On 12/30/20 4:13 AM, Jakub Jelinek wrote:
Hi!On the following testcase we ICE during constexpr evaluation (for warnings), because the IL has ADDR_EXPR of BIT_CAST_EXPR and ADDR_EXPR case asserts the result is not a CONSTRUCTOR. I've tried to force a temporary for those in call.c next to: if (convs->need_temporary_p || TREE_CODE (expr) == CONSTRUCTOR || TREE_CODE (expr) == VA_ARG_EXPR) but that resulted in a lot of ICEs, so this patch just punts on lval evaluation of BIT_CAST_EXPR instead, normally __builtin_bit_cast is called from std::bit_cast which is constexpr and therefore the BIT_CAST_EXPR isn't evaluated there during parsing or tsubst and when evaluating the call to std::bit_cast the NRV optimized return is assigned to some temporary or variable and so BIT_CAST_EXPR is not evaluated as lval.
This change is OK, but part of the problem is that we're trying to do overload resolution for an S copy/move constructor, which we shouldn't be because bit_cast is a prvalue, so in C++17 and up we should use it to directly initialize the target without any implied constructor call.
It seems we're mishandling this because the code in build_special_member_call specifically looks for TARGET_EXPR or CONSTRUCTOR, and BIT_CAST_EXPR is neither of those.
Wrapping a BIT_CAST_EXPR of aggregate type in a TARGET_EXPR would address this, and any other places that expect a class prvalue to come in the form of a TARGET_EXPR.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk? 2020-12-30 Jakub Jelinek <ja...@redhat.com> PR c++/98469 * constexpr.c (cxx_eval_constant_expression) <case BIT_CAST_EXPR>: Punt if lval is true. * g++.dg/cpp2a/bit-cast8.C: New test. * g++.dg/cpp2a/bit-cast9.C: New test. --- gcc/cp/constexpr.c.jj 2020-12-23 22:44:05.398093175 +0100 +++ gcc/cp/constexpr.c 2020-12-29 10:32:44.865030881 +0100 @@ -6900,6 +6900,15 @@ cxx_eval_constant_expression (const cons return t;case BIT_CAST_EXPR:+ if (lval) + { + if (!ctx->quiet) + error_at (EXPR_LOCATION (t), + "address of a call to %qs is not a constant expression", + "__builtin_bit_cast"); + *non_constant_p = true; + return t; + } r = cxx_eval_bit_cast (ctx, t, non_constant_p, overflow_p); break;--- gcc/testsuite/g++.dg/cpp2a/bit-cast8.C.jj 2020-12-29 10:35:31.547140723 +0100+++ gcc/testsuite/g++.dg/cpp2a/bit-cast8.C 2020-12-29 10:34:26.431879120 +0100 @@ -0,0 +1,11 @@ +// PR c++/98469 +// { dg-do compile { target c++20 } } +// { dg-options "-Wall" } + +struct S { int s; }; + +S +foo () +{ + return __builtin_bit_cast (S, 0); +} --- gcc/testsuite/g++.dg/cpp2a/bit-cast9.C.jj 2020-12-29 10:35:35.018101365 +0100 +++ gcc/testsuite/g++.dg/cpp2a/bit-cast9.C 2020-12-29 10:35:05.905431494 +0100 @@ -0,0 +1,15 @@ +// PR c++/98469 +// { dg-do compile { target c++20 } } +// { dg-options "-Wall" } + +template<typename T, typename F> +constexpr T +bit_cast (const F &f) noexcept +{ + return __builtin_bit_cast (T, f); +} +struct S { int s; }; +constexpr int foo (const S &x) { return x.s; } +constexpr int bar () { return foo (bit_cast<S> (0)); } +constexpr int x = bar (); +static_assert (!x); Jakub
