1 //===- CodeGenCommonISel.h - Common code between ISels ---------*- C++ -*--===//
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 // This file declares common utilities that are shared between SelectionDAG and
10 // GlobalISel frameworks.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #ifndef LLVM_CODEGEN_CODEGENCOMMONISEL_H
15 #define LLVM_CODEGEN_CODEGENCOMMONISEL_H
16 
17 #include "llvm/CodeGen/MachineBasicBlock.h"
18 #include <cassert>
19 namespace llvm {
20 
21 class BasicBlock;
22 class MachineBasicBlock;
23 /// Encapsulates all of the information needed to generate a stack protector
24 /// check, and signals to isel when initialized that one needs to be generated.
25 ///
26 /// *NOTE* The following is a high level documentation of SelectionDAG Stack
27 /// Protector Generation. This is now also ported be shared with GlobalISel,
28 /// but without any significant changes.
29 ///
30 /// High Level Overview of ISel Stack Protector Generation:
31 ///
32 /// Previously, the "stack protector" IR pass handled stack protector
33 /// generation. This necessitated splitting basic blocks at the IR level to
34 /// create the success/failure basic blocks in the tail of the basic block in
35 /// question. As a result of this, calls that would have qualified for the
36 /// sibling call optimization were no longer eligible for optimization since
37 /// said calls were no longer right in the "tail position" (i.e. the immediate
38 /// predecessor of a ReturnInst instruction).
39 ///
40 /// Since the sibling call optimization causes the callee to reuse the caller's
41 /// stack, if we could delay the generation of the stack protector check until
42 /// later in CodeGen after the sibling call decision was made, we get both the
43 /// tail call optimization and the stack protector check!
44 ///
45 /// A few goals in solving this problem were:
46 ///
47 ///   1. Preserve the architecture independence of stack protector generation.
48 ///
49 ///   2. Preserve the normal IR level stack protector check for platforms like
50 ///      OpenBSD for which we support platform-specific stack protector
51 ///      generation.
52 ///
53 /// The main problem that guided the present solution is that one can not
54 /// solve this problem in an architecture independent manner at the IR level
55 /// only. This is because:
56 ///
57 ///   1. The decision on whether or not to perform a sibling call on certain
58 ///      platforms (for instance i386) requires lower level information
59 ///      related to available registers that can not be known at the IR level.
60 ///
61 ///   2. Even if the previous point were not true, the decision on whether to
62 ///      perform a tail call is done in LowerCallTo in SelectionDAG (or
63 ///      CallLowering in GlobalISel) which occurs after the Stack Protector
64 ///      Pass. As a result, one would need to put the relevant callinst into the
65 ///      stack protector check success basic block (where the return inst is
66 ///      placed) and then move it back later at ISel/MI time before the
67 ///      stack protector check if the tail call optimization failed. The MI
68 ///      level option was nixed immediately since it would require
69 ///      platform-specific pattern matching. The ISel level option was
70 ///      nixed because SelectionDAG only processes one IR level basic block at a
71 ///      time implying one could not create a DAG Combine to move the callinst.
72 ///
73 /// To get around this problem:
74 ///
75 ///   1. SelectionDAG can only process one block at a time, we can generate
76 ///      multiple machine basic blocks for one IR level basic block.
77 ///      This is how we handle bit tests and switches.
78 ///
79 ///   2. At the MI level, tail calls are represented via a special return
80 ///      MIInst called "tcreturn". Thus if we know the basic block in which we
81 ///      wish to insert the stack protector check, we get the correct behavior
82 ///      by always inserting the stack protector check right before the return
83 ///      statement. This is a "magical transformation" since no matter where
84 ///      the stack protector check intrinsic is, we always insert the stack
85 ///      protector check code at the end of the BB.
86 ///
87 /// Given the aforementioned constraints, the following solution was devised:
88 ///
89 ///   1. On platforms that do not support ISel stack protector check
90 ///      generation, allow for the normal IR level stack protector check
91 ///      generation to continue.
92 ///
93 ///   2. On platforms that do support ISel stack protector check
94 ///      generation:
95 ///
96 ///     a. Use the IR level stack protector pass to decide if a stack
97 ///        protector is required/which BB we insert the stack protector check
98 ///        in by reusing the logic already therein.
99 ///
100 ///     b. After we finish selecting the basic block, we produce the validation
101 ///        code with one of these techniques:
102 ///          1) with a call to a guard check function
103 ///          2) with inlined instrumentation
104 ///
105 ///        1) We insert a call to the check function before the terminator.
106 ///
107 ///        2) We first find a splice point in the parent basic block
108 ///        before the terminator and then splice the terminator of said basic
109 ///        block into the success basic block. Then we code-gen a new tail for
110 ///        the parent basic block consisting of the two loads, the comparison,
111 ///        and finally two branches to the success/failure basic blocks. We
112 ///        conclude by code-gening the failure basic block if we have not
113 ///        code-gened it already (all stack protector checks we generate in
114 ///        the same function, use the same failure basic block).
