On Mon, 3 May 2021, Michael Matz wrote:
> Hello,
>
> On Mon, 3 May 2021, Jan Hubicka wrote:
>
> > > (it should not abort). The documentation of 'pure' must be read
> > > as that 'pure' is not valid for 'foo' since throwing an exception
> > > is obviously an observable effect on the state of the program
> > > (in particular for the testcase at hand). But for example
> > > IPA pure const does not cause us to infer nothrow on pure
> > > declared functions and the C++ program
> > >
> > > ...
> > >
> > > So - what is it? Are pure functions allowed to throw or not?
> >
> > I was one adding pure functions and it was really intended to be same
> > thing as const+reading memory. So does const function throw or not?
>
> I really want to say that const/pure and throw are mutually exclusive.
> But in reality they are orthogonal, so a const function may throw. (It
> certainly can in C++).
>
> This is because while we implement exceptions with memory state, that
> state in not accessible to the application. Exceptions could for instance
> also be implemented via return type extension. And then, as long as other
> guarantees of const or pure functions are adhered to (same input -> same
> output), throwing an exception from a const function would be completely
> natural. E.g. if a const function, given a specific set of arguments,
> always throws the same value that would still allow to CSE two calls to it
> in a row, and it would still allow to assume that no reachable memory was
> changed by that call.
>
> So, I think const and pure functions may validly throw (but then must
> always do so given the same inputs).
I've filed PR100394 since it appearantly never worked (for C++).
Richard.
>
> Ciao,
> Michael.
>
>
> > Internally we definitly want to make this separate - with symbol
> > summaries it is now reasonably easy to do. I was thinking of doing
> > similar summary as modref handles to pass down separate info tracked by
> > pure-const patch rather than trying to re-synthetize it to rather random
> > attributes we have now...
> >
> > Honza
> > >
> > > 2021-05-03 Richard Biener <rguent...@suse.de>
> > >
> > > * calls.c (expand_call): Preserve possibly throwing calls.
> > > * cfgexpand.c (expand_call_stmt): When a call can throw signal
> > > RTL expansion there are side-effects.
> > > * tree-ssa-dce.c (mark_stmt_if_obviously_necessary): Simplify.
> > > * tree-ssa-dse.c (pass_dse::execute): Preserve exceptions unless
> > > -fdelete-dead-exceptions.
> > >
> > > * g++.dg/tree-ssa/pr100382.C: New testcase.
> > > ---
> > > gcc/calls.c | 1 +
> > > gcc/cfgexpand.c | 5 ++++-
> > > gcc/testsuite/g++.dg/tree-ssa/pr100382.C | 25 ++++++++++++++++++++++++
> > > gcc/tree-ssa-dce.c | 21 +++-----------------
> > > gcc/tree-ssa-dse.c | 3 ++-
> > > 5 files changed, 35 insertions(+), 20 deletions(-)
> > > create mode 100644 gcc/testsuite/g++.dg/tree-ssa/pr100382.C
> > >
> > > diff --git a/gcc/calls.c b/gcc/calls.c
> > > index 883d08ba5f2..f3da1839dc5 100644
> > > --- a/gcc/calls.c
> > > +++ b/gcc/calls.c
> > > @@ -3808,6 +3808,7 @@ expand_call (tree exp, rtx target, int ignore)
> > > side-effects. */
> > > if ((flags & (ECF_CONST | ECF_PURE))
> > > && (!(flags & ECF_LOOPING_CONST_OR_PURE))
> > > + && (flags & ECF_NOTHROW)
> > > && (ignore || target == const0_rtx
> > > || TYPE_MODE (rettype) == VOIDmode))
> > > {
> > > diff --git a/gcc/cfgexpand.c b/gcc/cfgexpand.c
> > > index a6b48d3e48f..556a0b22ed6 100644
> > > --- a/gcc/cfgexpand.c
> > > +++ b/gcc/cfgexpand.c
> > > @@ -2795,7 +2795,10 @@ expand_call_stmt (gcall *stmt)
> > > CALL_EXPR_ARG (exp, i) = arg;
> > > }
> > >
> > > - if (gimple_has_side_effects (stmt))
