llvm/llvm/lib/CodeGen/CFIFixup.cpp

//===------ CFIFixup.cpp - Insert CFI remember/restore instructions -------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//

// This pass inserts the necessary  instructions to adjust for the inconsistency
// of the call-frame information caused by final machine basic block layout.
// The pass relies in constraints LLVM imposes on the placement of
// save/restore points (cf. ShrinkWrap) and has certain preconditions about
// placement of CFI instructions:
// * For any two CFI instructions of the function prologue one dominates
//   and is post-dominated by the other.
// * The function possibly contains multiple epilogue blocks, where each
//   epilogue block is complete and self-contained, i.e. CSR restore
//   instructions (and the corresponding CFI instructions)
//   are not split across two or more blocks.
// * CFI instructions are not contained in any loops.

// Thus, during execution, at the beginning and at the end of each basic block,
// following the prologue, the function can be in one of two states:
//  - "has a call frame", if the function has executed the prologue, and
//    has not executed any epilogue
//  - "does not have a call frame", if the function has not executed the
//    prologue, or has executed an epilogue
// which can be computed by a single RPO traversal.

// The location of the prologue is determined by finding the first block in the
// reverse traversal which contains CFI instructions.

// In order to accommodate backends which do not generate unwind info in
// epilogues we compute an additional property "strong no call frame on entry",
// which is set for the entry point of the function and for every block
// reachable from the entry along a path that does not execute the prologue. If
// this property holds, it takes precedence over the "has a call frame"
// property.

// From the point of view of the unwind tables, the "has/does not have call
// frame" state at beginning of each block is determined by the state at the end
// of the previous block, in layout order. Where these states differ, we insert
// compensating CFI instructions, which come in two flavours:

//   - CFI instructions, which reset the unwind table state to the initial one.
//     This is done by a target specific hook and is expected to be trivial
//     to implement, for example it could be:
//       .cfi_def_cfa <sp>, 0
//       .cfi_same_value <rN>
//       .cfi_same_value <rN-1>
//       ...
//     where <rN> are the callee-saved registers.
//   - CFI instructions, which reset the unwind table state to the one
//     created by the function prologue. These are
//       .cfi_restore_state
//       .cfi_remember_state
//     In this case we also insert a `.cfi_remember_state` after the last CFI
//     instruction in the function prologue.
//
// Known limitations:
//  * the pass cannot handle an epilogue preceding the prologue in the basic
//    block layout
//  * the pass does not handle functions where SP is used as a frame pointer and
//    SP adjustments up and down are done in different basic blocks (TODO)
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/CFIFixup.h"

#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SmallBitVector.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetFrameLowering.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/MC/MCDwarf.h"
#include "llvm/Target/TargetMachine.h"

usingnamespacellvm;

#define DEBUG_TYPE …

char CFIFixup::ID = …;

INITIALIZE_PASS(…)
FunctionPass *llvm::createCFIFixup() { … }

static bool isPrologueCFIInstruction(const MachineInstr &MI) { … }

static bool containsEpilogue(const MachineBasicBlock &MBB) { … }

static MachineBasicBlock *
findPrologueEnd(MachineFunction &MF, MachineBasicBlock::iterator &PrologueEnd) { … }

bool CFIFixup::runOnMachineFunction(MachineFunction &MF) { … }