Re: [PATCH] c++: consider built-in operator candidates first

[Jason Merrill via Gcc-patches]

On 9/20/21 15:32, Patrick Palka wrote:
On Mon, 20 Sep 2021, Jason Merrill wrote:
On 9/20/21 12:46, Patrick Palka wrote:
During operator overload resolution, we currently consider non-member
candidates before built-in candidates.  This didn't make a difference
before r12-3346, but after this change add_candidates will avoid
computing excess argument conversions if we've already seen a strictly
viable candidate, so it's better to consider built-in candidates first.

Doesn't r12-3346 stop considering conversions after it sees a bad one, and
later return to the bad candidate if there is no strictly viable candidate?
How does this patch change that?

Yes, but add_candidates also looks for a strictly viable candidate among
the already-considered candidates in the 'candidates' list via the line:

   bool seen_strictly_viable = any_strictly_viable (*candidates);

So by considering the built-in candidates first, the subsequent call to
add_candidates that considers the non-member functions in will be aware
of any (built-in) strictly viable candidate.

Ah, I get it, the problem is that the first add_candidates can't see any 
strictly-viable candidates.

Depending on the order of the candidates seems fragile.

Yeah.. :/  I guess in general it'd be better to build up the entire
overload set first and then call add_candidates exactly once (which
would also make the perfect candidate optimization more consistent/effective).
But I'm not sure if we can easily build up such an overload set in this
case since built-in candidates are represented and handled differently
than non-built-in candidates..

Or as another way of getting the same effect, add another possible value 
of shortcut_bad_convs to mean leave bad candidates incomplete in the 
candidates list, and then once we're done adding candidates and still 
don't have a viable one, we can go back and finish processing the bad 
candidates?

Either way, this could also help when there are both member and 
non-member candidates for the operator.

FWIW, although the test case added by this patch is contrived, this
opportunity was found in the real world by instrumenting the 'bad_fns'
mechanism added by r12-3346 to look for situations where we still end up
using it (and thus end up redundantly considering some candidates twice),
and this built-in operator situation was the most common in the
codebases that I tested (although still quite rare in the codebases that
I tested).


Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

gcc/cp/ChangeLog:

        * call.c (add_operator_candidates): Consider built-in operator
        candidates before considering non-member candidates.

gcc/testsuite/ChangeLog:

        * g++.dg/template/conv17.C: Extend test.
---
   gcc/cp/call.c                          | 13 +++++++------
   gcc/testsuite/g++.dg/template/conv17.C |  7 +++++++
   2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index c5601d96ab8..c0da083758f 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -6321,7 +6321,6 @@ add_operator_candidates (z_candidate **candidates,
                         vec<tree, va_gc> *arglist,
                         int flags, tsubst_flags_t complain)
   {
-  z_candidate *start_candidates = *candidates;
     bool ismodop = code2 != ERROR_MARK;
     tree fnname = ovl_op_identifier (ismodop, ismodop ? code2 : code);
   @@ -6333,6 +6332,12 @@ add_operator_candidates (z_candidate **candidates,
     if (rewritten && code != EQ_EXPR && code != SPACESHIP_EXPR)
       flags &= ~LOOKUP_REWRITTEN;
   +  /* Add built-in candidates to the candidate set.  The standard says to
+     rewrite built-in candidates, too, but there's no point.  */
+  if (!rewritten)
+    add_builtin_candidates (candidates, code, code2, fnname, arglist,
+                           flags, complain);
+
     bool memonly = false;
     switch (code)
       {
@@ -6352,6 +6357,7 @@ add_operator_candidates (z_candidate **candidates,
       /* Add namespace-scope operators to the list of functions to
        consider.  */
+  z_candidate *start_candidates = *candidates;
     if (!memonly)
       {
         tree fns = lookup_name (fnname, LOOK_where::BLOCK_NAMESPACE);
@@ -6423,11 +6429,6 @@ add_operator_candidates (z_candidate **candidates,
       if (!rewritten)
       {
-      /* The standard says to rewrite built-in candidates, too,
-        but there's no point.  */
-      add_builtin_candidates (candidates, code, code2, fnname, arglist,
-                             flags, complain);
-
         /* Maybe add C++20 rewritten comparison candidates.  */
         tree_code rewrite_code = ERROR_MARK;
         if (cxx_dialect >= cxx20
diff --git a/gcc/testsuite/g++.dg/template/conv17.C
b/gcc/testsuite/g++.dg/template/conv17.C
index f0f10f2ef4f..87ecefb8de3 100644
--- a/gcc/testsuite/g++.dg/template/conv17.C
+++ b/gcc/testsuite/g++.dg/template/conv17.C
@@ -61,3 +61,10 @@ concept E = requires { T().h(nullptr); };
     static_assert(!E<C>);
   #endif
+
+// Verify that the strictly viable built-in operator+ candidate precludes
+// us from computing all argument conversions for the below non-strictly
+// viable non-member candidate.
+enum N { n };
+int operator+(N&, B);
+int f = n + 42;