[llvm-dev] Purpose of various register classes in X86 target

Craig Topper via llvm-dev llvm-dev at lists.llvm.org
Thu Jul 27 23:23:23 PDT 2017

The "main" X86 register classes are pretty much

GR8 - 8-bit general purpose registers
GR16 - 16-bit general purpose registers
GR32 - 32-bit general purpose registers
GR64 - 64-bit general purpose registers
VR64 - mmx registers
VR128 - xmm0-xmm15 (vex and legacy sse encodable registers)
VR256 - ymm0-ymm15 (vex encodable registers)
VR512 - zmm0-zmm31(evex encodable registers)
VR128X - xmm0-xmm31 (evex encodable)
VR256X - ymm0-ymm31 (evex encodable)
FR32 - xmm0-xmm15 used for single precision floating point
FR32X - xmm0-xmm31 used for single precision floating point with evex
FR64 - xmm0-xmm15 used for double precision floating point
FR64X - xmm0-xmm31 used for double precision floating point with evex
VK* - mask registers
VK*WM - mask registers excluding k0 which can be used for write masking
RFP32 - single precision x87 floating point
RFP64 - double precision x87 floating point
RFP80 - extended precision x87 floating point
FR128 - Some 128-bit integer and floating point in xmm registers. Only
partially supported since there is no hardware support for those types.

The following are used for specific instrutions that take certain registers
that aren't in the normal classes, but those instructions are never
selected and the registers are never allocated. The instructions are only
usable through inline assembly.

The RST class is after the FP stackifier has converted RFP*.

CCR and FPCCR just contain the integer and floating point flag registers.
These aren't used for register allocation. But they are used by name in
other places.

The others that contain subsets of the above exist for specific purposes.
The best way to find their purpose if they don't have a comment is to grep
for them in the X86 directory. If you have specific questions about any of
these I can try to help.


On Thu, Jul 27, 2017 at 10:47 PM, Arvind via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello Matthias,
> On 28 July 2017 at 04:13, Matthias Braun <mbraun at apple.com> wrote:
>> It's not that hard in principle:
>> - A register class is a set of registers.
>> - Virtual Registers have a register class assigned.
>> - If you have register constraints (like x86 8bit operations only work on
>> al,ah,etc.) then you have to create a new register class to express that.
>> (The only exception being limited to a single register, which instead we
>> express by assigning the physreg directly instead of using a vreg).
>> - Tablegen may create more regsiter classes for register coalescing where
>> we want to accomodate constraints of multiple instructions at the same time.
>> - All the information is in the .td file; you just have to put some
>> effort into learning tablegen as the information is often expressed by
>> using functions (i.e. the use add/sub/rotate/etc.) instead of just writing
>> a table/list of registers).
> Thanks a lot for the response! The TableGen language is fairly
> straightforward(at least commands used in X86 td file). However, some of
> the comments about the classes didn't fully make sense: I suppose the
> constraints are probably derived from the X86 assembly language and I
> should look there? In addition, some classes don't have any comments(eg:
> GR64_TCW64) and I couldn't find much info elsewhere.
> Also, I don't know how the TableGen'erated classes are derived. Would
> reading TableGen's code help understand why those were generated and what
> constraints they encode? Or should I take another approach?
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