https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80536
--- Comment #13 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #12)
> (In reply to Marek Polacek from comment #11)
> > (In reply to Jakub Jelinek from comment #5)
> > > To expand on that, I think we want to drop that call from there and
> > > instead
> > > be able to simplify somehow a SAVE_EXPR if after c_fully_fold or cp_fold
> > > it
> > > becomes simple enough not to require any saving.
> >
> > Hmm, I'm not sure what you mean. save_expr has
> >
> > 3351 if (tree_invariant_p_1 (inner))
> > 3352 return expr;
>
> Sure, it has and also has skip_simple_arithmetic. But without the fold
> there is a chance (though small, as fold isn't recursive) that it previously
> would turn something non-invariant/simple arithmetics into invariant/simple
> arith and we wouldn't create the SAVE_EXPR, but now do. Besides increased
> memory footprint that wouldn't be bad, the problem is that I don't see any
> of the recursive folders being able to undo that, so we end up with them
> until gimplification.
This is true, but it happens very rarely. It can happen e.g. when the fold()
call in save_expr() folds away the first operand of a COMPOUND_EXPR, and the
second operand is e.g.
(long unsigned int) ((sizetype) SAVE_EXPR <n> * 4)
then skip_simple_arithmetic can pull out "SAVE_EXPR <n>" out of it, which is
tree_invariant_p_1.
> Thus, it would be nice if e.g. cp_fold, or fold, or c_fully_fold_internal
> was able to fold a SAVE_EXPR where:
> inner = skip_simple_arithmetic (TREE_OPERAND (save_expr, 0));
> if (TREE_CODE (inner) == ERROR_MARK)
> return inner;
>
> if (tree_invariant_p_1 (inner))
> return TREE_OPERAND (save_expr, 0);
But even if I add this to fold or c_fully_fold, we don't have any guarantees
that any of these will be called before gimplification, right? So most likely
we'd end up with the new SAVE_EXPR in the gimplifier, which, as you point out,
is not that bad.
> The problem on the C FE side (that would be nice to fix) is that it has its
> own c_save_expr that wants the operand to be c_fully_folded already when
> creating the SAVE_EXPR, it would be better if we could post-pone that and
> perhaps use some flag on the SAVE_EXPR to indicate whether we've
> c_fully_folded the operand already or not and only fully fold it once (C++
> FE does that through its hash maps) the first time something calls
> c_fully_fold on the SAVE_EXPR.
> So maybe you should start just with the C++ FE for now, or do it in fold too.
But c_fully_fold nor cp_fold step into SAVE_EXPRs, they just return them
unmodified. What can happen though is that c_save_expr gets something that
c_fully_fold folds into an invariant/simple arith, in which case we don't wrap
it in SAVE_EXPR, so the same expression might be folded multiple times, right?
And that could be solved by adding a hash map to c_fully_fold.
So shouldn't we first apply just this?
Comments very appreciated.
--- gcc/tree.c
+++ gcc/tree.c
@@ -3337,7 +3337,6 @@ tree_invariant_p (tree t)
tree
save_expr (tree expr)
{
- tree t = fold (expr);
tree inner;
/* If the tree evaluates to a constant, then we don't want to hide that
@@ -3345,33 +3344,33 @@ save_expr (tree expr)
However, a read-only object that has side effects cannot be bypassed.
Since it is no problem to reevaluate literals, we just return the
literal node. */
- inner = skip_simple_arithmetic (t);
+ inner = skip_simple_arithmetic (expr);
if (TREE_CODE (inner) == ERROR_MARK)
return inner;
if (tree_invariant_p_1 (inner))
- return t;
+ return expr;
/* If INNER contains a PLACEHOLDER_EXPR, we must evaluate it each time,
since
it means that the size or offset of some field of an object depends on
the value within another field.
- Note that it must not be the case that T contains both a PLACEHOLDER_EXPR
+ Note that it must not be the case that EXPR contains both a
PLACEHOLDER_EXPR
and some variable since it would then need to be both evaluated once and
evaluated more than once. Front-ends must assure this case cannot
happen by surrounding any such subexpressions in their own SAVE_EXPR
and forcing evaluation at the proper time. */
if (contains_placeholder_p (inner))
- return t;
+ return expr;
- t = build1 (SAVE_EXPR, TREE_TYPE (expr), t);
- SET_EXPR_LOCATION (t, EXPR_LOCATION (expr));
+ expr = build1 (SAVE_EXPR, TREE_TYPE (expr), expr);
+ SET_EXPR_LOCATION (expr, EXPR_LOCATION (expr));
/* This expression might be placed ahead of a jump to ensure that the
value was computed on both sides of the jump. So make sure it isn't
eliminated as dead. */
- TREE_SIDE_EFFECTS (t) = 1;
- return t;
+ TREE_SIDE_EFFECTS (expr) = 1;
+ return expr;
}
/* Look inside EXPR into any simple arithmetic operations. Return the
