http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57723
Bug ID: 57723
Summary: Missed optimization: recursion around empty function
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: petschy at gmail dot com
Background: freeing nodes of a tree allocated with custom allocators. One of
the allocators can't free individual pointers, so free() is NOP in that case
(the whole pool will be freed at once when the allocator is destroyed). With
this allocator, the whole recursive traversal can be eliminated in theory.
Examining the disasm of the generated code revealed that gcc unfolds the
recursion many levels, just to do the unneeded node traversal; the actual call
to the empty free() fn is eliminated.
In the test case, loop() does a simple linear traversal of the linked nodes.
The pointers are not volatile, and are only read, so there should not be any
side effects. Why can't the compiler optimize away the whole loop?
Clang does a somewhat better job, the recursion is optimized away, and one
function is completely reduced to NOP (free_all2()), but the others still have
the node traversal loop.
Tried with gcc 4.6, 4.7.3, 4.9.0 with the same results.
g++-4.9.0 -v:
Using built-in specs.
COLLECT_GCC=g++-4.9.0
COLLECT_LTO_WRAPPER=/home/usr-local/bin/../libexec/gcc/x86_64-unknown-linux-gnu/4.9.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ./configure --enable-languages=c,c++ --program-suffix=-4.9.0
Thread model: posix
gcc version 4.9.0 20130626 (experimental) (GCC)
commit 944f42fc29289812416f34d7b0c497ee79065396
command line:
g++-4.9.0 -std=c++11 -O3 -Wall 20130626-free_node.cpp
Regards, Peter