> > > + if (gimple_has_side_effects (stmt)
> > > + /* ??? Downstream in expand_expr_real_1 we assume that expressions
> > > + w/o side-effects do not throw so work around this here. */
> > > + || stmt_could_throw_p (cfun, stmt))
> > > TREE_SIDE_EFFECTS (exp) = 1;
> > >
> > > if (gimple_call_nothrow_p (stmt))
> > > diff --git a/gcc/testsuite/g++.dg/tree-ssa/pr100382.C
> > > b/gcc/testsuite/g++.dg/tree-ssa/pr100382.C
> > > new file mode 100644
> > > index 00000000000..b9948d3034a
> > > --- /dev/null
> > > +++ b/gcc/testsuite/g++.dg/tree-ssa/pr100382.C
> > > @@ -0,0 +1,25 @@
> > > +// { dg-do run }
> > > +// { dg-options "-O2" }
> > > +
> > > +int x, y;
> > > +int __attribute__((pure,noinline)) foo () { if (x) throw; return y; }
> > > +
> > > +int __attribute__((noinline)) bar()
> > > +{
> > > + int a[2];
> > > + x = 1;
> > > + try {
> > > + int res = foo ();
> > > + a[0] = res;
> > > + } catch (...) {
> > > + return 0;
> > > + }
> > > + return 1;
> > > +}
> > > +
> > > +int main()
> > > +{
> > > + if (bar ())
> > > + __builtin_abort ();
> > > + return 0;
> > > +}
> > > diff --git a/gcc/tree-ssa-dce.c b/gcc/tree-ssa-dce.c
> > > index 096cfc8721d..ff0389af36f 100644
> > > --- a/gcc/tree-ssa-dce.c
> > > +++ b/gcc/tree-ssa-dce.c
> > > @@ -250,14 +250,6 @@ mark_stmt_if_obviously_necessary (gimple *stmt, bool
> > > aggressive)
> > > && DECL_IS_REPLACEABLE_OPERATOR_NEW_P (callee))
> > > return;
> > >
> > > - /* Most, but not all function calls are required. Function calls that
> > > - produce no result and have no side effects (i.e. const pure
> > > - functions) are unnecessary. */
> > > - if (gimple_has_side_effects (stmt))
> > > - {
> > > - mark_stmt_necessary (stmt, true);
> > > - return;
> > > - }
> > > /* IFN_GOACC_LOOP calls are necessary in that they are used to
> > > represent parameter (i.e. step, bound) of a lowered OpenACC
> > > partitioned loop. But this kind of partitioned loop might not
> > > @@ -269,8 +261,6 @@ mark_stmt_if_obviously_necessary (gimple *stmt, bool
> > > aggressive)
> > > mark_stmt_necessary (stmt, true);
> > > return;
> > > }
> > > - if (!gimple_call_lhs (stmt))
> > > - return;
> > > break;
> > > }
> > >
> > > @@ -312,19 +302,14 @@ mark_stmt_if_obviously_necessary (gimple *stmt,
> > > bool aggressive)
> > > /* If the statement has volatile operands, it needs to be preserved.
> > > Same for statements that can alter control flow in unpredictable
> > > ways. */
> > > - if (gimple_has_volatile_ops (stmt) || is_ctrl_altering_stmt (stmt))
> > > - {
> > > - mark_stmt_necessary (stmt, true);
> > > - return;
> > > - }
> > > -
> > > - if (stmt_may_clobber_global_p (stmt))
> > > + if (gimple_has_side_effects (stmt) || is_ctrl_altering_stmt (stmt))
> > > {
> > > mark_stmt_necessary (stmt, true);
> > > return;
> > > }
> > >
> > > - if (gimple_vdef (stmt) && keep_all_vdefs_p ())
> > > + if ((gimple_vdef (stmt) && keep_all_vdefs_p ())
> > > + || stmt_may_clobber_global_p (stmt))
> > > {
> > > mark_stmt_necessary (stmt, true);
> > > return;
> > > diff --git a/gcc/tree-ssa-dse.c b/gcc/tree-ssa-dse.c
> > > index dfa6d314727..386667b9f7f 100644
> > > --- a/gcc/tree-ssa-dse.c
> > > +++ b/gcc/tree-ssa-dse.c
> > > @@ -1219,7 +1219,8 @@ pass_dse::execute (function *fun)
> > > dead SSA defs. */
> > > if (has_zero_uses (DEF_FROM_PTR (def_p))
> > > && !gimple_has_side_effects (stmt)
> > > - && !stmt_unremovable_because_of_non_call_eh_p (cfun, stmt))
> > > + && (!stmt_could_throw_p (fun, stmt)
> > > + || fun->can_delete_dead_exceptions))
> > > {
> > > if (dump_file && (dump_flags & TDF_DETAILS))
> > > {
> > > --
> > > 2.26.2
> >
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
