Re: [PATCH] c++: Don't cache calls which rethrow etc. [PR126508]

On 7/30/26 8:49 AM, Jakub Jelinek wrote:
> Hi!
> 
> The first 3 testcase below are miscompiled, we happily cache
> calls during constant evaluation which don't depend just on their
> arguments, but also on the current exceptions (uncaught or caught).
> If we decide to cache such functions and then try to evaluate them
> with different uncaught/caught exceptions (or none), we can get wrong
> results.
> We already don't cache calls which allocate and don't free all heap
> allocations, or free some heap allocations they haven't allocated,
> or which call (right now any) metafunctions, or have exited through
> exception, or aren't constant.
> This patch just adds the rethrow/__builtin_uncaught_exceptions/
> __builtin_current_exception calls to the set of non-cacheable operations
> (to be precise, e.g. rethrow would be safe to cache if we can prove
> that the current exception was always thrown from within that function,
> ditto __builtin_current_exception, but it is hard to figure out).
> The last testcase attempts to check if we don't need something similar
> also for __builtin_eh_ptr_adjust_ref, but the call to foo for some reason
> isn't cached and so I don't have a proof we need to handle it too.
> 
> So far lightly tested, ok for trunk/16.2 if it passes full
> bootstrap/regtest?

OK, though we might rename metafns_called to something like 
state_dependent and mention EH in its comment.  That can be a trunk-only 
followup.
> 2026-07-30  Jakub Jelinek  <[email protected]>
> 
> 	PR c++/126508
> 	* constexpr.cc (cxx_eval_cxa_builtin_fn): Avoid caching
> 	calls which rethrow or call __builtin_uncaught_exceptions
> 	or __builtin_current_exception.
> 
> 	* g++.dg/cpp26/constexpr-eh20.C: New test.
> 	* g++.dg/cpp26/constexpr-eh21.C: New test.
> 	* g++.dg/cpp26/constexpr-eh22.C: New test.
> 	* g++.dg/cpp26/constexpr-eh23.C: New test.
> 
> --- gcc/cp/constexpr.cc.jj	2026-07-27 18:51:44.636642818 +0200
> +++ gcc/cp/constexpr.cc	2026-07-30 14:03:20.410583194 +0200
> @@ -2118,6 +2118,9 @@ cxx_eval_cxa_builtin_fn (const constexpr
>         DECL_EXCEPTION_REFCOUNT (arg)
>   	= size_binop (PLUS_EXPR, DECL_EXCEPTION_REFCOUNT (arg), size_one_node);
>         ++ctx->global->uncaught_exceptions;
> +      /* Don't cache calls which rethrow, they depend on the current
> +	 exception which might be caught in the caller.  */
> +      ctx->global->metafns_called = true;
>         *jump_target = arg;
>         return void_node;
>       case CXA_BAD_CAST:
> @@ -2196,6 +2199,10 @@ cxx_eval_cxa_builtin_fn (const constexpr
>   	  *non_constant_p = true;
>   	  return call;
>   	}
> +      /* Don't cache calls which call __builtin_uncaught_exceptions (),
> +	 they depend on the current uncaught exceptions which might
> +	 be the state from their caller.  */
> +      ctx->global->metafns_called = true;
>         return build_int_cst (integer_type_node,
>   			    ctx->global->uncaught_exceptions);
>       case BUILTIN_CURRENT_EXCEPTION:
> @@ -2247,6 +2254,10 @@ cxx_eval_cxa_builtin_fn (const constexpr
>   			      size_one_node);
>   	      arg = fold_convert (ptr_type_node, build_address (arg));
>   	    }
> +	  /* Don't cache calls which call __builtin_current_exception (),
> +	     they depend on the current exception which might be caught
> +	     in the caller.  */
> +	  ctx->global->metafns_called = true;
>   	  return build_constructor_single (TREE_TYPE (decl), fld, arg);
>   	}
>       case STD_RETHROW_EXCEPTION:
> --- gcc/testsuite/g++.dg/cpp26/constexpr-eh20.C.jj	2026-07-30 13:04:03.278524380 +0200
> +++ gcc/testsuite/g++.dg/cpp26/constexpr-eh20.C	2026-07-30 13:03:39.287820983 +0200
> @@ -0,0 +1,31 @@
> +// PR c++/126508
> +// { dg-do compile { target c++26 } }
> +
> +constexpr int
> +foo ()
> +{
> +  try
> +    {
> +      throw;
> +    }
> +  catch (const int x)
> +    {
> +      return x;
> +    }
> +}
> +
> +constexpr int
> +bar (int x)
> +{
> +  try
> +    {
> +      throw x;
> +    }
> +  catch (...)
