On Mon, Sep 2, 2019 at 6:28 PM Ilya Leoshkevich <i...@linux.ibm.com> wrote:
>
> > Am 02.09.2019 um 12:37 schrieb Richard Biener <richard.guent...@gmail.com>:
> >
> > On Fri, Aug 30, 2019 at 5:25 PM Ilya Leoshkevich <i...@linux.ibm.com> wrote:
> >>
> >>> Am 30.08.2019 um 16:40 schrieb Ilya Leoshkevich <i...@linux.ibm.com>:
> >>>
> >>>> Am 30.08.2019 um 09:12 schrieb Richard Biener
> >>>> <richard.guent...@gmail.com>:
> >>>>
> >>>> On Thu, Aug 29, 2019 at 5:39 PM Ilya Leoshkevich <i...@linux.ibm.com>
> >>>> wrote:
> >>>>>
> >>>>>> Am 22.08.2019 um 15:45 schrieb Ilya Leoshkevich <i...@linux.ibm.com>:
> >>>>>>
> >>>>>> Bootstrap and regtest running on x86_64-redhat-linux and
> >>>>>> s390x-redhat-linux.
> >>>>>>
> >>>>>> This patch series adds signaling FP comparison support (both scalar and
> >>>>>> vector) to s390 backend.
> >>>>>
> >>>>> I'm running into a problem on ppc64 with this patch, and it would be
> >>>>> great if someone could help me figure out the best way to resolve it.
> >>>>>
> >>>>> vector36.C test is failing because gimplifier produces the following
> >>>>>
> >>>>> _5 = _4 > { 2.0e+0, 2.0e+0, 2.0e+0, 2.0e+0 };
> >>>>> _6 = VEC_COND_EXPR <_5, { -1, -1, -1, -1 }, { 0, 0, 0, 0 }>;
> >>>>>
> >>>>> from
> >>>>>
> >>>>> VEC_COND_EXPR < (*b > { 2.0e+0, 2.0e+0, 2.0e+0, 2.0e+0 }) ,
> >>>>> { -1, -1, -1, -1 } ,
> >>>>> { 0, 0, 0, 0 } >
> >>>>>
> >>>>> Since the comparison tree code is now hidden behind a temporary, my code
> >>>>> does not have anything to pass to the backend. The reason for creating
> >>>>> a temporary is that the comparison can trap, and so the following check
> >>>>> in gimplify_expr fails:
> >>>>>
> >>>>> if (gimple_seq_empty_p (internal_post) && (*gimple_test_f) (*expr_p))
> >>>>> goto out;
> >>>>>
> >>>>> gimple_test_f is is_gimple_condexpr, and it eventually calls
> >>>>> operation_could_trap_p (GT).
> >>>>>
> >>>>> My current solution is to simply state that backend does not support
> >>>>> SSA_NAME in vector comparisons, however, I don't like it, since it may
> >>>>> cause performance regressions due to having to fall back to scalar
> >>>>> comparisons.
> >>>>>
> >>>>> I was thinking about two other possible solutions:
> >>>>>
> >>>>> 1. Change the gimplifier to allow trapping vector comparisons. That's
> >>>>> a bit complicated, because tree_could_throw_p checks not only for
> >>>>> floating point traps, but also e.g. for array index out of bounds
> >>>>> traps. So I would have to create a tree_could_throw_p version which
> >>>>> disregards specific kinds of traps.
> >>>>>
> >>>>> 2. Change expand_vector_condition to follow SSA_NAME_DEF_STMT and use
> >>>>> its tree_code instead of SSA_NAME. The potential problem I see with
> >>>>> this is that there appears to be no guarantee that _5 will be inlined
> >>>>> into _6 at a later point. So if we say that we don't need to fall
> >>>>> back to scalar comparisons based on availability of vector >
> >>>>> instruction and inlining does not happen, then what's actually will
> >>>>> be required is vector selection (vsel on S/390), which might not be
> >>>>> available in general case.
> >>>>>
> >>>>> What would be a better way to proceed here?
