On Wed, 13 Oct 2021, Richard Sandiford wrote:
> Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> > The following makes sure to rewrite arithmetic with undefined behavior
> > on overflow to a well-defined variant when moving them to be always
> > executed as part of doing if-conversion for loop vectorization.
> >
> > Bootstrapped and tested on x86_64-unknown-linux-gnu.
> >
> > Any comments?
>
> LGTM FWIW, although…
>
> > Thanks,
> > Richard.
> >
> > 2021-10-11 Richard Biener <rguent...@suse.de>
> >
> > PR tree-optimization/102659
> > * tree-if-conv.c (need_to_rewrite_undefined): New flag.
> > (if_convertible_gimple_assign_stmt_p): Mark the loop for
> > rewrite when stmts with undefined behavior on integer
> > overflow appear.
> > (combine_blocks): Predicate also when we need to rewrite stmts.
> > (predicate_statements): Rewrite affected stmts to something
> > with well-defined behavior on overflow.
> > (tree_if_conversion): Initialize need_to_rewrite_undefined.
> >
> > * gcc.dg/torture/pr69760.c: Adjust the testcase.
> > * gcc.target/i386/avx2-vect-mask-store-move1.c: Expect to move
> > the conversions to unsigned as well.
> > ---
> > gcc/testsuite/gcc.dg/torture/pr69760.c | 3 +-
> > .../i386/avx2-vect-mask-store-move1.c | 2 +-
> > gcc/tree-if-conv.c | 28 ++++++++++++++++++-
> > 3 files changed, 29 insertions(+), 4 deletions(-)
> >
> > diff --git a/gcc/testsuite/gcc.dg/torture/pr69760.c
> > b/gcc/testsuite/gcc.dg/torture/pr69760.c
> > index 53733c7c6a4..47e01ae59bd 100644
> > --- a/gcc/testsuite/gcc.dg/torture/pr69760.c
> > +++ b/gcc/testsuite/gcc.dg/torture/pr69760.c
> > @@ -1,11 +1,10 @@
> > /* PR tree-optimization/69760 */
> > /* { dg-do run { target { { *-*-linux* *-*-gnu* *-*-uclinux* } && mmap } }
> > } */
> > -/* { dg-options "-O2" } */
> >
> > #include <unistd.h>
> > #include <sys/mman.h>
> >
> > -__attribute__((noinline, noclone)) void
> > +__attribute__((noinline, noclone)) static void
> > test_func (double *a, int L, int m, int n, int N)
> > {
> > int i, k;
> > diff --git a/gcc/testsuite/gcc.target/i386/avx2-vect-mask-store-move1.c
> > b/gcc/testsuite/gcc.target/i386/avx2-vect-mask-store-move1.c
> > index 989ba402e0e..6a47a09c835 100644
> > --- a/gcc/testsuite/gcc.target/i386/avx2-vect-mask-store-move1.c
> > +++ b/gcc/testsuite/gcc.target/i386/avx2-vect-mask-store-move1.c
> > @@ -78,4 +78,4 @@ avx2_test (void)
> > abort ();
> > }
> >
> > -/* { dg-final { scan-tree-dump-times "Move stmt to created bb" 6 "vect" }
> > } */
> > +/* { dg-final { scan-tree-dump-times "Move stmt to created bb" 10 "vect" }
> > } */
> > diff --git a/gcc/tree-if-conv.c b/gcc/tree-if-conv.c
> > index d7b7b309309..6a67acfeaae 100644
> > --- a/gcc/tree-if-conv.c
> > +++ b/gcc/tree-if-conv.c
> > @@ -132,6 +132,11 @@ along with GCC; see the file COPYING3. If not see
> > predicate_statements for the kinds of predication we support. */
> > static bool need_to_predicate;
> >
> > +/* True if we have to rewrite stmts that may invoke undefined behavior
> > + when a condition C was false so it doesn't if it is always executed.
