384104 - strict aliasing warnings in ALE macros

Status: RESOLVED FIXED

(Core :: JavaScript Engine, enhancement)

Description

[Igor Bukanov]

Attachment: implementation v1

When JS engine is compiled at -O2 with GCC, the compiler generates warnings like:

jsemit.c:1628: warning: dereferencing type-punned pointer will break strict-aliasing rules

They all happens inside ATOM_LIST_LOOKUP when it casts &_ale->entry.next to JSAtomListElement **:

#define ATOM_LIST_LOOKUP(_ale,_hep,_al,_atom)                                
...
    JSAtomListElement **_alep = &(_al)->list;
...
        _alep = (JSAtomListElement **)&_ale->entry.next;

Such assignments, although widely used and de-facto safe, indeed breaks the strict aliasing rules of the C standard so I suggest to fix this and suppress GCC warnings that currently pollute the compilation output at the minimal optimization level when GCC really detects potentially not initialized variables quite reliably.

The attached patch does that via declaring JSAtomList.list as JSHashEntry* and casting it to  (JSAtomListElement *) as necessary. The patch also replaces ALE_SET_VALUE and ALE_SET_NEXT via direct cast-free expansion.

Comment 1

[Igor Bukanov]

I committed the patch from comment 0 to the trunk:

Checking in js/src/jsatom.c;
/cvsroot/mozilla/js/src/jsatom.c,v  <--  jsatom.c
new revision: 3.95; previous revision: 3.94
done
Checking in js/src/jsatom.h;
/cvsroot/mozilla/js/src/jsatom.h,v  <--  jsatom.h
new revision: 3.60; previous revision: 3.59
done
Checking in js/src/jsemit.c;
/cvsroot/mozilla/js/src/jsemit.c,v  <--  jsemit.c
new revision: 3.256; previous revision: 3.255
done