We introduce a "dead code" optimization like pass on the AST.
The goal is to simplify away, in a `SequenceNode`, everything that
follows a node with sports a `nofallthrough behavior.
A `SwitchBreak` node should not have a specific associated
`FallThroughScopeType`, since its semantics represent the fact of a
`case` of the `switch` with no associated statements, and thus can be
represented with the `FallThrough` behavior.
Change the analysis in order to lift the assumption that a dispatcher
`switch` always coveris all the values of the variable state. This is
not true after the `simplifySwitchBreak` and `inlineDispatcherSwitch`
beautify passes, which can remove some of the `case`s.
The `FallThroughScope` associated to a `ScsNode` should represent the
fact that we have fallthrough from a loop.
Without any more advanced semantical analysis, we cannot conclude that
we do not have fallthrough.
Do not inline loop related `break` and `continue` statements.
Indeed, inlining them would mean moving from the scope of a cycle, to an
inner one, non-local control flow statements, and this would break the
semantics.
Delay the `SwitchBreaksFixer` beautify pass run.
Specifically, we run it last, as this pass computes information used
only by the backend, and the information it depends upon may be
invalidated by some passes that transform `switch`es (as the
`simplifyDualSwitch` pass).
Delay as much as possible the loop promotion passes, in order to catch
more opportunities.
Specifically, we now perform the loop promotion after the dual case
`switch` promotion pass.
Before this commit, it could happen that some duplicated dummy node
(used to mark backedges) could be left lingering in wrong regions when
collapsing a regions, if they were first iteration outlined.
This commit fixes the problem, by collecting them and letting
`updateNodes` take care of them, removing them from the containing
region and all its parents.
Before this commit, many passes in revng-c were skipping over
non-isolated functions.
Now revng-pipeline takes care of removing non-isolated functions so that
check can be omitted everywhere.
Implement a new beautify pass which simplifies away `SwitchBreakNode`s
that constitute the entire body of `case`s in `switch`es, that do not,
have a `default` case. In such situations indeed, the semantics is,
preserved by removing the `SwitchBreakNode`s.
We drop the assumption that each weaved `switch` must be nested inside
its related main `switch`, as a consequence of having generalized the
tiling algorithm in order to be able to emit a weaved `switch` as a AST
successors of the related main `switch`.
Various improvements to the debug graphs for `restructure-cfg` and
`beautify`:
- Normalize casing and syntax of debug graphs.
- Improve the graph folders name and layout.
- Implement `CFGDumper` and `ASTDumper` for when we need a serialization
with incremental indexes.
- Remove old and stale graph serializations.
Perform a complete rewrite of the `SwitchNode` tiling routine.
The tiling now works in the following way:
- When encountering a node which will produce a `SwitchNode` (either a
standard `switch` or a dispatcher `switch`), we look for the following
situations:
1) We have a node, a successor (case) of the `switch`, which in turn
is the successor of all the other successors (cases) of the
`switch`.
2) We have a node, not a successor (case) of the `switch`, which is
the successor of all the successors (cases) of the `switch`.
- If we find such candidate node, this node will be the fallthrough of
the `switch`. In addition, depending on whether the `switch` dominates
the candidate fallthrough, we can incorporate it as the immediate
successor of the `SwitchNode` we are building.
- There is currently an exception to the above, due to how we currently
handle weaved `switch`es. In such cases, we mandate that the weaved
`switch` is nested inside the main corresponding `switch`. For this
reason, we have a special casing handling the "all inlined but one"
situation in the new code, while this part could in theory be merged
in the common criterion below, at the cost of dropping the invariant
of the nesting of weaved switch`es.
Simplify `switch` cases that are simplified away during the dispatcher
`switch` inlining beautify.
Usually, we can simplify ast nodes by replacing them with `nullptr`s,
but due to how the `switch` cases work, we need to handle them in a
custom way.
We introduce the `InlineDispatcherSwitch` beautify pass. Its goal is to
try and inline the body of some of the `case`s of a exit dispatcher, in
place of the `SetNode` corresponding to that `case`, if this doesn't
introduce duplication in the code (i.e., a single `SetNode` for that
specific case value is present).
Additionally, if the inlining procedure is able to completely remove the
necessity of an exit dispatcher altogether, the pass removes it.
The pass is able to handle chains of weaved dispatcher `switch`es
referring to the same original dispatcher `switch`, by handling the
inline operation and the possible simplification level-wise.
The inlining procedure, cannot take place if a `SetNode` is contained in
the body of the case we are trying to inline, since this can possibly
break the semantics of the state variable of a loop, by placing a
`SetNode` in a more internal loop.
We introduce the PromoteCallNoReturn beautification pass. Its goal is to
restructure sequence of statements, in order to have `call`s to
`noreturn` functions as _inlined_ in the middle of the statement
sequence, and leave _non local control flow statements_ at the end of
that scope. E.g., we prefer:
```
if (cond)
call noreturnfunc();
return;
```
to
```
if (!cond)
return;
call noreturnfunc();
```
In order to do this, contextually, we restructure the routine computing
the `fallthrough` property, in order to be able to differentiate between
the _non local control flow statements_, a call to a `noreturn`
function, or a generic mix of the two (useful when combining results
from the two situations above).
