efriedma added inline comments.
================
Comment at: llvm/lib/Transforms/Scalar/TailRecursionElimination.cpp:825
+ // The local stack holds all alloca instructions and all byval arguments.
+ AllocaDerivedValueTracker Tracker;
+ for (Argument &Arg : F.args()) {
----------------
avl wrote:
> efriedma wrote:
> > avl wrote:
> > > efriedma wrote:
> > > > Do you have to redo the AllocaDerivedValueTracker analysis? Is it not
> > > > enough that the call you're trying to TRE is marked "tail"?
> > > >Do you have to redo the AllocaDerivedValueTracker analysis?
> > >
> > > AllocaDerivedValueTracker analysis(done in markTails) could be reused
> > > here.
> > > But marking, done in markTails(), looks like separate tasks. i.e. it is
> > > better
> > > to make TRE not depending on markTails(). There is a review for this -
> > > https://reviews.llvm.org/D60031
> > > Thus such separation looks useful(To not reuse result of markTails but
> > > have it computed inplace).
> > >
> > > > Is it not enough that the call you're trying to TRE is marked "tail"?
> > >
> > > It is not enough that call which is subject to TRE is marked "Tail".
> > > It also should be checked that other calls does not capture pointer to
> > > local stack:
> > >
> > > ```
> > > // do not do TRE if any pointer to local stack has escaped.
> > > if (!Tracker.EscapePoints.empty())
> > > return false;
> > >
> > > ```
> > >
> > > It is not enough that call which is subject to TRE is marked "Tail". It
> > > also should be checked that other calls does not capture pointer to local
> > > stack:
> >
> > If there's an escaped pointer to the local stack, we wouldn't infer "tail"
> > in the first place, would we?
> If function receives pointer to alloca then it would not be marked with
> "Tail". Then we do not have a possibility to understand whether this function
> receives pointer to alloca but does not capture it:
>
> ```
> void test(int recurseCount)
> {
> if (recurseCount == 0) return;
> int temp = 10;
> globalIncrement(&temp);
> test(recurseCount - 1);
> }
> ```
>
> test - marked with Tail.
> globalIncrement - not marked with Tail. But TRE could be done since it does
> not capture pointer. But if it will capture the pointer then we could not do
> TRE. So we need to check !Tracker.EscapePoints.empty().
>
>
>
> test - marked with Tail.
For the given code, TRE won't mark the recursive call "tail". That transform
isn't legal: the recursive call could access the caller's version of "temp".
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D82085/new/
https://reviews.llvm.org/D82085
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