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.
