Hi! This is a reason latest firefox is miscompiled by G++ 9, seems DR 1299 says in [class.temporary]/(6.7) that reference binding to a temporary object from the second or third operand of a conditional expression should have extended lifetime too, but extend_ref_init_temps_1 was apparently handling only comma expressions, some casts, . (COMPONENT_REF), [] (ARRAY_REF), but not COND_EXPR.
The following patch handles COND_EXPR in there too, in a similar way how the gimplifier handles the cleanups of the expressions in the COND_EXPR operand if there is no lifetime extension due to reference binding. In particular there are bool temporaries added, initialized to false and set to true if the particular (leaf) COND_EXPR argument has been invoked and the corresponding cleanup guarded that way. I admit I haven't tried to construct testcases for all the things CWG 1299 added, e.g. don't know if ( expression ) will work, if all the *_cast cases that need to be extended work (some of them do, as the testcase shows), or if e.g. .* works, so I'm not claiming the DR is completely implemented. Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk and after a while for release branches? Note, just to double check g++ doesn't ICE on expr1 ? : expr2 GNU extension, I also wrote the attached testcase, but unlike the testcase included in the patch which behaves the same with patched g++ as does recent clang++, the testcase with expr1 ? : expr2 (omitted second operand) actually never extends the lifetime of any temporary (that is the baz function), unlike clang++ which extends the lifetime of expr2 if expr1 is false, and doesn't extend lifetime of anything if expr1 is true. This is outside of the scope of the standard, so no idea what is correct, so I'm just asking how it should behave. extend_ref_init_temps_1 is never called in that case when the expression is COND_EXPR. 2019-12-06 Jakub Jelinek <ja...@redhat.com> PR c++/92831 - CWG 1299, not extending temporary lifetime for ?: * cp-tree.h (extend_ref_init_temps): Add a new argument with NULL default arg. * call.c (set_up_extended_ref_temp): Add COND_GUARD argument, pass it down to extend_ref_init_temps. Before pushing cleanup, if COND_GUARD is non-NULL, create a bool temporary if needed, initialize to false and guard the cleanup with the temporary being true. (extend_ref_init_temps_1): Add COND_GUARD argument, pass it down to recursive calls and set_up_extended_ref_temp. Handle COND_EXPR. (extend_ref_init_temps): Add COND_GUARD argument, pass it down to recursive calls and to extend_ref_init_temps_1. * g++.dg/cpp0x/temp-extend2.C: New test. --- gcc/cp/cp-tree.h.jj 2019-12-05 10:03:04.111181297 +0100 +++ gcc/cp/cp-tree.h 2019-12-06 11:00:43.230257916 +0100 @@ -6321,7 +6321,9 @@ extern tree convert_for_arg_passing (tr extern bool is_properly_derived_from (tree, tree); extern tree initialize_reference (tree, tree, int, tsubst_flags_t); -extern tree extend_ref_init_temps (tree, tree, vec<tree, va_gc>**); +extern tree extend_ref_init_temps (tree, tree, + vec<tree, va_gc>**, + tree * = NULL); extern tree make_temporary_var_for_ref_to_temp (tree, tree); extern bool type_has_extended_temps (tree); extern tree strip_top_quals (tree); --- gcc/cp/call.c.jj 2019-12-06 00:40:29.439856520 +0100 +++ gcc/cp/call.c 2019-12-06 11:00:17.111660971 +0100 @@ -11965,7 +11965,7 @@ make_temporary_var_for_ref_to_temp (tree static tree set_up_extended_ref_temp (tree decl, tree expr, vec<tree, va_gc> **cleanups, - tree *initp) + tree *initp, tree *cond_guard) { tree init; tree type; @@ -11996,7 +11996,8 @@ set_up_extended_ref_temp (tree decl, tre /* Recursively extend temps in this initializer. */ TARGET_EXPR_INITIAL (expr) - = extend_ref_init_temps (decl, TARGET_EXPR_INITIAL (expr), cleanups); + = extend_ref_init_temps (decl, TARGET_EXPR_INITIAL (expr), cleanups, + cond_guard); /* Any reference temp has a non-trivial initializer. */ DECL_NONTRIVIALLY_INITIALIZED_P (var) = true; @@ -12037,7 +12038,24 @@ set_up_extended_ref_temp (tree decl, tre { tree cleanup = cxx_maybe_build_cleanup (var, tf_warning_or_error); if (cleanup) - vec_safe_push (*cleanups, cleanup); + { + if (cond_guard && cleanup != error_mark_node) + { + if (*cond_guard == NULL_TREE) + { + *cond_guard = build_local_temp (boolean_type_node); + add_decl_expr (*cond_guard); + tree set = cp_build_modify_expr (UNKNOWN_LOCATION, + *cond_guard, NOP_EXPR, + boolean_false_node, + tf_warning_or_error); + finish_expr_stmt (set); + } + cleanup = build3 (COND_EXPR, void_type_node, + *cond_guard, cleanup, NULL_TREE); + } + vec_safe_push (*cleanups, cleanup); + } } /* We must be careful to destroy the temporary only @@ -12142,7 +12160,8 @@ initialize_reference (tree type, tree ex which is bound either to a reference or a std::initializer_list. */ static tree -extend_ref_init_temps_1 (tree decl, tree init, vec<tree, va_gc> **cleanups) +extend_ref_init_temps_1 (tree decl, tree init, vec<tree, va_gc> **cleanups, + tree *cond_guard) { tree sub = init; tree *p; @@ -12150,20 +12169,52 @@ extend_ref_init_temps_1 (tree decl, tree if (TREE_CODE (sub) == COMPOUND_EXPR) { TREE_OPERAND (sub, 1) - = extend_ref_init_temps_1 (decl, TREE_OPERAND (sub, 1), cleanups); + = extend_ref_init_temps_1 (decl, TREE_OPERAND (sub, 1), cleanups, + cond_guard); + return init; + } + if (TREE_CODE (sub) == COND_EXPR) + { + tree cur_cond_guard = NULL_TREE; + if (TREE_OPERAND (sub, 1)) + TREE_OPERAND (sub, 1) + = extend_ref_init_temps_1 (decl, TREE_OPERAND (sub, 1), cleanups, + &cur_cond_guard); + if (cur_cond_guard) + { + tree set = cp_build_modify_expr (UNKNOWN_LOCATION, cur_cond_guard, + NOP_EXPR, boolean_true_node, + tf_warning_or_error); + TREE_OPERAND (sub, 1) + = cp_build_compound_expr (set, TREE_OPERAND (sub, 1), + tf_warning_or_error); + } + cur_cond_guard = NULL_TREE; + if (TREE_OPERAND (sub, 2)) + TREE_OPERAND (sub, 2) + = extend_ref_init_temps_1 (decl, TREE_OPERAND (sub, 2), cleanups, + &cur_cond_guard); + if (cur_cond_guard) + { + tree set = cp_build_modify_expr (UNKNOWN_LOCATION, cur_cond_guard, + NOP_EXPR, boolean_true_node, + tf_warning_or_error); + TREE_OPERAND (sub, 2) + = cp_build_compound_expr (set, TREE_OPERAND (sub, 2), + tf_warning_or_error); + } return init; } if (TREE_CODE (sub) != ADDR_EXPR) return init; /* Deal with binding to a subobject. */ for (p = &TREE_OPERAND (sub, 0); - (TREE_CODE (*p) == COMPONENT_REF - || TREE_CODE (*p) == ARRAY_REF); ) + TREE_CODE (*p) == COMPONENT_REF || TREE_CODE (*p) == ARRAY_REF; ) p = &TREE_OPERAND (*p, 0); if (TREE_CODE (*p) == TARGET_EXPR) { tree subinit = NULL_TREE; - *p = set_up_extended_ref_temp (decl, *p, cleanups, &subinit); + *p = set_up_extended_ref_temp (decl, *p, cleanups, &subinit, cond_guard); recompute_tree_invariant_for_addr_expr (sub); if (init != sub) init = fold_convert (TREE_TYPE (init), sub); @@ -12178,13 +12229,14 @@ extend_ref_init_temps_1 (tree decl, tree lifetime to match that of DECL. */ tree -extend_ref_init_temps (tree decl, tree init, vec<tree, va_gc> **cleanups) +extend_ref_init_temps (tree decl, tree init, vec<tree, va_gc> **cleanups, + tree *cond_guard) { tree type = TREE_TYPE (init); if (processing_template_decl) return init; if (TYPE_REF_P (type)) - init = extend_ref_init_temps_1 (decl, init, cleanups); + init = extend_ref_init_temps_1 (decl, init, cleanups, cond_guard); else { tree ctor = init; @@ -12203,7 +12255,8 @@ extend_ref_init_temps (tree decl, tree i /* The temporary array underlying a std::initializer_list is handled like a reference temporary. */ tree array = CONSTRUCTOR_ELT (ctor, 0)->value; - array = extend_ref_init_temps_1 (decl, array, cleanups); + array = extend_ref_init_temps_1 (decl, array, cleanups, + cond_guard); CONSTRUCTOR_ELT (ctor, 0)->value = array; } else @@ -12212,7 +12265,8 @@ extend_ref_init_temps (tree decl, tree i constructor_elt *p; vec<constructor_elt, va_gc> *elts = CONSTRUCTOR_ELTS (ctor); FOR_EACH_VEC_SAFE_ELT (elts, i, p) - p->value = extend_ref_init_temps (decl, p->value, cleanups); + p->value = extend_ref_init_temps (decl, p->value, cleanups, + cond_guard); } recompute_constructor_flags (ctor); if (decl_maybe_constant_var_p (decl) && TREE_CONSTANT (ctor)) --- gcc/testsuite/g++.dg/cpp0x/temp-extend2.C.jj 2019-12-06 11:08:09.694368207 +0100 +++ gcc/testsuite/g++.dg/cpp0x/temp-extend2.C 2019-12-06 11:10:58.040768156 +0100 @@ -0,0 +1,36 @@ +// PR c++/92831 +// { dg-do run { target c++11 } } + +template<typename T> using id = T; +struct S { S () { s++; } ~S () { s--; } S (int) { s++; } static int s; }; +int S::s = 0; + +void +bar (bool cond, bool cond2) +{ + if (S::s != (cond ? cond2 ? 7 : 5 : cond2 ? 8 : 9)) + __builtin_abort (); +} + +void +foo (bool cond, bool cond2) +{ + int i = 1; + // temporary array has same lifetime as a + S&& a = id<S[3]>{1, 2, 3}[i]; + // temporary S has same lifetime as b + const S& b = static_cast<const S&>(0); + // exactly one of the four temporaries is lifetime-extended + S&& c = cond ? cond2 ? id<S[3]>{1, 2, 3}[i] : static_cast<S&&>(0) + : cond2 ? id<S[4]>{1, 2, 3, 4}[i] : id<S[5]>{1, 2, 3, 4, 5}[i]; + bar (cond, cond2); +} + +int +main () +{ + foo (true, true); + foo (true, false); + foo (false, true); + foo (false, false); +} Jakub
template<typename T> using id = T; struct S { S () : s (false) { ++c; } S (bool x) : s (x) { ++c; } ~S () { --c; }; bool s; static int c; }; int S::c = 0; extern "C" int printf (const char *, ...); void bar () { printf ("%d\n", S::c); } void foo (int i) { const bool&& a = id<S[3]>{false, true, false}[i].s ? id<S[2]>{true, false}[i].s : id<S[4]>{true, false, true, false}[i].s; bar (); } void baz (int i) { const bool&& a = id<S[3]>{false, true, false}[i].s ? : id<S[4]>{true, false, true, false}[i].s; bar (); } int main () { bar (); foo (0); bar (); foo (1); bar (); baz (0); bar (); baz (1); bar (); }