On 1/4/21 12:19 PM, Jakub Jelinek wrote:
> 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.
I would expect the expressions to be values of parameters (or objects in
static storage) and simple arithemetic on them. If there's other cases,
punting seems appropriate.
Martin -- are there nontrivial expressions we need to be worried about here?
Jeff
