[cfe-dev] Do we intentionally 'transform' non-dependent expressions?

[Richard Smith via cfe-dev]

On Fri, 20 Sep 2019, 10:14 Keane, Erich via cfe-dev, <cfe-dev at lists.llvm.org>
wrote:

> I decided to investigate https://bugs.llvm.org/show_bug.cgi?id=43370
>
>
>
> First, this code is in a dependent context (but is not dependent itself):
>
> uint64_t v[2];
>
>     __atomic_store_n(v, 0, 0);
>
>
>
> The function template declaration in the AST shows the first parameter as
> having an ArrayToPointerDecay implicit cast.
>
>
> However, the AtomicExpr transform calls “TransformExpr” on this, which
> then strips off all implicit casts. The expression then is NOT checked
> later, since it was already checked the first time.  If the expression is
> dependent, it gets properly checked, and the cast is added back in.
>
>
>
> It seems to me that transforming a dependent expression should be
> unnecessary at best, and a risk of crashing at worst (due to running out of
> Transform stack space).  SO, I put a test in the beginning of TransformExpr
> to just return the expr.
>
It is necessary in some cases (including this one): we need to rewrite the
reference to 'v' to refer to the instantiated variable rather than the
variable in the template.

> However, check-clang resulted in 600+ test failures.  I evaluated a
> couple, and all seem to be because they are re-checking a statement that
> doesn’t need to be (since it isn’t dependent!).  There is unfortunately no
> good way it seems to determine whether a Stmt is dependent, so I can’t find
> anything to put into TransformStmt to skip like before.  So, I think I will
> have to manage these tests 1 by 1.
>
>
> Before I deep-dive into this, I’d like to hear whether I’m missing
> something obvious and would just be wasting my time?  Does anyone know
> whether there is a good reason to still transform non-dependent expressions?
>
>
>
> Thanks,
> Erich
>
>
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