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:
> > > > 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".
> >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".
>
> it looks like recursive call could NOT access the caller's version of "temp":
>
> ```
> test(recurseCount - 1);
> ```
>
> Caller`s version of temp is accessed by non-recursive call:
>
> ```
> globalIncrement(&temp);
> ```
>
> If globalIncrement does not capture the "&temp" then TRE looks to be legal
> for that case.
>
> globalIncrement() would not be marked with "Tail". test() would be marked
> with Tail.
>
> Thus the pre-requisite for TRE would be: tail-recursive call must not receive
> pointer to local stack(Tail) and non-recursive calls must not capture the
> pointer to local stack.
Can you give a complete IR example where we infer "tail", but TRE is illegal?
Can you give a complete IR example, we we don't infer "tail", but we still do
the TRE transform here?
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D82085/new/
https://reviews.llvm.org/D82085
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