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The Register Allocation problem consists in mapping a +program Pv, that can use an unbounded number of +virtual registers, to a program Pp that contains a +finite (possibly small) number of physical registers. Each target +architecture has a different number of physical registers. If the +number of physical registers is not enough to accommodate all the +virtual registers, some of them will have to be mapped into +memory. These virtuals are called spilled virtuals.
+ +In LLVM, physical registers are denoted by integer numbers that +normally range from 1 to 1023. To see how this numbering is defined +for a particular architecture, you can read the +GenRegisterNames.inc file for that architecture. For +instance, by inspecting +lib/Target/X86/X86GenRegisterNames.inc we see that the 32-bit +register EAX is denoted by 15, and the MMX register +MM0 is mapped to 48.
+ +Some architectures contain registers that share the same physical +location. A notable example is the X86 platform. For instance, in the +X86 architecture, the registers EAX, AX and +AL share the first eight bits. These physical registers are +marked as aliased in LLVM. Given a particular architecture, you +can check which registers are aliased by inspecting its +RegisterInfo.td file. Moreover, the method +MRegisterInfo::getAliasSet(p_reg) returns an array containing +all the physical registers aliased to the register p_reg.
+ +Physical registers, in LLVM, are grouped in Register Classes. +Elements in the same register class are functionally equivalent, and can +be interchangeably used. Each virtual register can only be mapped to +physical registers of a particular class. For instance, in the X86 +architecture, some virtuals can only be allocated to 8 bit registers. +A register class is described by TargetRegisterClass objects. +To discover if a virtual register is compatible with a given physical, +this code can be used: +
+ +
+bool RegMapping_Fer::compatible_class(MachineFunction &mf,
+ unsigned v_reg,
+ unsigned p_reg) {
+ assert(MRegisterInfo::isPhysicalRegister(p_reg) &&
+ "Target register must be physical");
+ const TargetRegisterClass *trc = mf.getSSARegMap()->getRegClass(v_reg);
+ return trc->contains(p_reg);
+}
+
+Sometimes, mostly for debugging purposes, it is useful to change +the number of physical registers available in the target +architecture. This must be done statically, inside the +TargetRegsterInfo.td file. Just grep for +RegisterClass, the last parameter of which is a list of +registers. Just commenting some out is one simple way to avoid them +being used. A more polite way is to explicitly exclude some registers +from the allocation order. See the definition of the +GR register class in +lib/Target/IA64/IA64RegisterInfo.td for an example of this +(e.g., numReservedRegs registers are hidden.)
+ +Virtual registers are also denoted by integer numbers. Contrary to +physical registers, different virtual registers never share the same +number. The smallest virtual register is normally assigned the number +1024. This may change, so, in order to know which is the first virtual +register, you should access +MRegisterInfo::FirstVirtualRegister. Any register whose +number is greater than or equal to +MRegisterInfo::FirstVirtualRegister is considered a virtual +register. Whereas physical registers are statically defined in a +TargetRegisterInfo.td file and cannot be created by the +application developer, that is not the case with virtual registers. +In order to create new virtual registers, use the method +SSARegMap::createVirtualRegister(). This method will return a +virtual register with the highest code. +
+ +Before register allocation, the operands of an instruction are +mostly virtual registers, although physical registers may also be +used. In order to check if a given machine operand is a register, use +the boolean function MachineOperand::isRegister(). To obtain +the integer code of a register, use +MachineOperand::getReg(). An instruction may define or use a +register. For instance, ADD reg:1026 := reg:1025 reg:1024 +defines the registers 1024, and uses registers 1025 and 1026. Given a +register operand, the method MachineOperand::isUse() informs +if that register is being used by the instruction. The method +MachineOperand::isDef() informs if that registers is being +defined.
+ +We will call physical registers present in the LLVM bytecode before +register allocation pre-colored registers. Pre-colored +registers are used in many different situations, for instance, to pass +parameters of functions calls, and to store results of particular +instructions. There are two types of pre-colored registers: the ones +implicitly defined, and those explicitly +defined. Explicitly defined registers are normal operands, and can be +accessed with MachineInstr::getOperand(int)::getReg(). In +order to check which registers are implicitly defined by an +instruction, use the +TargetInstrInfo::get(opcode)::ImplicitDefs, where +opcode is the opcode of the target instruction. One important +difference between explicit and implicit physical registers is that +the latter are defined statically for each instruction, whereas the +former may vary depending on the program being compiled. For example, +an instruction that represents a function call will always implicitly +define or use the same set of physical registers. To read the +registers implicitly used by an instruction, use +TargetInstrInfo::get(opcode)::ImplicitUses. Pre-colored +registers impose constraints on any register allocation algorithm. The +register allocator must make sure that none of them is been +overwritten by the values of virtual registers while still alive.
