[gcc/devel/c++-modules] libstdc++: Optimize std::optional default constructor

[gcc/devel/c++-modules] libstdc++: Optimize std::optional default constructor

[Nathan Sidwell]

https://gcc.gnu.org/g:bafd12cb22e83b7da8946873513a897e48e2900f

commit bafd12cb22e83b7da8946873513a897e48e2900f
Author: Marc Glisse <marc.glisse@inria.fr>
Date:   Fri Jun 19 12:07:33 2020 +0100

    libstdc++: Optimize std::optional default constructor
    
    The attached patch 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, but there is no new testcase to verify the
    better codegen.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/optional (optional()): Explicitly define it.

Diff:
---
 libstdc++-v3/include/std/optional | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional
index b0184a4e1c0..923d45ae0e8 100644
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -687,7 +687,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     public:
       using value_type = _Tp;
 
-      constexpr optional() = default;
+      constexpr optional() noexcept { }
 
       constexpr optional(nullopt_t) noexcept { }