The new analysis is also used in the `promoteNoFallThrough` promotion
pass.
Introduce the `DispatcherKind` attribute as a field in the
`BasicBlockNode` and `ASTNode` classes, in order to be able to
distinguish entry and exit dispatchers, and the related set nodes.
This commits reorders the beautifiers so that do-while loops are matched
before while loops.
This has the effect that loops that can be matched both as while and
do-while loops end up matched as do-while.
The consequence is that we generate C code with a layout that is much
more similar to assembly, and prevents duplicating code to recompute the
condition of the while.
Handle the situation where the whole dispatcher `switch` is simplified
after the `SimplifyDualSwitch` is run.
Enforce check that when a `SwitchBreakNode` is removed due to the
`SimplifyDualSwitch` it must be pertinent to the containing dispatcher
`switch`.
Move the `simplifyImplicitContinue` at the end of the beautify pass,
after all the pass that modify the anatomy of the ASTTree.
Before this change, the pass was run before some changes that moved some
`continue`s in positions where they are not considered implicit.
Added an assertion which checks that during the `noFallThrough`
promotion phase, no implicit `ContinueNode` is present.
The container used to store the associated `ExprNode`s that we need to
flip when performing a `HybridNot` simplification, is changed from a
`llvm::SmallVector` to `llvm:SmallSet`.
Indeed, during the collection of the `ExprNode`s pointing to a
`llvm::BasicBlock`, it should not be possible to find a single
`ExprNode` twice. We now assert this fact.
Improve the `fallThroughScope` computation, in order to handle calls to
`NoReturn` functions in the analysis, representing the fact that they
induce a `noFallthrough` scope (i.e., execution will not ever resume
after the call, and therefore we can later improve the nesting of the
code with the `promoteNoFallthrough` pass).
We introduce a simplification step, which looks for `switch`es that can
be reduced to simpler `if` statements.
Specifically, the logic is the following:
1) When we identify a `switch` statement composed by a single `case` and
a possible default, we transform it into an `if` with the `case` now
corresponding to the `then`, and the `default` corresponding to the
`else`, if present.
2) When we identify a `switch` statement composed by two `case`s, and no
`default` is present, we can promote it to an `if` with `then` `else`
branches.
Other key details:
- The promotion happens only if we can identify at least one of the
`case`s that have a single element in the `case` label. If this is not
the case, we do not promote one to RHS of the `if` condition.
- A new `CompareNode` class, inheriting from `ExprNode`, is created, in
order to represent the equality or inequality condition of an `IfNode`
instance that is the result of the promotion. This `CompareNode` can
represent for the LHS both an `llvm::Value` or the `loop_state_var`,
while it embeds the RHS constant which completes the comparison.
- We remove `SwitchBreak` AST nodes that may now appear as children of
an `if` node promoted from a `switch`.
- We introduce in the `CompareNode` the `weaved` concept. Indeed, if a
promotion of a weaved `switch` happens, we should avoid the
serialization of the instructions leading to the computation of the
condition of the original `switch`, because they have been already
emitted by the main related dominating `switch`.
We also introduce an additional simplification step, which takes care
of:
- Promoting `!(==)` to `(!=)` and `!(!=)` to `(==)`, if the inner
equal/not equal is represented via a `CompareNode`.
- Promoting `x == 0` to `!x` and `x != 0` to `x`.
To be able to correctly emit (or not) the instructions computing a
condition of an `IfNode`, we need to add the `EmittBB` flag, an
additional parameter to the `buildGHASTCondition` function, which
controls the emission of the statements of a basic block computing a
condition.
Consequently, the `IfNode` acquires a `IsWeaved` field, which is used to
mirror the property having the same name on `SwitchNode`. Being now
possible a promotion from a dual `SwitchNode` to an `IfNode`, we need to
represent this property on the `IfNode` too.
The `default` `case` is now a standard `case`, and it is identified only
by having the `label` set empty.
Therefore, a list of beautify and transformation actions now do not need
special casing in order to handle the `default` `case`, which is reached
during the standard iteration over the `case`s.
A special accessor is still necessary, in order to correctly emit the
`default` `case` in the backend.
Introduce implicit statements simplification phase, specifically:
- A implicit `return` simplification: `return` statements in `void`
type functions, which are not followed by any other scope, can be
omitted.
- A implicit `continue` simplification: `continue` statements whose
fallthrough leads directly to the end of the cycle scope (i.e., to
execute another iteration of the enclosing loop), can be omitted.
In order to avoid the printing of the implicit `return`, we need an
additional `emitReturn` parameter in the `emitBasicBlock` method of the
`CCodeGenerator` class.
When copying nodes from nested AST to the root one, we should take care
of overwriting the mapping between the old `OldCFGNode`, so that it now
points to the newer AST node representing it.
Now, also `break` and `continue` on the AST, when printed, have the ID
number of the `ASTNode`, and in the name they preserve the original
`CFGNode` one also.
Alessandro Di Federico