From d99f80b48e1f3502357e21d071531432be333b8e Mon Sep 17 00:00:00 2001 From: Evgeniy Stepanov Date: Tue, 2 Aug 2016 23:21:30 +0000 Subject: [PATCH] [safestack] Layout large allocas first to reduce fragmentation. llvm-svn: 277544 --- llvm/lib/CodeGen/SafeStackLayout.cpp | 8 ++++++ llvm/test/Transforms/SafeStack/layout-frag.ll | 39 +++++++++++++++++++++++++++ 2 files changed, 47 insertions(+) create mode 100644 llvm/test/Transforms/SafeStack/layout-frag.ll diff --git a/llvm/lib/CodeGen/SafeStackLayout.cpp b/llvm/lib/CodeGen/SafeStackLayout.cpp index fb433c1..7d4dbd1 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 0000000..125eb0f --- /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*) -- 2.7.4