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Reintroduce trivially copyable maybe_t impl

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This reverts commit 1c92d4c5db and
reintroduces support for trivially copyable `maybe_t` impls but with a
GCC version check to disable the optimization for GNU GCC compiler
versions 9 and below.

GCC 8.3.0 armhf builds seem to have a problem with the trivially
copyable `maybe_t` impl that introduces odd heisenbugs that cause the
tests to fail. GDB reveals that `maybe_t` function parameters received
in the callee differ from what was passed-in by the caller.

This behavior appears to be (but has not been confirmed as) a
platform-specific compiler bug. Under the same system (32-bit Debian 10
armhf), compiling with clang 7.0.1 does not result in any bugs and
causes all the tests to pass while compiling with GCC 10.2 under 32-bit
Debian 11 armhf also doesn't run into any problems, so just expand the
existing GCC version check that gates support for trivially copyable
`maybe_t` impls to encompass both the troublesome GCC 8 version and the
untested GCC 9 version.
This commit is contained in:
Mahmoud Al-Qudsi 2022-10-29 11:26:34 -05:00
parent 1c92d4c5db
commit 4f46abec9d
1 changed files with 41 additions and 16 deletions
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57
src/maybe.h
View file

@ -7,10 +7,11 @@
#include <utility>
namespace maybe_detail {
// Template magic to make maybe_t<T> copyable iff T is copyable.
// maybe_impl_t is the "too aggressive" implementation: it is always copyable.
// Template magic to make maybe_t<T> (trivially) copyable iff T is.
// This is an unsafe implementation: it is always trivially copyable.
template <typename T>
struct maybe_impl_t {
struct maybe_impl_trivially_copyable_t {
alignas(T) char storage[sizeof(T)];
bool filled = false;
@ -38,11 +39,13 @@ struct maybe_impl_t {
return res;
}
maybe_impl_t() = default;
maybe_impl_trivially_copyable_t() = default;
// Move construction/assignment from a T.
explicit maybe_impl_t(T &&v) : filled(true) { new (storage) T(std::forward<T>(v)); }
maybe_impl_t &operator=(T &&v) {
explicit maybe_impl_trivially_copyable_t(T &&v) : filled(true) {
new (storage) T(std::forward<T>(v));
}
maybe_impl_trivially_copyable_t &operator=(T &&v) {
if (filled) {
value() = std::move(v);
} else {
@ -53,8 +56,8 @@ struct maybe_impl_t {
}
// Copy construction/assignment from a T.
explicit maybe_impl_t(const T &v) : filled(true) { new (storage) T(v); }
maybe_impl_t &operator=(const T &v) {
explicit maybe_impl_trivially_copyable_t(const T &v) : filled(true) { new (storage) T(v); }
maybe_impl_trivially_copyable_t &operator=(const T &v) {
if (filled) {
value() = v;
} else {
@ -63,15 +66,27 @@ struct maybe_impl_t {
}
return *this;
}
};
// Move construction/assignment from a maybe_impl.
maybe_impl_t(maybe_impl_t &&v) : filled(v.filled) {
// This is an unsafe implementation: it is always copyable.
template <typename T>
struct maybe_impl_not_trivially_copyable_t : public maybe_impl_trivially_copyable_t<T> {
using base_t = maybe_impl_trivially_copyable_t<T>;
using base_t::maybe_impl_trivially_copyable_t;
using base_t::operator=;
using base_t::filled;
using base_t::reset;
using base_t::storage;
// Move construction/assignment from another instance.
maybe_impl_not_trivially_copyable_t(maybe_impl_not_trivially_copyable_t &&v) {
filled = v.filled;
if (filled) {
new (storage) T(std::move(v.value()));
}
}
maybe_impl_t &operator=(maybe_impl_t &&v) {
maybe_impl_not_trivially_copyable_t &operator=(maybe_impl_not_trivially_copyable_t &&v) {
if (!v.filled) {
reset();
} else {
@ -80,14 +95,15 @@ struct maybe_impl_t {
return *this;
}
// Copy construction/assignment from a maybe_impl.
maybe_impl_t(const maybe_impl_t &v) : filled(v.filled) {
// Copy construction/assignment from another instance.
maybe_impl_not_trivially_copyable_t(const maybe_impl_not_trivially_copyable_t &v) : base_t() {
filled = v.filled;
if (v.filled) {
new (storage) T(v.value());
}
}
maybe_impl_t &operator=(const maybe_impl_t &v) {
maybe_impl_not_trivially_copyable_t &operator=(const maybe_impl_not_trivially_copyable_t &v) {
if (&v == this) return *this;
if (!v.filled) {
reset();
@ -97,7 +113,8 @@ struct maybe_impl_t {
return *this;
}
~maybe_impl_t() { reset(); }
maybe_impl_not_trivially_copyable_t() = default;
~maybe_impl_not_trivially_copyable_t() { reset(); }
};
struct copyable_t {};
@ -130,7 +147,15 @@ inline constexpr none_t none() { return none_t::none; }
// This is a value-type class that stores a value of T in aligned storage.
template <typename T>
class maybe_t : private maybe_detail::conditionally_copyable_t<T> {
maybe_detail::maybe_impl_t<T> impl_;
#if __GNUG__ && __GNUC__ < 5
using maybe_impl_t = maybe_detail::maybe_impl_not_trivially_copyable_t<T>;
#else
using maybe_impl_t =
typename std::conditional<std::is_trivially_copyable<T>::value,
maybe_detail::maybe_impl_trivially_copyable_t<T>,
maybe_detail::maybe_impl_not_trivially_copyable_t<T> >::type;
#endif
maybe_impl_t impl_;
public:
// return whether the receiver contains a value.