[Mem2Reg] Lexical allocs create lexical borrows. #39460
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Previously, there was only a frontend flag to influence the LangOpts struct. Now, the flag can be set when running sil-opt which is important for testing.
Previously, the addArgumentToBranch only allowed one to add a single additional argument to a branch. It then verified the argument count. That is a problem if multiple arguments have to be added to arrive at the correct argument count. Specifically, that was a problem when running Mem2Reg on a lexical alloc_stack, where three new phi arguments are added. Here, the function name is changed to addArgumentsToBranch (plural arguments) and the function accepts a SmallVector<SILValue> rather than a single SILValue, allowing one to add all the arguments that are necessary in order to verify that the resulting number of arguments is correct.
When the -enable-experimental-lexical-lifetimes flag is enabled, the alloc_stack instructions which the pass replaces alloc_box instructions with have the lexical attribute.
SILGen turns vars into alloc_boxes. When possible, AllocBoxToStack turns those into alloc_stacks. In order to preserve the lexical lifetime of those vars, the alloc_stacks are annotated with the [lexical] attribute. When Mem2Reg runs, it promotes the loads from and stores into those alloc_stacks to uses of registers. In order to preserve the lexical lifetime during that transformation, lexical borrow scopes must be introduces that encode the same lifetime as the alloc_stacks did. Here, that is done.
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SILGen turns vars into
alloc_boxes. When possible,AllocBoxToStackturns those intoalloc_stacks. In order to preserve the lexical lifetime of those vars, thealloc_stacks are annotated with the[lexical]attribute. WhenMem2Regruns, it promotes theloads from andstores into thosealloc_stacks to uses of registers. In order to preserve the lexical lifetime during that transformation, lexical borrow scopes must be introduced to encode the same lifetime as thealloc_stacks did. Here, that is done.