js/src/jit/Label.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 jit_Label_h
   8 #define jit_Label_h
   9
  10 #include "mozilla/Assertions.h"
  11
  12 #include <stdint.h>
  13
  14 namespace js {
  15 namespace jit {
  16
  17 struct LabelBase {
  18  private:
  19   // We use uint32_t instead of bool to ensure MSVC packs these fields
  20   // correctly.
  21   uint32_t bound_ : 1;
  22
  23   // offset_ < INVALID_OFFSET means that the label is either bound or has
  24   // incoming uses and needs to be bound.
  25   uint32_t offset_ : 31;
  26
  27   void operator=(const LabelBase& label) = delete;
  28
  29 #if defined(JS_CODEGEN_MIPS32) || defined(JS_CODEGEN_MIPS64) || \
  30     defined(JS_CODEGEN_LOONG64) || defined(JS_CODEGEN_RISCV64)
  31  public:
  32 #endif
  33   static const uint32_t INVALID_OFFSET = 0x7fffffff;  // UINT31_MAX.
  34
  35  public:
  36   LabelBase() : bound_(false), offset_(INVALID_OFFSET) {}
  37
  38   // If the label is bound, all incoming edges have been patched and any
  39   // future incoming edges will be immediately patched.
  40   bool bound() const { return bound_; }
  41   int32_t offset() const {
  42     MOZ_ASSERT(bound() || used());
  43     return offset_;
  44   }
  45   // Returns whether the label is not bound, but has incoming uses.
  46   bool used() const { return !bound() && offset_ < INVALID_OFFSET; }
  47   // Binds the label, fixing its final position in the code stream.
  48   void bind(int32_t offset) {
  49     MOZ_ASSERT(!bound());
  50     MOZ_ASSERT(offset >= 0);
  51     MOZ_ASSERT(uint32_t(offset) < INVALID_OFFSET);
  52     offset_ = offset;
  53     bound_ = true;
  54     MOZ_ASSERT(offset_ == offset, "offset fits in 31 bits");
  55   }
  56   // Marks the label as neither bound nor used.
  57   void reset() {
  58     offset_ = INVALID_OFFSET;
  59     bound_ = false;
  60   }
  61   // Sets the label's latest used position.
  62   void use(int32_t offset) {
  63     MOZ_ASSERT(!bound());
  64     MOZ_ASSERT(offset >= 0);
  65     MOZ_ASSERT(uint32_t(offset) < INVALID_OFFSET);
  66     offset_ = offset;
  67     MOZ_ASSERT(offset_ == offset, "offset fits in 31 bits");
  68   }
  69 };
  70
  71 // A label represents a position in an assembly buffer that may or may not have
  72 // already been generated. Labels can either be "bound" or "unbound", the
  73 // former meaning that its position is known and the latter that its position
  74 // is not yet known.
  75 //
  76 // A jump to an unbound label adds that jump to the label's incoming queue. A
  77 // jump to a bound label automatically computes the jump distance. The process
  78 // of binding a label automatically corrects all incoming jumps.
  79 class Label : public LabelBase {
  80  public:
  81 #ifdef DEBUG
  82   ~Label();
  83 #endif
  84 };
  85
  86 static_assert(sizeof(Label) == sizeof(uint32_t),
  87               "Label should have same size as uint32_t");
  88
  89 // Label's destructor asserts that if it has been used it has also been bound.
  90 // In the case long-lived labels, however, failed compilation (e.g. OOM) will
  91 // trigger this failure innocuously. This Label silences the assertion.
  92 class NonAssertingLabel : public Label {
  93  public:
  94 #ifdef DEBUG
  95   ~NonAssertingLabel() {
  96     if (used()) {
  97       bind(0);
  98     }
  99   }
 100 #endif
 101 };
 102
 103 }  // namespace jit
 104 }  // namespace js
 105
 106 #endif  // jit_Label_h