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authorBjörn Gustavsson <[email protected]>2011-09-29 08:13:24 +0200
committerBjörn Gustavsson <[email protected]>2011-11-07 14:00:05 +0100
commitaf5edfe20b24116fa892a37a746f4053fde6039b (patch)
tree1c859cd2b793bf61898ae00aca72b13c094588ef /erts/doc
parent4097f85c41d81b2a535cf95ba56aa807c1256beb (diff)
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beam_asm: Strenghten the calculation of Uniq for funs
Funs are identified by a triple, <Module,Uniq,Index>, where Module is the module name, Uniq is a 27 bit hash value of some intermediate representation of the code for the fun, and index is a small integer. When a fun is loaded, the triple for the fun will be compared to previously loaded funs. If all elements in the triple in the newly loaded fun are the same, the newly loaded fun will replace the previous fun. The idea is that if Uniq are the same, the code for the fun is also the same. The problem is that Uniq is only based on the intermediate representation of the fun itself. If the fun calls local functions in the same module, Uniq may remain the same even if the behavior of the fun has been changed. See http://erlang.org/pipermail/erlang-bugs/2007-June/000368.htlm for an example. As a long-term plan to fix this problem, the NewIndex and NewUniq fields was added to each fun in the R8 release (where NewUniq is the MD5 of the BEAM code for the module). Unfortunately, it turns out that the compiler does not assign unique value to NewIndex (if it isn't tested, it doesn't work), so we cannot use the <Module,NewUniq,NewIndex> triple as identification. It would be possible to use <Module,NewUniq,Index>, but that seems ugly. Therefore, fix the problem by making Uniq more unique by taking 27 bits from the MD5 for the BEAM code. That only requires a change to the compiler. Also update a test case for cover, which now fails because of the stronger Uniq calculation. (The comment in test case about why the Pid2 process survived is not correct.)
Diffstat (limited to 'erts/doc')
-rw-r--r--erts/doc/src/erlang.xml9
1 files changed, 7 insertions, 2 deletions
diff --git a/erts/doc/src/erlang.xml b/erts/doc/src/erlang.xml
index 45b66a4909..5d7ff7613b 100644
--- a/erts/doc/src/erlang.xml
+++ b/erts/doc/src/erlang.xml
@@ -1196,11 +1196,16 @@ true
</item>
<tag><c>{new_uniq, Uniq}</c></tag>
<item>
- <p><c>Uniq</c> (a binary) is a unique value for this fun.</p>
+ <p><c>Uniq</c> (a binary) is a unique value for this fun.
+ It is calculated from the compiled code for the entire module.</p>
</item>
<tag><c>{uniq, Uniq}</c></tag>
<item>
- <p><c>Uniq</c> (an integer) is a unique value for this fun.</p>
+ <p><c>Uniq</c> (an integer) is a unique value for this fun.
+ Starting in the R15 release, this integer is calculated from
+ the compiled code for the entire module. Before R15, this
+ integer was based on only the body of the fun.
+ </p>
</item>
</taglist>
</desc>