> +    {
> +      return foo ();
> +    }
> +}
> +
> +static_assert (bar (42) == 42);
> +static_assert (bar (43) == 43);
> --- gcc/testsuite/g++.dg/cpp26/constexpr-eh21.C.jj	2026-07-30 13:05:39.040340448 +0200
> +++ gcc/testsuite/g++.dg/cpp26/constexpr-eh21.C	2026-07-30 13:09:35.632415397 +0200
> @@ -0,0 +1,43 @@
> +// PR c++/126508
> +// { dg-do compile { target c++26 } }
> +
> +constexpr int
> +foo ()
> +{
> +  return __builtin_uncaught_exceptions ();
> +}
> +
> +constexpr int
> +bar ()
> +{
> +  return __builtin_uncaught_exceptions ();
> +}
> +
> +struct A { constexpr A () : a (0) {} constexpr ~A () { if (foo () != a) asm (""); } int a; };
> +struct B { constexpr B () : b (0) {} constexpr ~B () { if (bar () != b) asm (""); } int b; };
> +
> +constexpr bool
> +baz ()
> +{
> +  {
> +    A a;
> +  }
> +  try
> +    {
> +      A a;
> +      B b;
> +      a.a = 1;
> +      b.b = 1;
> +      throw 42;
> +    }
> +  catch (...)
> +    {
> +    }
> +  {
> +    A a;
> +    B b;
> +  }
> +  return true;
> +}
> +
> +static_assert (baz ());
> --- gcc/testsuite/g++.dg/cpp26/constexpr-eh22.C.jj	2026-07-30 13:15:19.463164518 +0200
> +++ gcc/testsuite/g++.dg/cpp26/constexpr-eh22.C	2026-07-30 13:17:06.057846657 +0200
> @@ -0,0 +1,41 @@
> +// PR c++/126508
> +// { dg-do compile { target c++26 } }
> +
> +#include <exception>
> +
> +constexpr bool
> +foo ()
> +{
> +  return __builtin_current_exception () != nullptr;
> +}
> +
> +constexpr int
> +bar ()
> +{
> +  return __builtin_current_exception () != nullptr;
> +}
> +
> +constexpr bool
> +baz ()
> +{
> +  if (foo ())
> +    return false;
> +  try
> +    {
> +      throw 42;
> +    }
> +  catch (...)
> +    {
> +      if (!foo ())
> +	return false;
> +      if (!bar ())
> +	return false;
> +    }
> +  if (foo ())
> +    return false;
> +  if (bar ())
> +    return false;
> +  return true;
> +}
> +
> +static_assert (baz ());
> --- gcc/testsuite/g++.dg/cpp26/constexpr-eh23.C.jj	2026-07-30 13:28:07.676488300 +0200
> +++ gcc/testsuite/g++.dg/cpp26/constexpr-eh23.C	2026-07-30 13:28:03.177545200 +0200
> @@ -0,0 +1,22 @@
> +// PR c++/126508
> +// { dg-do compile { target c++26 } }
> +
> +#include <exception>
> +
> +constexpr std::exception_ptr
> +foo (const std::exception_ptr &x)
> +{
> +  return x;
> +}
> +
> +constexpr bool
> +baz ()
> +{
> +  std::exception_ptr a = std::make_exception_ptr (42);
> +  auto b = foo (a);
> +  auto c = foo (a);
> +  auto d = foo (a);
> +  return true;
> +}
> +
> +static_assert (baz ());
> 
> 	Jakub
>