Inside a register allocator

We are going to be discussing LLVM's Fast Register Allocator: you might like to open RegAllocFast.cpp and refer to it as we go through the article.

FastRegAlloc allocates linearly, going through instructions and their operands in order. It uses PhysRegState to keep track of the state of various physical registers: they can be 0 (regDisabled), 1 (regFree), 2 (regReserved), or a virtual register number (a large number). At the time of allocation, the full UseDef information is available (so you have information like LR.LastUse); a register can either be a <def> or a use. If IsImplicit is flipped, it could also be a implicit-def (<imp-def>) or implicit-use (<imp-use>).

%vreg349 = MOVZX32rm8 %vreg351, 1, %noreg, 0, %noreg

turns into

%ESI = MOVZX32rm8 %RDX, 1, %noreg, 0, %noreg

after allocation. The <kill> indicates that the instruction is the last use of %RDX. In another example,


turns into


Notice that the instruction references physical registers even before the allocation. That's because this instruction specifically wants to sign extend %RAX into %RDX (not any other register) for use in a later IDIV. Remember that IDIV operates on %RDX:%RAX as the numerator, and writes the quotient in %RAX, reminder in %RDX.

The instruction is modeled as MachineInstr, and the operands as MachineOperand. Note that a MO doesn't have to be a register: it could also be an immediate value; we use MO.isReg() to find out if it's a register, physical or virtual. In addition to the states shown in the pretty-print, MO can also be EarlyClobber, Dead, or Undef. A Dead MO implies Def, and indicates that the value defined is used no longer.

AllocateBasicBlock, which operates on MachineBasicBlock, can be separated into three different scans, that operate on register MOs. Before the first scan, we set up the LiveIns (registers coming in live from the previous MBB) as regReserved so they aren't clobbered. The first scan operates on physical registers that are allocatable Uses; it calls usePhysReg on them. At this stage, the physical register must be either regDisabled, regReserved, or regFree. It cannot be allocated to a virtual register. Why? Imagine you see:


Now, if %RAX were already allocated to a virtual register, we're basically screwed in this linear walk. Otherwise, kill it, mark it as regFree, and move on: we have done our part by completing the use of the register that was reserved earlier.

The second scan operates only on virtual register Uses. We add the register to LiveVirtRegs, via reloadVirtReg. If it didn't already exist in the map, we allocVirtReg it, which essentially gets the first regFree register not used in an instruction, and calls assignVirtToPhysReg on it. assignVirtToPhysReg is very simple: it just updates the PhysRegState mapping. Finally, reloadVirtReg updates the UsedInInstr map.

The third scan operates on physical and virtual registers that are Defs. If the register is a physical register, it does definePhysReg with regReserved unless MO.isDead(), in which case it's regFree. definePhysReg is very simple: it just puts the register in the state that was requested (regReserved or regFree, in this case).

To think about the problem, if we have,

%RAX = COPY %vreg342

we should always regReserve %RAX, right? Except if %RAX is dead. What about if it's an <imp-def>?


We didn't pass CQO %RAX explicitly, but that doesn't mean that a later instruction (for instance, IDIV) is not relying on this register's value. If we have an instruction between CQO and IDIV, that can potentially clobber %RAX, leading to a regalloc crash. Hence, we regReserve even if MO.isImplicit().

The third scan does defineVirtReg on virtual registers, to grab the first free physical register for the virtual register.

Note: the "pretty-prints" are generated by setting a breakpoint in AllocateBasicBlock and executing p MBB->dump().