[safestack] Layout large allocas first to reduce fragmentation.

llvm-svn: 277544

2 files changed, 47 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);
 
diff --git a/llvm/test/Transforms/SafeStack/layout-frag.ll b/llvm/test/Transforms/SafeStack/layout-frag.ll
new file mode 100644
index 00000000000..125eb0f8be9
--- /dev/null
+++ b/llvm/test/Transforms/SafeStack/layout-frag.ll
@@ -0,0 +1,39 @@
+; Test that safestack layout reuses a region w/o fragmentation.
+; RUN: opt -safe-stack -S -mtriple=x86_64-pc-linux-gnu < %s -o - | FileCheck %s
+
+define void @f() safestack {
+; CHECK-LABEL: define void @f
+entry:
+; CHECK:  %[[USP:.*]] = load i8*, i8** @__safestack_unsafe_stack_ptr
+; CHECK:   getelementptr i8, i8* %[[USP]], i32 -16
+
+  %x0 = alloca i64, align 8
+  %x1 = alloca i8, align 1
+  %x2 = alloca i64, align 8
+
+  %x0a = bitcast i64* %x0 to i8*
+  %x2a = bitcast i64* %x2 to i8*
+
+  call void @llvm.lifetime.start(i64 4, i8* %x0a)
+  call void @capture64(i64* %x0)
+  call void @llvm.lifetime.end(i64 4, i8* %x0a)
+
+  call void @llvm.lifetime.start(i64 4, i8* %x1)
+  call void @llvm.lifetime.start(i64 4, i8* %x2a)
+  call void @capture8(i8* %x1)
+  call void @capture64(i64* %x2)
+  call void @llvm.lifetime.end(i64 4, i8* %x1)
+  call void @llvm.lifetime.end(i64 4, i8* %x2a)
+
+; Test that i64 allocas share space.
+; CHECK: getelementptr i8, i8* %unsafe_stack_ptr, i32 -8
+; CHECK: getelementptr i8, i8* %unsafe_stack_ptr, i32 -9
+; CHECK: getelementptr i8, i8* %unsafe_stack_ptr, i32 -8
+
+  ret void
+}
+
+declare void @llvm.lifetime.start(i64, i8* nocapture)
+declare void @llvm.lifetime.end(i64, i8* nocapture)
+declare void @capture8(i8*)
+declare void @capture64(i64*)