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cgraph node profile update in cgraph_rebuild_references causes a performance issue


We have a program like this:

A() {    // hot func
  ...
}

B() {
  A();    // very hot
  if (i) {
    A();  // very cold
  }
}

Both callsites of A will be inlined into B. In gcc func
save_inline_function_body in inline_transform stage, A's first clone
will be choosen and turned into a real func. For our case, the clone
node choosen corresponds to the cold callsite of A.
cgraph_rebuild_references in tree_function_versioning will reset the
cgraph node count of the choosen clone to the entry bb count of func A
(A is hot). So the cgraph node count of the choosen clone becomes hot
while its inline edge count is still cold. It breaks the assumption
described here:
https://gcc.gnu.org/ml/gcc-patches/2014-05/msg01366.html:
for inline node, bb->count == edge->count == edge->callee->count

For the patch committed in the thread above (it is listed below),
cg_edge->callee->count is used for profile update to its inline
instance, which leads to a hot BB in func B which is actually very
cold. The wrong profile information causes performance regression in
one of our internal benchmarks.

Index: gcc/tree-inline.c
===================================================================
--- gcc/tree-inline.c (revision 210535)
+++ gcc/tree-inline.c (working copy)
@@ -4355,7 +4355,7 @@ expand_call_inline (basic_block bb, gimple stmt, c
      function in any way before this point, as this CALL_EXPR may be
      a self-referential call; if we're calling ourselves, we need to
      duplicate our body before altering anything.  */
-  copy_body (id, bb->count,
+  copy_body (id, cg_edge->callee->count,
        GCOV_COMPUTE_SCALE (cg_edge->frequency, CGRAPH_FREQ_BASE),
      bb, return_block, NULL);

I don't understand why we need to reset the cgraph node count of the
first clone to entry bb count in cgraph_rebuild_references, could you
shed some light on it? The above patch may just uncover the problem?

Thanks,
Wei.


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