http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49498
bin.cheng <amker.cheng at gmail dot com> changed:
What |Removed |Added
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CC| |amker.cheng at gmail dot
| |com
--- Comment #17 from bin.cheng <amker.cheng at gmail dot com> 2012-11-20
07:08:18 UTC ---
Hi,
I spent some time analyzing this bug and I think I understand the problem now.
For below dump file from trunk/cris-elf when compiling the attached k.c:
;; Function foo (foo, funcdef_no=0, decl_uid=1323, cgraph_uid=0)
;; 1 loops found
;;
;; Loop 0
;; header 0, latch 1
;; depth 0, outer -1
;; nodes: 0 1 2 3 4 5 6 7 8 9 10 11
;; 2 succs { 10 3 }
;; 3 succs { 11 4 }
;; 4 succs { 11 }
;; 5 succs { 6 7 }
;; 6 succs { 9 }
;; 7 succs { 6 8 }
;; 8 succs { 9 }
;; 9 succs { 1 }
;; 10 succs { 5 6 }
;; 11 succs { 5 8 }
foo (int n, int l, int m, int r)
{
int v;
int g.1;
int g.0;
<bb 2>:
if (n_4(D) <= 9)
goto <bb 10>;
else
goto <bb 3>;
<bb 3>:
if (m_5(D) > 100)
goto <bb 11>;
else
goto <bb 4>;
<bb 4>:
goto <bb 11>;
<bb 5>:
# v_14 = PHI <v_13(11), r_7(D)(10)>
g.0_9 = g;
g.1_10 = g.0_9 + 1;
g = g.1_10;
if (n_4(D) <= 9)
goto <bb 6>;
else
goto <bb 7>;
<bb 6>:
# v_17 = PHI <v_14(5), v_14(7), r_7(D)(10)>
blah (v_17);
goto <bb 9>;
<bb 7>:
if (m_5(D) > 100)
goto <bb 6>;
else
goto <bb 8>;
<bb 8>:
<bb 9>:
return 0;
<bb 10>:
if (m_5(D) != 0)
goto <bb 5>;
else
goto <bb 6>;
<bb 11>:
# v_13 = PHI <r_7(D)(3), v_6(D)(4)>
if (m_5(D) != 0)
goto <bb 5>;
else
goto <bb 8>;
}
There are two flaws in tree-ssa-uninit.c revealing this bug.
1. GCC try to find def_chains from cd_root(which is the closest dominating bb
for phi_bb) to phi_bb, but only find use_predicates from phi_bb to use_bb. In
general case with canonical CFG, this is fine, but in non-canonical CFG, it's
possible to have ancestor basic block of phi_bb in def_chains which have branch
that never reach to phi_bb, like basic block 10 reported in this PR. In this
scenario the corresponding condition should not be counted in
def_chains(edge<10, 5> in this case).
There are two methods to fix this:
a) find use predicates from dom(phi_bb), rather than phi_bb in non-canonical
CFGs.
b) prune branch conditions that are irrelevant to this use/def in
def_chains.
Method a is simpler, but the problem is it results in more dep_chains which
might exceeds the limit MAX_NUM_CHAINS. As for method b), I haven't got any
clue to implement it.
2. When calling is_use_properly_guarded in find_uninit_use, GCC finds
predicates from source basic block if the use_stmt is a phi node. This results
in missing condition at the end of each def_chain. Different from the first
issue, this can be easily fixed.
