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c808a49b50
There's a couple minor deletions amongst this but 99% of it is just moving the documentation around to prepare the way for more meaningful changes.
99 lines
3.6 KiB
ReStructuredText
99 lines
3.6 KiB
ReStructuredText
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.. _instructionselect:
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InstructionSelect
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-----------------
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This pass transforms generic machine instructions into equivalent
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target-specific instructions. It traverses the ``MachineFunction`` bottom-up,
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selecting uses before definitions, enabling trivial dead code elimination.
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.. _api-instructionselector:
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API: InstructionSelector
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^^^^^^^^^^^^^^^^^^^^^^^^
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The target implements the ``InstructionSelector`` class, containing the
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target-specific selection logic proper.
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The instance is provided by the subtarget, so that it can specialize the
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selector by subtarget feature (with, e.g., a vector selector overriding parts
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of a general-purpose common selector).
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We might also want to parameterize it by MachineFunction, to enable selector
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variants based on function attributes like optsize.
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The simple API consists of:
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.. code-block:: c++
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virtual bool select(MachineInstr &MI)
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This target-provided method is responsible for mutating (or replacing) a
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possibly-generic MI into a fully target-specific equivalent.
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It is also responsible for doing the necessary constraining of gvregs into the
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appropriate register classes as well as passing through COPY instructions to
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the register allocator.
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The ``InstructionSelector`` can fold other instructions into the selected MI,
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by walking the use-def chain of the vreg operands.
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As GlobalISel is Global, this folding can occur across basic blocks.
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SelectionDAG Rule Imports
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^^^^^^^^^^^^^^^^^^^^^^^^^
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TableGen will import SelectionDAG rules and provide the following function to
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execute them:
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.. code-block:: c++
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bool selectImpl(MachineInstr &MI)
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The ``--stats`` option can be used to determine what proportion of rules were
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successfully imported. The easiest way to use this is to copy the
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``-gen-globalisel`` tablegen command from ``ninja -v`` and modify it.
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Similarly, the ``--warn-on-skipped-patterns`` option can be used to obtain the
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reasons that rules weren't imported. This can be used to focus on the most
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important rejection reasons.
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PatLeaf Predicates
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^^^^^^^^^^^^^^^^^^
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PatLeafs cannot be imported because their C++ is implemented in terms of
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``SDNode`` objects. PatLeafs that handle immediate predicates should be
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replaced by ``ImmLeaf``, ``IntImmLeaf``, or ``FPImmLeaf`` as appropriate.
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There's no standard answer for other PatLeafs. Some standard predicates have
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been baked into TableGen but this should not generally be done.
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Custom SDNodes
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^^^^^^^^^^^^^^
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Custom SDNodes should be mapped to Target Pseudos using ``GINodeEquiv``. This
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will cause the instruction selector to import them but you will also need to
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ensure the target pseudo is introduced to the MIR before the instruction
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selector. Any preceding pass is suitable but the legalizer will be a
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particularly common choice.
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ComplexPatterns
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^^^^^^^^^^^^^^^
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ComplexPatterns cannot be imported because their C++ is implemented in terms of
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``SDNode`` objects. GlobalISel versions should be defined with
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``GIComplexOperandMatcher`` and mapped to ComplexPattern with
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``GIComplexPatternEquiv``.
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The following predicates are useful for porting ComplexPattern:
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* isBaseWithConstantOffset() - Check for base+offset structures
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* isOperandImmEqual() - Check for a particular constant
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* isObviouslySafeToFold() - Check for reasons an instruction can't be sunk and folded into another.
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There are some important points for the C++ implementation:
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* Don't modify MIR in the predicate
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* Renderer lambdas should capture by value to avoid use-after-free. They will be used after the predicate returns.
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* Only create instructions in a renderer lambda. GlobalISel won't clean up things you create but don't use.
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