[PATCH] D120086: Explicitly document that `__is_trivially_relocatable(T)` implies that `T` is implicit-lifetime.

[Devin Jeanpierre via Phabricator via cfe-commits]

devin.jeanpierre updated this revision to Diff 409794.
devin.jeanpierre added a comment.

Rebase off of D119385 . Sorry, I'm bad at git.


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Files:
  clang/docs/LanguageExtensions.rst


Index: clang/docs/LanguageExtensions.rst
===
--- clang/docs/LanguageExtensions.rst
+++ clang/docs/LanguageExtensions.rst
@@ -1372,8 +1372,9 @@
 * ``__is_trivially_relocatable`` (Clang): Returns true if moving an object
   of the given type, and then destroying the source object, is known to be
   functionally equivalent to copying the underlying bytes and then dropping the
-  source object on the floor. This is true of trivial types and types which
-  were made trivially relocatable via the ``clang::trivial_abi`` attribute.
+  source object on the floor. (Note: any such type is, therefore, 
implicit-lifetime.)
+  This is true of trivial types and types which were made trivially relocatable
+  via the ``clang::trivial_abi`` attribute.
 * ``__is_union`` (C++, GNU, Microsoft, Embarcadero)
 * ``__is_unsigned`` (C++, Embarcadero):
   Returns false for enumeration types. Note, before Clang 13, returned true for


Index: clang/docs/LanguageExtensions.rst
===
--- clang/docs/LanguageExtensions.rst
+++ clang/docs/LanguageExtensions.rst
@@ -1372,8 +1372,9 @@
 * ``__is_trivially_relocatable`` (Clang): Returns true if moving an object
   of the given type, and then destroying the source object, is known to be
   functionally equivalent to copying the underlying bytes and then dropping the
-  source object on the floor. This is true of trivial types and types which
-  were made trivially relocatable via the ``clang::trivial_abi`` attribute.
+  source object on the floor. (Note: any such type is, therefore, implicit-lifetime.)
+  This is true of trivial types and types which were made trivially relocatable
+  via the ``clang::trivial_abi`` attribute.
 * ``__is_union`` (C++, GNU, Microsoft, Embarcadero)
 * ``__is_unsigned`` (C++, Embarcadero):
   Returns false for enumeration types. Note, before Clang 13, returned true for
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[PATCH] D120086: Explicitly document that `__is_trivially_relocatable(T)` implies that `T` is implicit-lifetime.

[Arthur O'Dwyer via Phabricator via cfe-commits]

Quuxplusone added a comment.

FWIW, I think this is a good idea. I even think that the parentheses and 
"Note:" are too self-deprecating, and this deserves to be a bigger deal. 
Perhaps `__is_implicit_lifetime(T)` should be an intrinsic in its own right! 
Then you could say very concretely that `__is_trivially_relocatable(T)` implies 
`__is_implicit_lifetime(T)`.


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[PATCH] D120086: Explicitly document that `__is_trivially_relocatable(T)` implies that `T` is implicit-lifetime.

[Devin Jeanpierre via Phabricator via cfe-commits]

devin.jeanpierre created this revision.
devin.jeanpierre added reviewers: rsmith, rjmccall.
devin.jeanpierre requested review of this revision.
Herald added a project: clang.
Herald added a subscriber: cfe-commits.

Previous change: D114732 

The wording here might need some tweaking. What I want to guarantee is that 
everything valid for an implicit-lifetime type is also valid for these types. 
Either they're added to the set of implicit-lifetime types as an extension, or 
the operations are valid for additional non-implicit-lifetime types -- 
something like that.

Something to this effect is required for it to be defined behavior to `memcpy` 
to a new location, and then use the type at that new location. And so, this 
guarantee is necessary for `__is_trivially_relocatable` to be actually useful 
in practice. See e.g. D119385  for how this 
plays out.

AIUI, Clang is already fine with this and needs no code changes. Indeed, we see 
many types which are not what the standard would call implicit-lifetime, yet 
which use memcpy for copying: `std::vector` only checks for trivial move, and 
does not pair this with requiring trivial destroy. (See e.g. 
`__construct_forward_with_exception_guarantees`)

(Specifically: I believe that Clang is already fine with treating any 
non-polymorphic type as if it were implicit-lifetime, but I am not sure how to 
check this belief. There's not much in the way of explicit references to 
"implicit-lifetime" types in the codebase. Had a brief out of band conversation 
with rsmith, but may of course have misunderstood the conclusions here.)


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D120086

Files:
  clang/docs/LanguageExtensions.rst


Index: clang/docs/LanguageExtensions.rst
===
--- clang/docs/LanguageExtensions.rst
+++ clang/docs/LanguageExtensions.rst
@@ -1372,8 +1372,9 @@
 * ``__is_trivially_relocatable`` (Clang): Returns true if moving an object
   of the given type, and then destroying the source object, is known to be
   functionally equivalent to copying the underlying bytes and then dropping the
-  source object on the floor. This is true of trivial types and types which
-  were made trivially relocatable via the ``clang::trivial_abi`` attribute.
+  source object on the floor. (Note: any such type is, therefore, 
implicit-lifetime.)
+  This is true of trivial types and types which were made trivially relocatable
+  via the ``clang::trivial_abi`` attribute.
 * ``__is_union`` (C++, GNU, Microsoft, Embarcadero)
 * ``__is_unsigned`` (C++, Embarcadero):
   Returns false for enumeration types. Note, before Clang 13, returned true for


Index: clang/docs/LanguageExtensions.rst
===
--- clang/docs/LanguageExtensions.rst
+++ clang/docs/LanguageExtensions.rst
@@ -1372,8 +1372,9 @@
 * ``__is_trivially_relocatable`` (Clang): Returns true if moving an object
   of the given type, and then destroying the source object, is known to be
   functionally equivalent to copying the underlying bytes and then dropping the
-  source object on the floor. This is true of trivial types and types which
-  were made trivially relocatable via the ``clang::trivial_abi`` attribute.
+  source object on the floor. (Note: any such type is, therefore, implicit-lifetime.)
+  This is true of trivial types and types which were made trivially relocatable
+  via the ``clang::trivial_abi`` attribute.
 * ``__is_union`` (C++, GNU, Microsoft, Embarcadero)
 * ``__is_unsigned`` (C++, Embarcadero):
   Returns false for enumeration types. Note, before Clang 13, returned true for
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