1276140 - Avoid undefined behavior in frame properties table

Status: RESOLVED FIXED

(Core :: Layout, defect)

Description

[Xidorn Quan [:xidorn] UTC+11]

The current method that using union to reinterpret value is actually an undefined behavior per C++ spec, since it requires to access inactive union member.

Although many compilers implement a language extension to allow that, we may still want to avoid undefined behavior as much as possible.

Using memcpy is the safest way to do reinterpreting between unknown types, and most compilers are able to optimize it properly when possible.

Comment 1

[Xidorn Quan [:xidorn] UTC+11]

Attachment: MozReview Request: Bug 1276140 - Use memcpy rather than union to reinterpret in frame properties table. r?froydnj

Review commit: https://reviewboard.mozilla.org/r/55682/diff/#index_header
See other reviews: https://reviewboard.mozilla.org/r/55682/

Comment 2

[Nathan Froyd [:froydnj]]

Comment on attachment 8757140 [details]
MozReview Request: Bug 1276140 - Use memcpy rather than union to reinterpret in frame properties table. r?froydnj

https://reviewboard.mozilla.org/r/55682/#review52462

r=me with the changes below.

::: layout/base/FramePropertyTable.h:298
(Diff revision 1)
> +  template<typename T>
> +  struct ReinterpretHelper<T*>
> +  {
> +    static void* ToPointer(T* aValue)
> +    {
> +      return reinterpret_cast<void*>(aValue);

Casts to and from void pointers can be static_cast, here and below.

Comment 3

[Xidorn Quan [:xidorn] UTC+11]

https://hg.mozilla.org/integration/mozilla-inbound/rev/bc8ea177c0d8303861f04c6009582acf40c692bb
Bug 1276140 - Use memcpy rather than union to reinterpret in frame properties table. r=froydnj

Comment 4

[Carsten Book [:Tomcat]]

https://hg.mozilla.org/mozilla-central/rev/bc8ea177c0d8