[safestack] Layout large allocas first to reduce fragmentation.

llvm-svn: 277544

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SafeStackLayout.cpp b/llvm/lib/CodeGen/SafeStackLayout.cpp
index fb433c1856a..7d4dbd13abf 100644
--- a/llvm/lib/CodeGen/SafeStackLayout.cpp
+++ b/llvm/lib/CodeGen/SafeStackLayout.cpp
@@ -132,6 +132,14 @@ void StackLayout::computeLayout() {
   // If this is replaced with something smarter, it must preserve the property
   // that the first object is always at the offset 0 in the stack frame (for
   // StackProtectorSlot), or handle stack protector in some other way.
+
+  // Sort objects by size (largest first) to reduce fragmentation.
+  if (StackObjects.size() > 2)
+    std::stable_sort(StackObjects.begin() + 1, StackObjects.end(),
+                     [](const StackObject &a, const StackObject &b) {
+                       return a.Size > b.Size;
+                     });
+
   for (auto &Obj : StackObjects)
     layoutObject(Obj);