Get rid of the per-value Vec in the LiveRange data type and use a
bforest::Map instead to represent the live-in intervals for non-local
live ranges.
This has some advantages:
- The memory footprint of a local live range is reduced from 40 to 20
bytes, and
- Clearing the Liveness data structure is now a constant time operation
which doesn't call free().
- The potentially quadratic behavior when computing large live ranges is
controlled by the logarithmic B-tree operations.
Define two public iterator types in the flowgraph module, PredIter and
SuccIter, which are by-value iterators over an EBB's predecessors and
successors respectively.
Provide matching pred_iter() and succ_iter() methods for inspecting the
CFG. Remove the get_predecessors() method which returned a slice.
Update the uses of get_predecessors(), none of which depended on it
being a slice.
This abstraction makes it possible to change the internal representation
of the CFG.
Rename the ArgumentType type to AbiParam since it describes the ABI
characteristics of a parameter or return value, not just the value type.
In Signature, rename members argument_types and return_types to "params"
and "returns". Again, they are not just types.
Fix a couple lingering references to "EBB arguments".
Add EBB parameter and EBB argument to the langref glossary to clarify
the distinction between formal EBB parameter values and arguments passed
to branches.
- Replace "ebb_arg" with "ebb_param" in function names that deal with
EBB parameters.
- Rename the ValueDef variants to Result and Param.
- A bunch of other small langref fixes.
No functional changes intended.
When the return value from a call has been spilled, the reload pass
needs to insert a spill instruction right after the call instruction
which returns its results in registers.
A priory, an EBB argument value only gets an affinity if it is used
directly by a non-ghost instruction. A use by a branch passing arguments
to an EBB doesn't count.
When an EBB argument value does have an affinity, the values passed by
all the predecessors must also have affinities. This can cause EBB
argument values to get affinities recursively.
- Add a second pass to the liveness computation for propagating EBB
argument affinities, possibly recursively.
- Verify EBB argument affinities correctly: A value passed to a branch
must have an affinity only if the corresponding EBB argument value in
the destination has an affinity.
When an EBB argument value is used only as a return value, it still
needs to be given a register affinity. Otherwise it would appear as a
ghost value with no affinity.
Do the same to call arguments.
Add a spilling pass which lowers register pressure by assigning SSA
values to the stack. Important missing features:
- Resolve conflicts where an instruction uses the same value more than
once in incompatible ways.
- Deal with EBB arguments.
Fix bugs in the reload pass exposed by the first test case:
- Create live ranges for temporary registers.
- Set encodings on created spill and fill instructions.
The create_dead() methods can create a live range for a new value, and
extend_local() can extend a live range within an EBB where it is already
live.
This is enough to update liveness for new values as long as they stay
local to their EBB.
The live value tracker expects them to be there.
We may eventually delete dead arguments from internal EBBs, but at least
the entry block needs to be able to handle dead function arguments.
Now we can access instruction results and arguments as well as EBB
arguments as slices.
Delete the Values iterator which was traversing the linked lists of
values. It is no longer needed.
The tables returned by recipe_names() and recipe_constraints() are now
collected into an EncInfo struct that is available from
TargetIsa::encoding_info(). This is equivalent to the register bank
tables available fro TargetIsa::register_info().
This cleans of the TargetIsa interface and makes it easier to add
encoding-related information.
Now that variable arguments are always stored in a value list with the
fixed arguments, we no longer need the arcane [&[Value]; 2] return type.
Arguments are always stored contiguously, so just return a &[Value]
slice.
Also remove the each_arg() methods which were just trying to make it
easier to work with the old slice pair.
Add a new kind of instruction format that keeps all of its value
arguments in a value list. These value lists are all allocated out of
the dfg.value_lists memory pool.
Instruction formats with the value_list property set store *all* of
their value arguments in a single value list. There is no distinction
between fixed arguments and variable arguments.
Change the Call instruction format to use the value list representation
for its arguments.
This change is only the beginning. The intent is to eliminate the
boxed_storage instruction formats completely. Value lists use less
memory, and when the transition is complete, InstructionData will have a
trivial Drop implementation.
When the liveness pass implements dead code elimination, missing live
ranges can be used to indicate unused values that it may be possible to
remove. But even then, we may have to keep dead defs around if the
instruction has side effects or other live defs.
Each live range has an affinity hint containing the preferred register
class (or stack slot). Compute the affinity by merging the constraints
of the def and all uses.
