Re: [PATCH] c++: Fix ICE with __builtin_bit_cast [PR98469]

[Jason Merrill <jason@redhat.com>]

On 1/4/21 3:48 PM, Jakub Jelinek wrote:
> On Mon, Jan 04, 2021 at 03:44:46PM -0500, Jason Merrill wrote:
>> 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.
> 
> I can try that tomorrow.  Won't that cause copying through extra temporary
> in some cases though, or is that guaranteed to be optimized?

It won't cause any extra copying when it's used to initialize another 
object (like the return value of std::bit_cast).  Class prvalues are 
always expressed with a TARGET_EXPR in the front end; the TARGET_EXPR 
melts away when used as an initializer, it only creates a temporary when 
it's used in another way.

Jason