This commit changes MarkAssignments (and related tests) so that now
instruction with more than one use are not assigned to a new variable
by default.
In some cases it might still be good to serialize them for readability,
but for those cases we will implementa a standalone pass that only
implements heuristics based on readability.
MarkAssignments now only takes decisions based on semantics of
side-effects.
This commit does various things oriented at reducing the number of local
variables emitted in C:
- MarkAssignments now know that @Copy and @Assign involving
@LocalVariable only have side effects that affect the local variable
itself; this enables to reduce the number of times we're forced to
emit a local variable due to interfering side effects
- Drop the @AssignmentMarker FunctionTag; AddAssignmentMarkerPass now
doesn't emit @AssignmentMarker anymore; instead it emits groups of
@LocalVariable, @Copy, and @Assign, which benefit from the previous
point
- Drop 2 MarkAssignments::Reasons: HasManyUses and HasUsesOutsideOfBB;
both these have now been aggregated into the AlwaysAssign reason for
simplicity, representing all reasons non involving side effects
- Update BeautifyGHAST and how it reasons about side effects when
beautifying; before this commit it used @AssignmentMarker, now it
looks at @Assign
- Simplify ExitSSA; before this commit it was trying hard to be smart on
where it emitted the store instructions representing the incoming
values of the PHI that was being destroyed; this seemed smart when we
originally did it but it generated C code that was not really better
to read, so this useless complexity is finally gone
If we find values to be used outside their scope, we need to always
mark them for assignment, so that they have a dedicated variable that
is declared in the right scope. To do this we:
1. Export the logic that decides if a value needs a top-scope variable
in a public header, to be used by both `VariableScopeAnalysis` and
`MarkAssignments`.
2. Add the `HasUsesOutsideBB` reason for assignment markers
Add `ReadsMemory` and `WritesMemory` Tags and restructure the
logic that decides whether two instructions are interfering
as follows:
1. Consider `Store`s and anything with a `WritesMemory` tag as
having side effects
2. Consider all the `TaintSet` of an instruction when deciding
if two instructions interfere
3. Consider both `ReadsMemory` and `WritesMemory` Tags when
deciding interference
This library replaces the old AddIRSerializationMarkers, cleaning up a
lot of historcal baggage, merging Liveness and MarkAnalysis, and
adopting a more accurate naming across all the codebase.
Alessandro Di Federico