Register Indexing

CPU register indexing in gem5 is a complicated by the need to support multiple ISAs with sometimes very different register semantics (register windows, condition codes, mode-based alternate register sets, etc.). In addition, this support has evolved gradually as new ISAs have been added, so older code may not take advantage of newer features or terminology.

Types of Register Indices

There are three types of register indices used internally in the CPU models: relative, unified, and flattened.

Relative

A relative register index is the index that is encoded in a machine instruction. There is a separate index space for each class of register (integer, floating point, etc.), starting at 0. The register class is implied by the opcode. Thus a value of "1" in a source register field may mean integer register 1 (e.g., "%r1") or floating point register 1 (e.g., "%f1") depending on the type of the instruction.

Unified

While relative register indices are good for keeping instruction encodings compact, they are ambiguous, and thus not convenient for things like managing dependencies. To avoid this ambiguity, the decoder maps the relative register indices into a unified register space by adding class-specific offsets to relocate each relative index range into a unique position. Integer registers are unmodified, and continue to start at zero. Floating-point register indices are offset by (at least) the number of integer registers, so that the first FP register (e.g., "%f0") gets a unified index that is greater than that of the last integer register. Similarly, miscellaneous (a.k.a. control) registers are mapped past the end of the FP register index space.

Flattened

Unified register indices provide an unambiguous description of all the registers that are accessible as instruction operands at a given point in the execution. Unfortunately, due to the complex features of some ISAs, they do not always unambiguously identify the actual state that the instruction is referencing. For example, in ISAs with register windows (notably SPARC), a particular register identifier such as "%o0" will refer to a different register after a "save" or "restore" operation than it did previously. Several ISAs have registers that are hidden in normal operation, but get mapped on top of ordinary registers when an interrupt occurs (e.g., ARM's mode-specific registers), or under explicit supervisor control (e.g., SPARC's "alternate globals").

We solve this problem by maintaining a flattened register space which provides a distinct index for every unique register storage location. For example, the integer portion of the SPARC flattened register space has distinct indices for the globals and the alternate globals, as well as for each of the available register windows. The "flattening" process of translating from a unified or relative register index to a flattened register index varies by ISA. On some ISAs, the mapping is trivial, while others use table lookups to do the translation.

A key distinction between the generation of unified and flattened register indices is that the former can always be done statically while the latter often depends on dynamic processor state. That is, the translation from relative to unified indices depends only on the context provided by the instruction itself (which is convenient as the translation is done in the decoder). In contrast, the mapping to a flattened register index may depend on processor state such as the interrupt level or the current window pointer on SPARC.

Combining Register Index Types

Although the typical progression for modifying register indices is relative -> unified -> flattened, it turns out that relative vs. unified and flattened vs. unflattened are orthogonal attributes. Relative vs. unified indicates whether the index is relative to the base register for its register class (integer, FP, or misc) or has the base offset for its class added in. Flattened vs. unflattened indicates whether the index has been adjusted to account for runtime context such as register window adjustments or alternate register file modes. Thus a relative flattened register index is one in which the runtime context has been accounted for, but is still expressed relative to the base offset for its class.

A single set of class-specific offsets is used to generate unified indices from relative indices regardless of whether the indices are flattened or unflattened. Thus the offsets must be large enough to separate the register classes even when flattened addresses are being used. As a result, the unflattened unified register space is often discontiguous.

Illustration

As an illustration, consider a hypothetical architecture with four integer registers (%r0-%r4), three FP registers (%f0-%f2), and two misc/control registers (%msr0-%msr1). In addition, the architecture supports a complete set of alternate integer and FP registers for fast context switching.

The resulting register file layout, along with the unified flattened register file indices, is shown at right. Although the indices in the picture range from 0 to 15, the actual set of valid indices depends on the type of index and (for relative indices) the register class as well:

Relative unflattened	Int: 0-3; FP: 0-2; Misc: 0-1
Unified unflattened	0-3, 8-10, 14-15
Relative flattened	Int: 0-7; FP: 0-5; Misc: 0-1
Unified flattened	0-15

In this example, register %f1 in the alternate FP register file could be referred to via the relative flattened index 4 as well as the relative unflattened index 1, the unified unflattened index 9, or the unified flattened index 12. Note that the difference between the relative and unified indices is always 8 (regardless of flattening), and the difference between the unflattened and flattened indices is 3 (regardless of relative vs. unified status).

Caveats

• Although the gem5 code is unfortunately not always clear about which type of register index is expected by a particular function, functions whose name incorporates a register class (e.g., readIntReg()) expect a relative register index, and functions that expect a flattened index often have "flat" in the function name.
• Although the general case is complicated, the common case can be deceptively simple. For example, because integer registers start at the beginning of the unified register space, relative and unified register indices are identical for integer registers. Furthermore, in an architecture with no (or rarely-used) alternate integer registers, the unflattened and flattened indices are (almost always) the same as well, meaning that all four types of register indices are interchangeable in this case. While this situation seems to be a simplification, it also tends to hide bugs where the wrong register index type is used.
• The description above is intended to illustrate the typical usage of these index types. There may be exceptions that don't precisely follow this description, but I got tired of writing "typically" in every sentence.
• The terms 'relative' and 'unified' were invented for use in this documentation, so you are unlikely see them in the code until the code starts catching up with this page.
• This discussion pertains only to the *architectural* registers. An out-of-order CPU model such as O3 adds another layer of complexity by renaming these architectural registers (using the flattened register indices) to an underlying physical register file.

Outstanding Questions

• The SPARC code looks broken, in that the FP_Base_DepTag value is much smaller than NumIntRegs (see src/arch/sparc/registers.hh). Is it really broken, or is there some subtlety not captured here that allows it to work?