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//===- SafeStackLayout.cpp - SafeStack frame layout -----------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "SafeStackLayout.h"
#include "llvm/IR/Value.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cassert>
using namespace llvm;
using namespace llvm::safestack;
#define DEBUG_TYPE "safestacklayout"
static cl::opt<bool> ClLayout("safe-stack-layout",
cl::desc("enable safe stack layout"), cl::Hidden,
cl::init(true));
LLVM_DUMP_METHOD void StackLayout::print(raw_ostream &OS) {
OS << "Stack regions:\n";
for (unsigned i = 0; i < Regions.size(); ++i) {
OS << " " << i << ": [" << Regions[i].Start << ", " << Regions[i].End
<< "), range " << Regions[i].Range << "\n";
}
OS << "Stack objects:\n";
for (auto &IT : ObjectOffsets) {
OS << " at " << IT.getSecond() << ": " << *IT.getFirst() << "\n";
}
}
void StackLayout::addObject(const Value *V, unsigned Size, Align Alignment,
const StackLifetime::LiveRange &Range) {
StackObjects.push_back({V, Size, Alignment, Range});
ObjectAlignments[V] = Alignment;
MaxAlignment = std::max(MaxAlignment, Alignment);
}
static unsigned AdjustStackOffset(unsigned Offset, unsigned Size,
Align Alignment) {
return alignTo(Offset + Size, Alignment) - Size;
}
void StackLayout::layoutObject(StackObject &Obj) {
if (!ClLayout) {
// If layout is disabled, just grab the next aligned address.
// This effectively disables stack coloring as well.
unsigned LastRegionEnd = Regions.empty() ? 0 : Regions.back().End;
unsigned Start = AdjustStackOffset(LastRegionEnd, Obj.Size, Obj.Alignment);
unsigned End = Start + Obj.Size;
Regions.emplace_back(Start, End, Obj.Range);
ObjectOffsets[Obj.Handle] = End;
return;
}
LLVM_DEBUG(dbgs() << "Layout: size " << Obj.Size << ", align "
<< Obj.Alignment.value() << ", range " << Obj.Range
<< "\n");
assert(Obj.Alignment <= MaxAlignment);
unsigned Start = AdjustStackOffset(0, Obj.Size, Obj.Alignment);
unsigned End = Start + Obj.Size;
LLVM_DEBUG(dbgs() << " First candidate: " << Start << " .. " << End << "\n");
for (const StackRegion &R : Regions) {
LLVM_DEBUG(dbgs() << " Examining region: " << R.Start << " .. " << R.End
<< ", range " << R.Range << "\n");
assert(End >= R.Start);
if (Start >= R.End) {
LLVM_DEBUG(dbgs() << " Does not intersect, skip.\n");
continue;
}
if (Obj.Range.overlaps(R.Range)) {
// Find the next appropriate location.
Start = AdjustStackOffset(R.End, Obj.Size, Obj.Alignment);
End = Start + Obj.Size;
LLVM_DEBUG(dbgs() << " Overlaps. Next candidate: " << Start << " .. "
<< End << "\n");
continue;
}
if (End <= R.End) {
LLVM_DEBUG(dbgs() << " Reusing region(s).\n");
break;
}
}
unsigned LastRegionEnd = Regions.empty() ? 0 : Regions.back().End;
if (End > LastRegionEnd) {
// Insert a new region at the end. Maybe two.
if (Start > LastRegionEnd) {
LLVM_DEBUG(dbgs() << " Creating gap region: " << LastRegionEnd << " .. "
<< Start << "\n");
Regions.emplace_back(LastRegionEnd, Start, StackLifetime::LiveRange(0));
LastRegionEnd = Start;
}
LLVM_DEBUG(dbgs() << " Creating new region: " << LastRegionEnd << " .. "
<< End << ", range " << Obj.Range << "\n");
Regions.emplace_back(LastRegionEnd, End, Obj.Range);
LastRegionEnd = End;
}
// Split starting and ending regions if necessary.
for (unsigned i = 0; i < Regions.size(); ++i) {
StackRegion &R = Regions[i];
if (Start > R.Start && Start < R.End) {
StackRegion R0 = R;
R.Start = R0.End = Start;
Regions.insert(&R, R0);
continue;
}
if (End > R.Start && End < R.End) {
StackRegion R0 = R;
R0.End = R.Start = End;
Regions.insert(&R, R0);
break;
}
}
// Update live ranges for all affected regions.
for (StackRegion &R : Regions) {
if (Start < R.End && End > R.Start)
R.Range.join(Obj.Range);
if (End <= R.End)
break;
}
ObjectOffsets[Obj.Handle] = End;
}
void StackLayout::computeLayout() {
// Simple greedy algorithm.
// 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)
llvm::stable_sort(drop_begin(StackObjects),
[](const StackObject &a, const StackObject &b) {
return a.Size > b.Size;
});
for (auto &Obj : StackObjects)
layoutObject(Obj);
LLVM_DEBUG(print(dbgs()));
}
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