e1bb7f6dde
- Add command line option `num-to-skip-size` to parameterize the size of `NumToSkip` bytes in the decoder table. Default value will be 2, and targets that need larger size can use 3. - Keep all existing targets, except AArch64, to use size 2, and change AArch64 to use size 3 since it run into the "disassembler decoding table too large" error with size 2. - Additional fixes on top of earlier revert: mark `decodeNumToSkip` as static (not necessary anymore as the generated code is now in anonymous namespace, but doing it for consistency) and incorporate Bazel build changes from https://github.com/llvm/llvm-project/pull/136212 - Following is a rough reduction in size for the decoder tables by switching to size 2. ``` Target Old Size New Size % Reduction ================================================ AArch64 153254 153254 0.00 AMDGPU 471566 412805 12.46 ARC 5724 5061 11.58 ARM 84936 73831 13.07 AVR 1497 1306 12.76 BPF 2172 1927 11.28 CSKY 10064 8692 13.63 Hexagon 47967 41965 12.51 Lanai 1108 982 11.37 LoongArch 24446 21621 11.56 MSP430 4200 3716 11.52 Mips 36330 31415 13.53 PPC 31897 28098 11.91 RISCV 37979 32790 13.66 Sparc 8331 7252 12.95 SystemZ 36722 32248 12.18 VE 48296 42873 11.23 XCore 2590 2316 10.58 Xtensa 3827 3316 13.35 ```
LLVM TableGen
The purpose of TableGen is to generate complex output files based on information from source files that are significantly easier to code than the output files would be, and also easier to maintain and modify over time.
The information is coded in a declarative style involving classes and records, which are then processed by TableGen.
class Hello <string _msg> {
string msg = !strconcat("Hello ", _msg);
}
def HelloWorld: Hello<"world!"> {}
------------- Classes -----------------
class Hello<string Hello:_msg = ?> {
string msg = !strconcat("Hello ", Hello:_msg);
}
------------- Defs -----------------
def HelloWorld { // Hello
string msg = "Hello world!";
}
Try this example on Compiler Explorer.
The internalized records are passed on to various backends, which extract information from a subset of the records and generate one or more output files.
These output files are typically .inc files for C++, but may be any type of file that the backend developer needs.
Resources for learning the language:
- TableGen Overview
- Programmer's reference guide
- Tutorial
- Tools for Learning LLVM TableGen
- Lessons in TableGen (video), slides
- Improving Your TableGen Descriptions (video), slides
Writing TableGen backends:
- TableGen Backend Developer's Guide
- How to write a TableGen backend (video), slides, also available as a notebook.
TableGen in MLIR:
Useful tools: