js/src/frontend/JumpList.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
   2  * vim: set ts=8 sts=2 et sw=2 tw=80:
   3  * This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef frontend_JumpList_h
   8 #define frontend_JumpList_h
   9
  10 #include <stddef.h>  // ptrdiff_t
  11
  12 #include "frontend/BytecodeOffset.h"  // BytecodeOffset
  13 #include "js/TypeDecls.h"             // jsbytecode
  14
  15 namespace js {
  16 namespace frontend {
  17
  18 // Linked list of jump instructions that need to be patched. The linked list is
  19 // stored in the bytes of the incomplete bytecode that will be patched, so no
  20 // extra memory is needed, and patching the instructions destroys the list.
  21 //
  22 // Example:
  23 //
  24 //     JumpList brList;
  25 //     if (!emitJump(JSOp::JumpIfFalse, &brList)) {
  26 //         return false;
  27 //     }
  28 //     ...
  29 //     JumpTarget label;
  30 //     if (!emitJumpTarget(&label)) {
  31 //         return false;
  32 //     }
  33 //     ...
  34 //     if (!emitJump(JSOp::Goto, &brList)) {
  35 //         return false;
  36 //     }
  37 //     ...
  38 //     patchJumpsToTarget(brList, label);
  39 //
  40 //                      +-> (the delta is END_OF_LIST_DELTA (=0) for the last
  41 //                      |    item)
  42 //                      |
  43 //                      |
  44 //    JumpIfFalse .. <+ +                +-+   JumpIfFalse ..
  45 //    ..              |                  |     ..
  46 //  label:            |                  +-> label:
  47 //    JumpTarget      |                  |     JumpTarget
  48 //    ..              |                  |     ..
  49 //    Goto .. <+ +----+                  +-+   Goto .. <+
  50 //             |                                   |
  51 //             |                                   |
  52 //             +                                   +
  53 //           brList                              brList
  54 //
  55 //       |                                  ^
  56 //       +------- patchJumpsToTarget -------+
  57 //
  58
  59 // Offset of a jump target instruction, used for patching jump instructions.
  60 struct JumpTarget {
  61   BytecodeOffset offset = BytecodeOffset::invalidOffset();
  62 };
  63
  64 struct JumpList {
  65   // Delta value for pre-patchJumpsToTarget that marks the end of the link.
  66   static const ptrdiff_t END_OF_LIST_DELTA = 0;
  67
  68   // -1 is used to mark the end of jump lists.
  69   JumpList() : offset(BytecodeOffset::invalidOffset()) {}
  70
  71   BytecodeOffset offset;
  72
  73   // Add a jump instruction to the list.
  74   void push(jsbytecode* code, BytecodeOffset jumpOffset);
  75
  76   // Patch all jump instructions in this list to jump to `target`.  This
  77   // clobbers the list.
  78   void patchAll(jsbytecode* code, JumpTarget target);
  79 };
  80
  81 } /* namespace frontend */
  82 } /* namespace js */
  83
  84 #endif /* frontend_JumpList_h */