> > + See predicate_statements for the kinds of predication we support. */
> > +static bool need_to_rewrite_undefined;
> > +
> > /* Indicate if there are any complicated PHIs that need to be handled in
> > if-conversion. Complicated PHI has more than two arguments and can't
> > be degenerated to two arguments PHI. See more information in comment
> > @@ -1042,6 +1047,12 @@ if_convertible_gimple_assign_stmt_p (gimple *stmt,
> > fprintf (dump_file, "tree could trap...\n");
> > return false;
> > }
> > + else if (INTEGRAL_TYPE_P (TREE_TYPE (lhs))
> > + && TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (lhs))
> > + && arith_code_with_undefined_signed_overflow
> > + (gimple_assign_rhs_code (stmt)))
> > + /* We have to rewrite stmts with undefined overflow. */
> > + need_to_rewrite_undefined = true;
> >
> > /* When if-converting stores force versioning, likewise if we
> > ended up generating store data races. */
> > @@ -2563,6 +2574,20 @@ predicate_statements (loop_p loop)
> >
> > gsi_replace (&gsi, new_stmt, true);
> > }
> > + else if (INTEGRAL_TYPE_P (TREE_TYPE (gimple_assign_lhs (stmt)))
> > + && TYPE_OVERFLOW_UNDEFINED
> > + (TREE_TYPE (gimple_assign_lhs (stmt)))
> > + && arith_code_with_undefined_signed_overflow
> > + (gimple_assign_rhs_code (stmt)))
> > + {
> > + gsi_remove (&gsi, true);
> > + gsi_insert_seq_before (&gsi, rewrite_to_defined_overflow (stmt),
> > + GSI_SAME_STMT);
> > + if (gsi_end_p (gsi))
> > + gsi = gsi_last_bb (gimple_bb (stmt));
> > + else
> > + gsi_prev (&gsi);
>
> …perhaps there should be a GSI_* for this idiom.
Yeah, the issue is that gsi_remove might put gsi to one after the
last stmt (gsi_end_p) which gsi_insert_seq_before happily
handles (insert at the end of the block) but gsi_prev chokes on.
I wonder what GSI_CONTINUE_LINKING would do with gsi_insert[_seq]_before.
Hmm, it puts us at the first stmt inserted which might be OK in this
case but the intention was to put us as the last. So we could
eventually add GSI_LAST_NEW_STMT or so ;)
I'll see to do that as followup if you think that makes sense.
Note we currently document
enum gsi_iterator_update
{
GSI_NEW_STMT, /* Only valid when single statement is added, move
iterator to it. */
GSI_SAME_STMT, /* Leave the iterator at the same statement. */
GSI_CONTINUE_LINKING /* Move iterator to whatever position is suitable
for linking other statements in the same
direction. */
};
where the "Only valid when single statement" part is a bit odd. It
seems to consistently point at the earliest inserted stmt (I guess
first/last is a bit confusing and maybe earlierst/latest is better?)
Richard.
> Thanks,
> Richard
>
> > + }
> > else if (gimple_vdef (stmt))
> > {
> > tree lhs = gimple_assign_lhs (stmt);
> > @@ -2647,7 +2672,7 @@ combine_blocks (class loop *loop)
> > insert_gimplified_predicates (loop);
> > predicate_all_scalar_phis (loop);
> >
> > - if (need_to_predicate)
> > + if (need_to_predicate || need_to_rewrite_undefined)
> > predicate_statements (loop);
> >
> > /* Merge basic blocks. */
> > @@ -3148,6 +3173,7 @@ tree_if_conversion (class loop *loop, vec<gimple *>
> > *preds)
> > rloop = NULL;
> > ifc_bbs = NULL;
> > need_to_predicate = false;
> > + need_to_rewrite_undefined = false;
> > any_complicated_phi = false;
> >
> > /* Apply more aggressive if-conversion when loop or its outer loop were
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