> >>>>
> >>>> On GIMPLE there isn't a good reason to split out trapping comparisons
> >>>> from [VEC_]COND_EXPR - the gimplifier does this for GIMPLE_CONDs
> >>>> where it is important because we'd have no way to represent EH info
> >>>> when not done. It might be a bit awkward to preserve EH across RTL
> >>>> expansion though in case the [VEC_]COND_EXPR are not expanded
> >>>> as a single pattern, but I'm not sure.
> >>>
> >>> Ok, so I'm testing the following now - for the problematic test that
> >>> helped:
> >>>
> >>> diff --git a/gcc/gimple-expr.c b/gcc/gimple-expr.c
> >>> index b0c9f9b671a..940aa394769 100644
> >>> --- a/gcc/gimple-expr.c
> >>> +++ b/gcc/gimple-expr.c
> >>> @@ -602,17 +602,33 @@ is_gimple_lvalue (tree t)
> >>> || TREE_CODE (t) == BIT_FIELD_REF);
> >>> }
> >>>
> >>> -/* Return true if T is a GIMPLE condition. */
> >>> +/* Helper for is_gimple_condexpr and
> >>> is_possibly_trapping_gimple_condexpr. */
> >>>
> >>> -bool
> >>> -is_gimple_condexpr (tree t)
> >>> +static bool
> >>> +is_gimple_condexpr_1 (tree t, bool allow_traps)
> >>> {
> >>> return (is_gimple_val (t) || (COMPARISON_CLASS_P (t)
> >>> - && !tree_could_throw_p (t)
> >>> + && (allow_traps || !tree_could_throw_p (t))
> >>> && is_gimple_val (TREE_OPERAND (t, 0))
> >>> && is_gimple_val (TREE_OPERAND (t, 1))));
> >>> }
> >>>
> >>> +/* Return true if T is a GIMPLE condition. */
> >>> +
> >>> +bool
> >>> +is_gimple_condexpr (tree t)
> >>> +{
> >>> + return is_gimple_condexpr_1 (t, false);
> >>> +}
> >>> +
> >>> +/* Like is_gimple_condexpr, but allow the T to trap. */
> >>> +
> >>> +bool
> >>> +is_possibly_trapping_gimple_condexpr (tree t)
> >>> +{
> >>> + return is_gimple_condexpr_1 (t, true);
> >>> +}
> >>> +
> >>> /* Return true if T is a gimple address. */
> >>>
> >>> bool
> >>> diff --git a/gcc/gimple-expr.h b/gcc/gimple-expr.h
> >>> index 1ad1432bd17..20546ca5b99 100644
> >>> --- a/gcc/gimple-expr.h
> >>> +++ b/gcc/gimple-expr.h
> >>> @@ -41,6 +41,7 @@ extern void gimple_cond_get_ops_from_tree (tree, enum
> >>> tree_code *, tree *,
> >>> tree *);
> >>> extern bool is_gimple_lvalue (tree);
> >>> extern bool is_gimple_condexpr (tree);
> >>> +extern bool is_possibly_trapping_gimple_condexpr (tree);
> >>> extern bool is_gimple_address (const_tree);
> >>> extern bool is_gimple_invariant_address (const_tree);
> >>> extern bool is_gimple_ip_invariant_address (const_tree);
> >>> diff --git a/gcc/gimplify.c b/gcc/gimplify.c
> >>> index daa0b71c191..4e6256390c0 100644
> >>> --- a/gcc/gimplify.c
> >>> +++ b/gcc/gimplify.c
> >>> @@ -12973,6 +12973,7 @@ gimplify_expr (tree *expr_p, gimple_seq *pre_p,
> >>> gimple_seq *post_p,
> >>> else if (gimple_test_f == is_gimple_val
> >>> || gimple_test_f == is_gimple_call_addr
> >>> || gimple_test_f == is_gimple_condexpr
> >>> + || gimple_test_f == is_possibly_trapping_gimple_condexpr
> >>> || gimple_test_f == is_gimple_mem_rhs
> >>> || gimple_test_f == is_gimple_mem_rhs_or_call
> >>> || gimple_test_f == is_gimple_reg_rhs
> >>> @@ -13814,7 +13815,7 @@ gimplify_expr (tree *expr_p, gimple_seq *pre_p,
> >>> gimple_seq *post_p,
> >>> enum gimplify_status r0, r1, r2;
> >>>
> >>> r0 = gimplify_expr (&TREE_OPERAND (*expr_p, 0), pre_p,
> >>> - post_p, is_gimple_condexpr, fb_rvalue);
> >>> + post_p,
> >>> is_possibly_trapping_gimple_condexpr, fb_rvalue);
> >>> r1 = gimplify_expr (&TREE_OPERAND (*expr_p, 1), pre_p,
