std::optional defaut constructor

[Marc Glisse]

Hello,

is there any drawback to the attached patch? It changes the code generated for

std::optional<std::array<int,1024>>f(){return{};}

from

 	movq	$0, (%rdi)
 	movq	%rdi, %r8
 	leaq	8(%rdi), %rdi
 	xorl	%eax, %eax
 	movq	$0, 4084(%rdi)
 	movq	%r8, %rcx
 	andq	$-8, %rdi
 	subq	%rdi, %rcx
 	addl	$4100, %ecx
 	shrl	$3, %ecx
 	rep stosq
 	movq	%r8, %rax

or with different tuning

 	subq	$8, %rsp
 	movl	$4100, %edx
 	xorl	%esi, %esi
 	call	memset
 	addq	$8, %rsp

to the much shorter

 	movb	$0, 4096(%rdi)
 	movq	%rdi, %rax

i.e. the same as the nullopt constructor.

The constructor was already non-trivial, so we don't lose that. It passes the
testsuite without regression. I thought I remembered there was a reason not to
do this...

2020-06-04  Marc Glisse  <marc.glisse@inria.fr>

 	* include/std/optional (optional()): Explicitly define it.

(I don't currently have a setup that would enable me to commit anything. I'll
try to fix it eventually, but likely not so soon)

-- 
Marc Glisse
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