The main changes are:
*) Rewrite everything in swift. So far, parts of memory-behavior analysis were already implemented in swift. Now everything is done in swift and lives in `AliasAnalysis.swift`. This is a big code simplification.
*) Support many more instructions in the memory-behavior analysis - especially OSSA instructions, like `begin_borrow`, `end_borrow`, `store_borrow`, `load_borrow`. The computation of end_borrow effects is now much more precise. Also, partial_apply is now handled more precisely.
*) Simplify and reduce type-based alias analysis (TBAA). The complexity of the old TBAA comes from old days where the language and SIL didn't have strict aliasing and exclusivity rules (e.g. for inout arguments). Now TBAA is only needed for code using unsafe pointers. The new TBAA handles this - and not more. Note that TBAA for classes is already done in `AccessBase.isDistinct`.
*) Handle aliasing in `begin_access [modify]` scopes. We already supported truly immutable scopes like `begin_access [read]` or `ref_element_addr [immutable]`. For `begin_access [modify]` we know that there are no other reads or writes to the access-address within the scope.
*) Don't cache memory-behavior results. It turned out that the hit-miss rate was pretty bad (~ 1:7). The overhead of the cache lookup took as long as recomputing the memory behavior.
This corresponds to the parameter-passing convention of the Itanium C++
ABI, in which the argument is passed indirectly and possibly modified,
but not destroyed, by the callee.
@in_cxx is handled the same way as @in in callers and @in_guaranteed in
callees. OwnershipModelEliminator emits the call to destroy_addr that is
needed to destroy the argument in the caller.
rdar://122707697
Layers:
- FunctionConvention: AST FunctionType: results, parameters
- ArgumentConventions: SIL function arguments
- ApplyOperandConventions: applied operands
The meaning of an integer index is determined by the collection
type. All the mapping between the various indices (results,
parameters, SIL argument, applied arguments) is restricted to the
collection type that owns that mapping. Remove the concept of a
"caller argument index".
We can ignore any memory writes in a program termination point, because it's not relevant for the caller.
But we need to consider memory reads, otherwise preceeding memory writes would be eliminated by dead-store-elimination in the caller.
E.g. String initialization for error strings which are printed by the program termination point.
Regarding ownership: a program termination point must not touch any reference counted objects.
Functions "are deinit barriers" (more pedantically, applies of functions
are deinit barriers) if any of their instructions are deinit barriers.
During side-effect analysis, when walking a function's instructions for
other global effects, also check for the deinit-barrier effect. If an
instruction is found to be a deinit barrier, mark the function's global
effects accordingly.
Add SILFunction::isDeinitBarrier to conveniently access the effects
computed during ComputeSideEffects.
Update the isBarrierApply predicate to iterate over the list of callees,
if complete, to check whether any is a deinit barrier. If none is, then
the apply is not a deinit barrier.
So far, function effects only included escape effects.
This change adds side-effects (but they are not computed, yet).
It also involves refactoring of the existing escape effects.
Also the SIL effect syntax changed a bit. Details are in docs/SIL.rst
So far, argument effects were printed in square brackets before the function name, e.g.
```
sil [escapes !%0.**, !%1, %1.c*.v** => %0.v**] @foo : $@convention(thin) (@guaranteed T) -> @out S {
bb0(%0 : $*S, %1 : @guaranteed $T):
...
```
As we are adding more argument effects, this becomes unreadable.
To make it more readable, print the effects after the opening curly brace, and print a separate line for each argument. E.g.
```
sil [ossa] @foo : $@convention(thin) (@guaranteed T) -> @out S {
[%0: noescape **]
[%1: noescape, escape c*.v** => %0.v**]
bb0(%0 : $*S, %1 : @guaranteed $T):
...
```
Add a flag `analyzeAddresses` for distinguishing address vs value escape analysis. This is simpler than handling that in the visitUse/visitDef closures.
Also, fix a related bug, which let an address, which is escaping to a function, get unnoticed.
Store a list of argument effects in a function, which specify if and how arguments escape.
Such effects can be specified in the Swift source code (for details see docs/ReferenceGuides/UnderscoredAttributes.md) or derived in an optimization pass.
For details see the documentation in SwiftCompilerSources/Sources/SIL/Effects.swift.
