d416289417
Fix the dependency of `CodeGenDAGPatterns::ParseDefaultOperands()` on
the particular order of SDNode definitions. Implicit usage of the first
definition as a placeholder makes `llvm-tblgen -gen-dag-isel` fail if
that SDNode is not usable as an output pattern operator and an instance
of `OperandWithDefaultOps` is used in a pattern.
Presently, each `OperandWithDefaultOps` record is processed by
constructing an instance of TreePattern from its `DefaultOps` argument
that has the form `(ops ...)`. Even though the result of processing the
root operator of that DAG is not inspected by `ParseDefaultOperands()`
function itself, that operator has to be supported by the underlying
`TreePattern::ParseTreePattern()` function. For that reason, a temporary
DAG is created by replacing the root operator of `DefaultOps` argument
with the first SDNode defined, which is usually `def imm : ...` defined
in `TargetSelectionDAG.td` file.
This results in misleading errors being reported when implementing new
`SDNode` types, if the new definition happens to be added before the
`def imm : ...` line. The error is reported by several test cases
executed by `check-llvm` target, as well as by the regular build, if one
of the enabled targets inherit one of its operand types from
`OperandWithDefaultOps`:
OptionalIntOperand: ../llvm/test/TableGen/DAGDefaultOps.td:28:5: error: In OptionalIntOperand: Cannot use 'unexpected_node' in an output pattern!
def OptionalIntOperand: OperandWithDefaultOps<i32, (ops (i32 0))>;
This commit implements a dedicated constructor of `TreePattern` to be
used if the caller does not care about the particular root operator of
the pattern being processed.
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: