From: Andrew Pinski <pinskia@gmail.com>
To: Richard Biener <richard.guenther@gmail.com>
Cc: Andrew Pinski <apinski@marvell.com>,
GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: [V2/PATCH] Fix tree-optimization/102216: missed optimization causing Warray-bounds
Date: Mon, 22 Nov 2021 17:30:18 -0800 [thread overview]
Message-ID: <CA+=Sn1nvBFxp=5Jn7Ft-APGOwba579js2qfz5=xeuYm81WaFLw@mail.gmail.com> (raw)
In-Reply-To: <CAFiYyc1RzuhZA3_KOxSjt1DwdhWLJd3vbVi-dDn9MCD2353N+w@mail.gmail.com>
On Mon, Nov 22, 2021 at 3:40 AM Richard Biener
<richard.guenther@gmail.com> wrote:
>
> On Mon, Nov 22, 2021 at 9:40 AM Andrew Pinski <pinskia@gmail.com> wrote:
> >
> > On Wed, Oct 27, 2021 at 3:42 AM Richard Biener via Gcc-patches
> > <gcc-patches@gcc.gnu.org> wrote:
> > >
> > > On Wed, Oct 27, 2021 at 12:00 PM apinski--- via Gcc-patches
> > > <gcc-patches@gcc.gnu.org> wrote:
> > > >
> > > > From: Andrew Pinski <apinski@marvell.com>
> > > >
> > > > The problem here is tree-ssa-forwprop.c likes to produce
> > > > &MEM <const char *> [(void *)_4 + 152B] which is the same as
> > > > _4 p+ 152 which the rest of GCC likes better.
> > > > This implements this transformation back to pointer plus to
> > > > improve better code generation later on.
> > >
> > > Why do you think so? Can you pin-point the transform that now
> > > fixes the new testcase?
> >
> > So we had originally:
> > language_names_p_9 = &MEM <const char *> [(void *)_4 + 24B];
> > ...
> > _2 = _4 + 40;
>
> Of course if that would have been
>
> _2 = &MEM [_4 + 40B];
>
> the issue would be fixed as well. That said, I agree that _4 + 40
> is better but I think we should canonicalize all &MEM[SSA + CST]
> this way. There is a canonicalization phase in fold_stmt_1:
>
> /* First do required canonicalization of [TARGET_]MEM_REF addresses
> after propagation.
> ??? This shouldn't be done in generic folding but in the
> propagation helpers which also know whether an address was
> propagated.
> Also canonicalize operand order. */
> switch (gimple_code (stmt))
> {
> case GIMPLE_ASSIGN:
> if (gimple_assign_rhs_class (stmt) == GIMPLE_SINGLE_RHS)
> {
> tree *rhs = gimple_assign_rhs1_ptr (stmt);
> if ((REFERENCE_CLASS_P (*rhs)
> || TREE_CODE (*rhs) == ADDR_EXPR)
> && maybe_canonicalize_mem_ref_addr (rhs))
> changed = true;
>
> where this could be done (apart from adding a match.pd pattern for this).
Yes that is a good idea, I now have a patch which I am testing to add
this canonicalization. It is actually simpler than the previous patch
too.
Thanks,
Andrew
>
> > if (_2 != language_names_p_9)
> >
> > Forwprop is able to figure out that the above if statement is now
> > always false as we have (_4 +p 40) != (_4 +p 24) which gets simplified
> > via a match-and-simplify pattern ().
> > Does that answer your question?
> >
> > I will look into the other comments in a new patch.
> >
> > Thanks,
> > Andrew
> >
> > >
> > > Comments below
> > >
> > > > OK? Bootstrapped and tested on aarch64-linux-gnu.
> > > >
> > > > Changes from v1:
> > > > * v2: Add comments.
> > > >
> > > > gcc/ChangeLog:
> > > >
> > > > PR tree-optimization/102216
> > > > * tree-ssa-forwprop.c (rewrite_assign_addr): New function.
> > > > (forward_propagate_addr_expr_1): Use rewrite_assign_addr
> > > > when rewriting into the addr_expr into an assignment.
> > > >
> > > > gcc/testsuite/ChangeLog:
> > > >
> > > > PR tree-optimization/102216
> > > > * g++.dg/tree-ssa/pr102216.C: New test.
> > > > ---
> > > > gcc/testsuite/g++.dg/tree-ssa/pr102216.C | 22 +++++++++
> > > > gcc/tree-ssa-forwprop.c | 58 ++++++++++++++++++------
> > > > 2 files changed, 67 insertions(+), 13 deletions(-)
> > > > create mode 100644 gcc/testsuite/g++.dg/tree-ssa/pr102216.C
> > > >
> > > > diff --git a/gcc/testsuite/g++.dg/tree-ssa/pr102216.C b/gcc/testsuite/g++.dg/tree-ssa/pr102216.C
> > > > new file mode 100644
> > > > index 00000000000..b903e4eb57d
> > > > --- /dev/null
> > > > +++ b/gcc/testsuite/g++.dg/tree-ssa/pr102216.C
> > > > @@ -0,0 +1,22 @@
> > > > +/* { dg-options "-O2 -fdump-tree-optimized" } */
> > > > +void link_error ();
> > > > +void g ()
> > > > +{
> > > > + const char **language_names;
> > > > +
> > > > + language_names = new const char *[6];
> > > > +
> > > > + const char **language_names_p = language_names;
> > > > +
> > > > + language_names_p++;
> > > > + language_names_p++;
> > > > + language_names_p++;
> > > > +
> > > > + if ( (language_names_p) - (language_names+3) != 0)
> > > > + link_error();
> > > > + delete[] language_names;
> > > > +}
> > > > +/* We should have removed the link_error on the gimple level as GCC should
> > > > + be able to tell that language_names_p is the same as language_names+3. */
> > > > +/* { dg-final { scan-tree-dump-times "link_error" 0 "optimized" } } */
> > > > +
> > > > diff --git a/gcc/tree-ssa-forwprop.c b/gcc/tree-ssa-forwprop.c
> > > > index a830bab78ba..e4331c60525 100644
> > > > --- a/gcc/tree-ssa-forwprop.c
> > > > +++ b/gcc/tree-ssa-forwprop.c
> > > > @@ -637,6 +637,47 @@ forward_propagate_into_cond (gimple_stmt_iterator *gsi_p)
> > > > return 0;
> > > > }
> > > >
> > > > +/* Rewrite the DEF_RHS as needed into the (plain) use statement. */
> > > > +
> > > > +static void
> > > > +rewrite_assign_addr (gimple_stmt_iterator *use_stmt_gsi, tree def_rhs)
> > > > +{
> > > > + tree def_rhs_base;
> > > > + poly_int64 def_rhs_offset;
> > > > +
> > > > + /* Get the base and offset. */
> > > > + if ((def_rhs_base = get_addr_base_and_unit_offset (TREE_OPERAND (def_rhs, 0),
> > > > + &def_rhs_offset)))
> > >
> > > So this will cause us to rewrite &MEM[p_1].a.b.c; to a pointer-plus,
> > > right? Don't
> > > we want to preserve that for object-size stuff? So maybe directly pattern
> > > match ADDR_EXPR <MEM_REF <SSA_NAME, ..>> only?
> > >
> > > > + {
> > > > + tree new_ptr;
> > > > + poly_offset_int off = 0;
> > > > +
> > > > + /* If the base was a MEM, then add the offset to the other
> > > > + offset and adjust the base. */
> > > > + if (TREE_CODE (def_rhs_base) == MEM_REF)
> > > > + {
> > > > + off += mem_ref_offset (def_rhs_base);
> > > > + new_ptr = TREE_OPERAND (def_rhs_base, 0);
> > > > + }
> > > > + else
> > > > + new_ptr = build_fold_addr_expr (def_rhs_base);
> > > > +
> > > > + /* If we have the new base is not an address express, then use a p+ expression
> > > > + as the new expression instead of &MEM[x, offset]. */
> > > > + if (TREE_CODE (new_ptr) != ADDR_EXPR)
> > > > + {
> > > > + tree offset = wide_int_to_tree (sizetype, off);
> > > > + def_rhs = build2 (POINTER_PLUS_EXPR, TREE_TYPE (def_rhs), new_ptr, offset);
> > >
> > > Ick. You should be able to use gimple_assign_set_rhs_with_ops.
> > >
> > > > + }
> > > > + }
> > > > +
> > > > + /* Replace the rhs with the new expression. */
> > > > + def_rhs = unshare_expr (def_rhs);
> > >
> > > and definitely no need to unshare anything here?
> > >
> > > > + gimple_assign_set_rhs_from_tree (use_stmt_gsi, def_rhs);
> > > > + gimple *use_stmt = gsi_stmt (*use_stmt_gsi);
> > > > + update_stmt (use_stmt);
> > > > +}
> > > > +
> > > > /* We've just substituted an ADDR_EXPR into stmt. Update all the
> > > > relevant data structures to match. */
> > > >
> > > > @@ -696,8 +737,8 @@ forward_propagate_addr_expr_1 (tree name, tree def_rhs,
> > > > if (single_use_p
> > > > && useless_type_conversion_p (TREE_TYPE (lhs), TREE_TYPE (def_rhs)))
> > > > {
> > > > - gimple_assign_set_rhs1 (use_stmt, unshare_expr (def_rhs));
> > > > - gimple_assign_set_rhs_code (use_stmt, TREE_CODE (def_rhs));
> > > > + rewrite_assign_addr (use_stmt_gsi, def_rhs);
> > > > + gcc_assert (gsi_stmt (*use_stmt_gsi) == use_stmt);
> > > > return true;
> > > > }
> > > >
> > > > @@ -741,14 +782,7 @@ forward_propagate_addr_expr_1 (tree name, tree def_rhs,
> > > > if (forward_propagate_addr_expr (lhs, new_def_rhs, single_use_p))
> > > > return true;
> > > >
> > > > - if (useless_type_conversion_p (TREE_TYPE (lhs),
> > > > - TREE_TYPE (new_def_rhs)))
> > > > - gimple_assign_set_rhs_with_ops (use_stmt_gsi, TREE_CODE (new_def_rhs),
> > > > - new_def_rhs);
> > > > - else if (is_gimple_min_invariant (new_def_rhs))
> > > > - gimple_assign_set_rhs_with_ops (use_stmt_gsi, NOP_EXPR, new_def_rhs);
> > > > - else
> > > > - return false;
> > > > + rewrite_assign_addr (use_stmt_gsi, new_def_rhs);
> > > > gcc_assert (gsi_stmt (*use_stmt_gsi) == use_stmt);
> > > > update_stmt (use_stmt);
> > > > return true;
> > > > @@ -951,9 +985,7 @@ forward_propagate_addr_expr_1 (tree name, tree def_rhs,
> > > > unshare_expr (def_rhs),
> > > > fold_convert (ptr_type_node,
> > > > rhs2)));
> > > > - gimple_assign_set_rhs_from_tree (use_stmt_gsi, new_rhs);
> > > > - use_stmt = gsi_stmt (*use_stmt_gsi);
> > > > - update_stmt (use_stmt);
> > > > + rewrite_assign_addr (use_stmt_gsi, new_rhs);
> > >
> > > so you only do this after addr_expr forwarding but not on stmts in
> > > general? You could
> > > do it that way in the 2nd loop over the BB.
> > >
> > > > tidy_after_forward_propagate_addr (use_stmt);
> > > > return true;
> > > > }
> > > > --
> > > > 2.17.1
> > > >
next prev parent reply other threads:[~2021-11-23 1:30 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-27 9:59 apinski
2021-10-27 10:22 ` Bernhard Reutner-Fischer
2021-10-27 10:41 ` Richard Biener
2021-11-22 8:40 ` Andrew Pinski
2021-11-22 11:39 ` Richard Biener
2021-11-23 1:30 ` Andrew Pinski [this message]
2021-10-27 16:21 ` Martin Sebor
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