Add 'cold' attribute to functions

[Diego Novillo]

On 2013-05-15 12:06 , Chandler Carruth wrote:
>
>
>
>     Should I add the following changes as well?  There don't seem to
>     be tests for these files (nothing I could find in a quick search,
>     anyway).
>
>
> I would add them, but maybe in a separate commit.

Will do.

>     Main patch is attached.  OK to commit?
>
>
> LGTM. Check w/ Duncan though since he looked at one version of it.

Sure.  Duncan, please let me know if that extra test case I added 
addresses your concerns.  Note that I could only address the analysis 
part.  I believe codegen is not using this information properly yet (I 
left a TODO in the test case).

>
> I'm not sure what the best way to phrase all of this is though...

How about something like this:

diff --git a/docs/LangRef.rst b/docs/LangRef.rst
index 1c0864d..77ef016 100644
--- a/docs/LangRef.rst
+++ b/docs/LangRef.rst
@@ -808,10 +808,15 @@ example:
      this function into callers whenever possible, ignoring any active
      inlining size threshold for this caller.
  ``cold``
-    This attribute indicates that this function is not called
-    frequently.  When computing edge weights, basic blocks
-    post-dominated by a cold function are also considered to be cold;
-    and, thus, given low weight.
+    This attribute indicates that this function is unlikely to be
+    executed. It indicates that performance of the code in paths
+    coming out and leading into that function call should not be a
+    concern.  Paths leading to the cold call are marked as unlikely by
+    the branch prediction analysis.  Example of functions that are
+    useful to mark cold are functions handling unlikely conditions,
+    such as fatal errors.  This will cause the optimizer to place more
+    emphasis on optimizing the hot paths in the code (e.g., code
+    layout will move cold code sections to the bottom).
  ``nonlazybind``
      This attribute suppresses lazy symbol binding for the function. This
      may make calls to the function faster, at the cost of extra program


Diego.
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