[PATCH] store VLA bounds in attribute access as strings (PR 97172)

[Jakub Jelinek]

On Mon, Jan 04, 2021 at 12:14:15PM -0700, Jeff Law via Gcc-patches wrote:
> > Doing the STRING_CST is certainly less fragile since the SSA names
> > created at gimplification time could even be ggc_freed when no longer
> > used in the IL.
> Obviously we can't use SSA_NAMEs as they're specific to each function as
> they get compiled.  But what's not as clear to me is why we can't use a
> SAVE_EXPR of the original expression that indicates the size of the
> parameter.

The gimplifier is destructive, so if the expressions are partly (e.g. in
those SAVE_EXPRs) shared with what is in the actual IL, we lose.
And if they aren't shared and there are side-effects, if we tried to
gimplify them again we'd get the side-effects duplicated.
So it all depends on what the code wants to handle, if e.g. just values of
parameters with simple arithmetics on those and punt on everything else,
then it is doable, but generally it is not.

	Jakub