middle-end/100394 - avoid DSE/DCE of pure call that throws

There is -fdelete-dead-exceptions now and we're tracking
nothrow and const/pure bits separately and I do remember that
const or pure does _not_ imply nothrow.

Now, in the light of the PR100382 fix which added a
stmt_unremovable_because_of_non_call_eh_p guard to DSEs "DCE"
I wondered how -fdelete-dead-exceptions applies to calls and
whether stmt_unremovable_because_of_non_call_eh_p doing

  return (fun->can_throw_non_call_exceptions
          && !fun->can_delete_dead_exceptions
          && stmt_could_throw_p (fun, stmt));

really should conditionalize itself on
fun->can_throw_non_call_exceptions.  In fact DCE happily elides
pure function calls that throw without a LHS (probably a
consistency bug).  The following testcase shows this:

int x, y;
int __attribute__((pure,noinline)) foo () { if (x) throw 1; return y; }

int main()
{
  int a[2];
  x = 1;
  try {
    int res = foo ();
    a[0] = res;
  } catch (...) {
      return 0;
  }
  return 1;
}

note that if you wrap foo () into another noinline
wrap_foo () { foo (); return 1; } function then we need to make
sure to not DCE this call either even though it only throws
externally.

2021-05-03  Richard Biener  <rguenther@suse.de>

	PR middle-end/100394
	* 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,
	mark all possibly throwing stmts necessary unless we can elide
	dead EH.
	* tree-ssa-dse.c (pass_dse::execute): Preserve exceptions unless
	-fdelete-dead-exceptions.
	* tree.h (DECL_PURE_P): Add note about exceptions.

	* g++.dg/torture/pr100382.C: New testcase.

1 files changed, 4 insertions, 1 deletions

diff --git a/gcc/tree.h b/gcc/tree.h
index 5a609b9..6d3cfc4 100644
--- a/gcc/tree.h
+++ b/gcc/tree.h
@@ -3133,7 +3133,10 @@ set_function_decl_type (tree decl, function_decl_type t, bool set)
   (FUNCTION_DECL_CHECK (NODE)->function_decl.returns_twice_flag)
 
 /* Nonzero in a FUNCTION_DECL means this function should be treated
-   as "pure" function (like const function, but may read global memory).  */
+   as "pure" function (like const function, but may read global memory).
+   Note that being pure or const for a function is orthogonal to being
+   nothrow, i.e. it is valid to have DECL_PURE_P set and TREE_NOTHROW
+   cleared.  */
 #define DECL_PURE_P(NODE) (FUNCTION_DECL_CHECK (NODE)->function_decl.pure_flag)
 
 /* Nonzero only if one of TREE_READONLY or DECL_PURE_P is nonzero AND