> >>> post_p, is_gimple_val, fb_rvalue);
> >>> r2 = gimplify_expr (&TREE_OPERAND (*expr_p, 2), pre_p,
> >>> diff --git a/gcc/tree-cfg.c b/gcc/tree-cfg.c
> >>> index b75fdb2e63f..175b858f56b 100644
> >>> --- a/gcc/tree-cfg.c
> >>> +++ b/gcc/tree-cfg.c
> >>> @@ -4121,8 +4121,11 @@ verify_gimple_assign_ternary (gassign *stmt)
> >>> return true;
> >>> }
> >>>
> >>> - if (((rhs_code == VEC_COND_EXPR || rhs_code == COND_EXPR)
> >>> - ? !is_gimple_condexpr (rhs1) : !is_gimple_val (rhs1))
> >>> + if ((rhs_code == VEC_COND_EXPR
> >>> + ? !is_possibly_trapping_gimple_condexpr (rhs1)
> >>> + : (rhs_code == COND_EXPR
> >>> + ? !is_gimple_condexpr (rhs1)
> >>> + : !is_gimple_val (rhs1)))
> >>> || !is_gimple_val (rhs2)
> >>> || !is_gimple_val (rhs3))
> >>> {
> >>>
> >>>>
> >>>> To go this route you'd have to split the is_gimple_condexpr check
> >>>> I guess and eventually users turning [VEC_]COND_EXPR into conditional
> >>>> code (do we have any?) have to be extra careful then.
> >>>>
> >>>
> >>> We have expand_vector_condition, which turns VEC_COND_EXPR into
> >>> COND_EXPR - but this should be harmless, right? I could not find
> >>> anything else.
> >>
> >> Ugh, I've realized I need to check not only VEC_COND_EXPR, but also
> >> COND_EXPR usages. There is, of course, a great deal more code, so I'm
> >> not sure whether I looked exhaustively through it, but there are at
> >> least store_expr and do_jump which do exactly this during expansion.
> >> Should we worry about EH edges at this point?
> >
> > Well, the EH edge needs to persist (and be rooted off the comparison,
> > not the selection).
>
> Ok, I'm trying to create some samples that may reveal problems with EH
> edges in these two cases. So far with these experiments I only managed
> to find and unrelated S/390 bug :-)
> https://gcc.gnu.org/ml/gcc-patches/2019-09/msg00065.html
>
> > That said, I'd simply stop using is_gimple_condexpr for GIMPLE_CONDs
> > (it allows is_gimple_val which isn't proper form for GIMPLE_COND). Of
> > course
> > there's code using it for GIMPLE_CONDs which would need to be adjusted.
>
> I'm sorry, I don't quite get this - what would that buy us? and what
> would you use instead? Right now we fix up non-conforming values
> accepted by is_gimple_val using gimple_cond_get_ops_from_tree - is
> there a problem with this approach?
>
> What I have in mind right now is to:
> - allow trapping conditions for COND_EXPR and VEC_COND_EXPR;
> - report them as trapping in operation_could_trap_p and
> tree_could_trap_p iff their condition is trapping;
> - find and adjust all places where this messes up EH edges.
>
> GIMPLE_COND logic appears to be already covered precisely because it
> uses is_gimple_condexpr.
>
> Am I missing something?
Not really - all I'm saying is that currently we use is_gimple_condexpr
to check whether a GENERIC tree is suitable for [VEC_]COND_EXPR
during for example forward propagation.
And GIMPLE_COND already uses its own logic (as you say) but
still passes use is_gimple_condexpr for it.
So my proposal would be to change is_gimple_condexpr to
allow trapping [VEC_]COND_EXPR and stop using is_gimple_condexpr
checks on conditions to be used for GIMPLE_CONDs (and substitute
another predicate there). For this to work and catch wrong-doings
we should amend gimple_cond_get_ops_from_tree to assert
that the extracted condition cannot trap.
Richard.
