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* [gcc(refs/users/marxin/heads/cond-expr-ssa-name)] libstdc++: Optimize std::optional default constructor
@ 2020-06-24 9:20 Martin Liska
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From: Martin Liska @ 2020-06-24 9:20 UTC (permalink / raw)
To: gcc-cvs, libstdc++-cvs
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 { }
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