https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96487
Bug ID: 96487
Summary: cddce1 optimizer depends on order of basic blocks
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: sandra at gcc dot gnu.org
Target Milestone: ---
Created attachment 49007
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49007&action=edit
c and c++ dump files
If the test case gcc.dg/tree-ssa/ssa-dce-3.c is compiled as C++ code
instead of as C, it fails to reduce to an empty loop as expected.
It seems to be triggered by a slight difference in the input coming
into the cddce1 pass. The C front end canonicalizes the test to the
end of the loop so the latch (bb 5) falls through to the header (bb
6). The C++ front end orders the latch (bb 6) last with a goto to the
header (bb 3).
I did some additional tracing of the flow through the pass beyond the
dump file output. Because the latch in the C input does not end with
a control statement, it is ignored by mark_last_stmt_necessary, via
the call to mark_control_dependent_edges_necessary at the end of
find_obviously_necessary_stmts. So in the C case, nothing gets added
to the work list, while for C++ it does process the latch block,
follow the control flow out of it, and ends up marking the loop end
test etc as necessary.
I am wondering if this is a bug in the way the C output is handled and
it is incorrectly optimizing away the loop body. It seems like it
should not matter if the control transfer between blocks is done via
explicit goto or via fallthrough, anyway; either it ought to handle
the fallthrough case like the explicit goto case, or vice versa.
I originally noticed this problem in conjunction with these patches to
unify the loop handling in the C and C++ front ends:
https://gcc.gnu.org/pipermail/gcc-patches/2019-November/534142.html
But it can be reproduced with unmodified sources just by compiling
with g++ instead of gcc. The commands used to produce the attached
dump files were
x86_64-linux-gnu-gcc /path/to/gcc/testsuite/gcc.dg/tree-ssa/ssa-dce-3.c -O2
-fdump-tree-dse1 -fdump-tree-cddce1-details -S
x86_64-linux-gnu-g++ /path/to/gcc/testsuite/gcc.dg/tree-ssa/ssa-dce-3.c -O2
-fdump-tree-dse1 -fdump-tree-cddce1-details -S