+ +There are two ways to map virtual registers to physical registers (or to +memory slots). The first way, that we will call direct mapping, +is based on the use of methods of the classes MRegisterInfo, +and MachineOperand. The second way, that we will call +indirect mapping, relies on the VirtRegMap class in +order to insert loads and stores sending and getting values to and from +memory.
+ +The direct mapping provides more flexibility to the developer of +the register allocator; however, it is more error prone, and demands +more implementation work. Basically, the programmer will have to +specify where load and store instructions should be inserted in the +target function being compiled in order to get and store values in +memory. To assign a physical register to a virtual register present in +a given operand, use MachineOperand::setReg(p_reg). To insert +a store instruction, use +MRegisterInfo::storeRegToStackSlot(...), and to insert a load +instruction, use MRegisterInfo::loadRegFromStackSlot.
+ +The indirect mapping shields the application developer from the +complexities of inserting load and store instructions. In order to map +a virtual register to a physical one, use +VirtRegMap::assignVirt2Phys(vreg, preg). In order to map a +certain virtual register to memory, use +VirtRegMap::assignVirt2StackSlot(vreg). This method will +return the stack slot where vreg's value will be located. If +it is necessary to map another virtual register to the same stack +slot, use VirtRegMap::assignVirt2StackSlot(vreg, +stack_location). One important point to consider when using the +indirect mapping, is that even if a virtual register is mapped to +memory, it still needs to be mapped to a physical register. This +physical register is the location where the virtual register is +supposed to be found before being stored or after being reloaded.
+ +If the indirect strategy is used, after all the virtual registers +have been mapped to physical registers or stack slots, it is necessary +to use a spiller object to place load and store instructions in the +code. Every virtual that has been mapped to a stack slot will be +stored to memory after been defined and will be loaded before being +used. The implementation of the spiller tries to recycle load/store +instructions, avoiding unnecessary instructions. For an example of how +to invoke the spiller, see +RegAllocLinearScan::runOnMachineFunction in +lib/CodeGen/RegAllocLinearScan.cpp.
+ +With very rare exceptions (e.g., function calls), the LLVM machine +code instructions are three address instructions. That is, each +instruction is expected to define at most one register, and to use at +most two registers. However, some architectures use two address +instructions. In this case, the defined register is also one of the +used register. For instance, an instruction such as ADD %EAX, +%EBX, in X86 is actually equivalent to %EAX = %EAX + +%EBX.
+ +In order to produce correct code, LLVM must convert three address +instructions that represent two address instructions into true two +address instructions. LLVM provides the pass +TwoAddressInstructionPass for this specific purpose. It must +be run before register allocation takes place. After its execution, +the resulting code may no longer be in SSA form. This happens, for +instance, in situations where an instruction such as %a = ADD %b +%c is converted to two instructions such as:
+ ++%a = MOVE %b +%a = ADD %a %b ++
Notice that, internally, the second instruction is represented as +ADD %a[def/use] %b. I.e., the register operand %a is +both used and defined by the instruction.
+ +An important transformation that happens during register allocation is called +the SSA Deconstruction Phase. The SSA form simplifies many +analyses that are performed on the control flow graph of +programs. However, traditional instruction sets do not implement +PHI instructions. Thus, in order to generate executable code, compilers +must replace PHI instructions with other instructions that preserve their +semantics.
+ +There are many ways in which PHI instructions can safely be removed +from the target code. The most traditional PHI deconstruction +algorithm replaces PHI instructions with copy instructions. That is +the strategy adopted by LLVM. The SSA deconstruction algorithm is +implemented in nlib/CodeGen/>PHIElimination.cpp. In order to +invoke this pass, the identifier PHIEliminationID must be +marked as required in the code of the register allocator.
+ +Instruction folding is an optimization performed during +register allocation that removes unnecessary copy instructions. For +instance, a sequence of instructions such as:
+ ++%EBX = LOAD %mem_address +%EAX = COPY %EBX ++
can be safely substituted by the single instruction: + +
+%EAX = LOAD %mem_address ++
Instructions can be folded with the +MRegisterInfo::foldMemoryOperand(...) method. Care must be +taken when folding instructions; a folded instruction can be quite +different from the original instruction. See +LiveIntervals::addIntervalsForSpills in +lib/CodeGen/LiveIntervalAnalysis.cpp for an example of its use.
+ +The LLVM infrastructure provides the application developer with +three different register allocators:
+ +The type of register allocator used in llc can be chosen with the +command line option -regalloc=...:
+ ++$ llc -f -regalloc=simple file.bc -o sp.s; +$ llc -f -regalloc=local file.bc -o lc.s; +$ llc -f -regalloc=linearscan file.bc -o ln.s; ++