviews.llvm.org/D149288

1 files changed, 4 insertions, 2 deletions

diff --git a/llvm/lib/Target/TargetMachine.cpp b/llvm/lib/Target/TargetMachine.cpp
index 0b4a251..2119167 100644
--- a/llvm/lib/Target/TargetMachine.cpp
+++ b/llvm/lib/Target/TargetMachine.cpp
@@ -39,14 +39,16 @@ TargetMachine::TargetMachine(const Target &T, StringRef DataLayoutString,
 
 TargetMachine::~TargetMachine() = default;
 
-bool TargetMachine::isLargeData() const {
+bool TargetMachine::isLargeData(const GlobalVariable *GV) const {
   if (getTargetTriple().getArch() != Triple::x86_64)
     return false;
   // Large data under the large code model still needs to be thought about, so
   // restrict this to medium.
   if (getCodeModel() != CodeModel::Medium)
     return false;
-  return true;
+  const DataLayout &DL = GV->getParent()->getDataLayout();
+  uint64_t Size = DL.getTypeSizeInBits(GV->getValueType()) / 8;
+  return Size == 0 || Size > LargeDataThreshold;
 }
 
 bool TargetMachine::isPositionIndependent() const {