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? 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
