lib/CodeGen/SafeStackLayout.cpp

   1 //===- SafeStackLayout.cpp - SafeStack frame layout -----------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8
   9 #include "SafeStackLayout.h"
  10 #include "SafeStackColoring.h"
  11 #include "llvm/IR/Value.h"
  12 #include "llvm/Support/CommandLine.h"
  13 #include "llvm/Support/Compiler.h"
  14 #include "llvm/Support/Debug.h"
  15 #include "llvm/Support/MathExtras.h"
  16 #include "llvm/Support/raw_ostream.h"
  17 #include <algorithm>
  18 #include <cassert>
  19
  20 using namespace llvm;
  21 using namespace llvm::safestack;
  22
  23 #define DEBUG_TYPE "safestacklayout"
  24
  25 static cl::opt<bool> ClLayout("safe-stack-layout",
  26                               cl::desc("enable safe stack layout"), cl::Hidden,
  27                               cl::init(true));
  28
  29 LLVM_DUMP_METHOD void StackLayout::print(raw_ostream &OS) {
  30   OS << "Stack regions:\n";
  31   for (unsigned i = 0; i < Regions.size(); ++i) {
  32     OS << "  " << i << ": [" << Regions[i].Start << ", " << Regions[i].End
  33        << "), range " << Regions[i].Range << "\n";
  34   }
  35   OS << "Stack objects:\n";
  36   for (auto &IT : ObjectOffsets) {
  37     OS << "  at " << IT.getSecond() << ": " << *IT.getFirst() << "\n";
  38   }
  39 }
  40
  41 void StackLayout::addObject(const Value *V, unsigned Size, unsigned Alignment,
  42                             const StackColoring::LiveRange &Range) {
  43   StackObjects.push_back({V, Size, Alignment, Range});
  44   ObjectAlignments[V] = Alignment;
  45   MaxAlignment = std::max(MaxAlignment, Alignment);
  46 }
  47
  48 static unsigned AdjustStackOffset(unsigned Offset, unsigned Size,
  49                                   unsigned Alignment) {
  50   return alignTo(Offset + Size, Alignment) - Size;
  51 }
  52
  53 void StackLayout::layoutObject(StackObject &Obj) {
  54   if (!ClLayout) {
  55     // If layout is disabled, just grab the next aligned address.
  56     // This effectively disables stack coloring as well.
  57     unsigned LastRegionEnd = Regions.empty() ? 0 : Regions.back().End;
  58     unsigned Start = AdjustStackOffset(LastRegionEnd, Obj.Size, Obj.Alignment);
  59     unsigned End = Start + Obj.Size;
  60     Regions.emplace_back(Start, End, Obj.Range);
  61     ObjectOffsets[Obj.Handle] = End;
  62     return;
  63   }
  64
  65   LLVM_DEBUG(dbgs() << "Layout: size " << Obj.Size << ", align "
  66                     << Obj.Alignment << ", range " << Obj.Range << "\n");
  67   assert(Obj.Alignment <= MaxAlignment);
  68   unsigned Start = AdjustStackOffset(0, Obj.Size, Obj.Alignment);
  69   unsigned End = Start + Obj.Size;
  70   LLVM_DEBUG(dbgs() << "  First candidate: " << Start << " .. " << End << "\n");
  71   for (const StackRegion &R : Regions) {
  72     LLVM_DEBUG(dbgs() << "  Examining region: " << R.Start << " .. " << R.End
  73                       << ", range " << R.Range << "\n");
  74     assert(End >= R.Start);
  75     if (Start >= R.End) {
  76       LLVM_DEBUG(dbgs() << "  Does not intersect, skip.\n");
  77       continue;
  78     }
  79     if (Obj.Range.Overlaps(R.Range)) {
  80       // Find the next appropriate location.
  81       Start = AdjustStackOffset(R.End, Obj.Size, Obj.Alignment);
  82       End = Start + Obj.Size;
  83       LLVM_DEBUG(dbgs() << "  Overlaps. Next candidate: " << Start << " .. "
  84                         << End << "\n");
  85       continue;
  86     }
  87     if (End <= R.End) {
  88       LLVM_DEBUG(dbgs() << "  Reusing region(s).\n");
  89       break;
  90     }
  91   }
  92
  93   unsigned LastRegionEnd = Regions.empty() ? 0 : Regions.back().End;
  94   if (End > LastRegionEnd) {
  95     // Insert a new region at the end. Maybe two.
  96     if (Start > LastRegionEnd) {
  97       LLVM_DEBUG(dbgs() << "  Creating gap region: " << LastRegionEnd << " .. "
  98                         << Start << "\n");
  99       Regions.emplace_back(LastRegionEnd, Start, StackColoring::LiveRange());
 100       LastRegionEnd = Start;
 101     }
 102     LLVM_DEBUG(dbgs() << "  Creating new region: " << LastRegionEnd << " .. "
 103                       << End << ", range " << Obj.Range << "\n");
 104     Regions.emplace_back(LastRegionEnd, End, Obj.Range);
 105     LastRegionEnd = End;
 106   }
 107
 108   // Split starting and ending regions if necessary.
 109   for (unsigned i = 0; i < Regions.size(); ++i) {
 110     StackRegion &R = Regions[i];
 111     if (Start > R.Start && Start < R.End) {
 112       StackRegion R0 = R;
 113       R.Start = R0.End = Start;
 114       Regions.insert(&R, R0);
 115       continue;
 116     }
 117     if (End > R.Start && End < R.End) {
 118       StackRegion R0 = R;
 119       R0.End = R.Start = End;
 120       Regions.insert(&R, R0);
 121       break;
 122     }
 123   }
 124
 125   // Update live ranges for all affected regions.
 126   for (StackRegion &R : Regions) {
 127     if (Start < R.End && End > R.Start)
 128       R.Range.Join(Obj.Range);
 129     if (End <= R.End)
 130       break;
 131   }
 132
 133   ObjectOffsets[Obj.Handle] = End;
 134 }
 135
 136 void StackLayout::computeLayout() {
 137   // Simple greedy algorithm.
 138   // If this is replaced with something smarter, it must preserve the property
 139   // that the first object is always at the offset 0 in the stack frame (for
 140   // StackProtectorSlot), or handle stack protector in some other way.
 141
 142   // Sort objects by size (largest first) to reduce fragmentation.
 143   if (StackObjects.size() > 2)
 144     std::stable_sort(StackObjects.begin() + 1, StackObjects.end(),
 145                      [](const StackObject &a, const StackObject &b) {
 146                        return a.Size > b.Size;
 147                      });
 148
 149   for (auto &Obj : StackObjects)
 150     layoutObject(Obj);
 151
 152   LLVM_DEBUG(print(dbgs()));
 153 }