MCFixup.h revision 5d672cfab096390690a1a5f33b0057c4cf252c55
1//===-- llvm/MC/MCFixup.h - Instruction Relocation and Patching -*- C++ -*-===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9 10#ifndef LLVM_MC_MCFIXUP_H 11#define LLVM_MC_MCFIXUP_H 12 13#include <cassert> 14 15namespace llvm { 16 17// Private constants, do not use. 18// 19// This is currently laid out so that the MCFixup fields can be efficiently 20// accessed, while keeping the offset field large enought that the assembler 21// backend can reasonably use the MCFixup representation for an entire fragment 22// (splitting any overly large fragments). 23// 24// The division of bits between the kind and the opindex can be tweaked if we 25// end up needing more bits for target dependent kinds. 26enum { 27 MCFIXUP_NUM_GENERIC_KINDS = 128, 28 MCFIXUP_NUM_KIND_BITS = 8, 29 MCFIXUP_NUM_OPINDEX_BITS = 8, 30 MCFIXUP_NUM_OFFSET_BITS = (32 - MCFIXUP_NUM_OPINDEX_BITS - 31 MCFIXUP_NUM_OPINDEX_BITS) 32}; 33 34/// MCFixupKind - Extensible enumeration to represent the type of a fixup. 35enum MCFixupKind { 36 FK_Data_1 = 0, ///< A one-byte fixup. 37 FK_Data_2, ///< A two-byte fixup. 38 FK_Data_4, ///< A four-byte fixup. 39 FK_Data_8, ///< A eight-byte fixup. 40 41 FirstTargetFixupKind = MCFIXUP_NUM_GENERIC_KINDS, 42 43 MaxTargetFixupKind = (1 << MCFIXUP_NUM_KIND_BITS) 44}; 45 46/// MCFixup - Encode information on a single operation to perform on an byte 47/// sequence (e.g., an encoded instruction) which requires assemble- or run- 48/// time patching. 49/// 50/// Fixups are used any time the target instruction encoder needs to represent 51/// some value in an instruction which is not yet concrete. The encoder will 52/// encode the instruction assuming the value is 0, and emit a fixup which 53/// communicates to the assembler backend how it should rewrite the encoded 54/// value. 55/// 56/// During the process of relaxation, the assembler will apply fixups as 57/// symbolic values become concrete. When relaxation is complete, any remaining 58/// fixups become relocations in the object file (or errors, if the fixup cannot 59/// be encoded on the target). 60class MCFixup { 61 static const unsigned MaxOffset = 1 << MCFIXUP_NUM_KIND_BITS; 62 63 /// The byte index of start of the relocation inside the encoded instruction. 64 unsigned Offset : MCFIXUP_NUM_OFFSET_BITS; 65 66 /// The index of the operand to encode into the instruction. 67 unsigned OpIndex : MCFIXUP_NUM_OPINDEX_BITS; 68 69 /// The target dependent kind of fixup item this is. The kind is used to 70 /// determine how the operand value should be encoded into the instruction. 71 unsigned Kind : MCFIXUP_NUM_KIND_BITS; 72 73public: 74 static MCFixup Create(unsigned Offset, unsigned OpIndex, MCFixupKind Kind) { 75 MCFixup FI; 76 FI.Offset = Offset; 77 FI.OpIndex = OpIndex; 78 FI.Kind = unsigned(Kind); 79 80 assert(Offset == FI.Offset && "Offset out of range!"); 81 assert(OpIndex == FI.OpIndex && "Operand index out of range!"); 82 assert(Kind == FI.Kind && "Kind out of range!"); 83 return FI; 84 } 85 86 unsigned getOffset() const { return Offset; } 87 88 unsigned getOpIndex() const { return OpIndex; } 89 90 MCFixupKind getKind() const { return MCFixupKind(Kind); } 91}; 92 93} // End llvm namespace 94 95#endif 96