115 class StackProtectorDescriptor {
116 public:
117   StackProtectorDescriptor() = default;
118 
119   /// Returns true if all fields of the stack protector descriptor are
120   /// initialized implying that we should/are ready to emit a stack protector.
121   bool shouldEmitStackProtector() const {
122     return ParentMBB && SuccessMBB && FailureMBB;
123   }
124 
125   bool shouldEmitFunctionBasedCheckStackProtector() const {
126     return ParentMBB && !SuccessMBB && !FailureMBB;
127   }
128 
129   /// Initialize the stack protector descriptor structure for a new basic
130   /// block.
131   void initialize(const BasicBlock *BB, MachineBasicBlock *MBB,
132                   bool FunctionBasedInstrumentation) {
133     // Make sure we are not initialized yet.
134     assert(!shouldEmitStackProtector() && "Stack Protector Descriptor is "
135                                           "already initialized!");
136     ParentMBB = MBB;
137     if (!FunctionBasedInstrumentation) {
138       SuccessMBB = addSuccessorMBB(BB, MBB, /* IsLikely */ true);
139       FailureMBB = addSuccessorMBB(BB, MBB, /* IsLikely */ false, FailureMBB);
140     }
141   }
142 
143   /// Reset state that changes when we handle different basic blocks.
144   ///
145   /// This currently includes:
146   ///
147   /// 1. The specific basic block we are generating a
148   /// stack protector for (ParentMBB).
149   ///
150   /// 2. The successor machine basic block that will contain the tail of
151   /// parent mbb after we create the stack protector check (SuccessMBB). This
152   /// BB is visited only on stack protector check success.
153   void resetPerBBState() {
154     ParentMBB = nullptr;
155     SuccessMBB = nullptr;
156   }
157 
158   /// Reset state that only changes when we switch functions.
159   ///
160   /// This currently includes:
161   ///
162   /// 1. FailureMBB since we reuse the failure code path for all stack
163   /// protector checks created in an individual function.
164   ///
165   /// 2.The guard variable since the guard variable we are checking against is
166   /// always the same.
167   void resetPerFunctionState() { FailureMBB = nullptr; }
168 
169   MachineBasicBlock *getParentMBB() { return ParentMBB; }
170   MachineBasicBlock *getSuccessMBB() { return SuccessMBB; }
171   MachineBasicBlock *getFailureMBB() { return FailureMBB; }
172 
173 private:
174   /// The basic block for which we are generating the stack protector.
175   ///
176   /// As a result of stack protector generation, we will splice the
177   /// terminators of this basic block into the successor mbb SuccessMBB and
178   /// replace it with a compare/branch to the successor mbbs
179   /// SuccessMBB/FailureMBB depending on whether or not the stack protector
180   /// was violated.
181   MachineBasicBlock *ParentMBB = nullptr;
182 
183   /// A basic block visited on stack protector check success that contains the
184   /// terminators of ParentMBB.
185   MachineBasicBlock *SuccessMBB = nullptr;
186 
187   /// This basic block visited on stack protector check failure that will
188   /// contain a call to __stack_chk_fail().
189   MachineBasicBlock *FailureMBB = nullptr;
190 
191   /// Add a successor machine basic block to ParentMBB. If the successor mbb
192   /// has not been created yet (i.e. if SuccMBB = 0), then the machine basic
193   /// block will be created. Assign a large weight if IsLikely is true.
194   MachineBasicBlock *addSuccessorMBB(const BasicBlock *BB,
195                                      MachineBasicBlock *ParentMBB,
196                                      bool IsLikely,
197                                      MachineBasicBlock *SuccMBB = nullptr);
198 };
199 
200 /// Find the split point at which to splice the end of BB into its success stack
201 /// protector check machine basic block.
202 ///
203 /// On many platforms, due to ABI constraints, terminators, even before register
204 /// allocation, use physical registers. This creates an issue for us since
205 /// physical registers at this point can not travel across basic
206 /// blocks. Luckily, selectiondag always moves physical registers into vregs
207 /// when they enter functions and moves them through a sequence of copies back
208 /// into the physical registers right before the terminator creating a
209 /// ``Terminator Sequence''. This function is searching for the beginning of the
210 /// terminator sequence so that we can ensure that we splice off not just the
211 /// terminator, but additionally the copies that move the vregs into the
212 /// physical registers.
213 MachineBasicBlock::iterator
214 findSplitPointForStackProtector(MachineBasicBlock *BB,
215                                 const TargetInstrInfo &TII);
216 
217 } // namespace llvm
218 
219 #endif // LLVM_CODEGEN_CODEGENCOMMONISEL_H
220