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