Operand Collector

After an instruction is decoded, a hardware unit called a collector unit is allocated to buffer the source operands of the instruction.

指令译码后，一个叫Collector Unit的硬件单元开始工作，它负责将指令的源操作数缓冲起来。

The collector units are not used to eliminate name dependencies via register renaming, but rather as a way to space register operand accesses out in time so that no more than one access to a bank occurs in a single cycle. In the organization shown in the figure, each of the four collector units contains three operand entries. Each operand entry has four fields: a valid bit, a register identifier, a ready bit, and operand data. Each operand data field can hold a single 128 byte source operand composed of 32 four byte elements (one four byte value for each scalar thread in a warp). In addition, the collector unit contains an identifier indicating which warp the instruction belongs to. The arbitrator contains a read request queue for each bank to hold access requests until they are granted.

Collector Unit的功能不是通过寄存器重命名的方式来消除寄存器的数据依赖，而是将寄存器文件分成多个Bank，并且尽量保证让一条指令中的寄存器访问的是不同的Bank

When an instruction is received from the decode stage and a collector unit is available it is allocated to the instruction and the operand, warp, register identifier and valid bits are set. In addition, source operand read requests are queued in the arbiter. To simplify the design, data being written back by the execution units is always prioritized over read requests. The arbitrator selects a group of up to four non-conflicting accesses to se2nd to the register file. To reduce crossbar and collector unit area the selection is made so that each collector unit only receives one operand per cycle.

As each operand is read out of the register file and placed into the corresponding collector unit, a “ready bit” is set. Finally, when all the operands are ready the instruction is issued to a SIMD execution unit.

In our model, each back-end pipeline (SP, SFU, MEM) has a set of dedicated collector units, and they share a pool of general collector units. The number of units available to each pipeline and the capacity of the pool of the general units are configurable.