http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55424
Bug #: 55424
Summary: [4.8 Regression]gcc.dg/uninit-pred-8_b.c bogus warning
line 23 on ARM/Cortex-M0/-Os
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: tree-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: amker.ch...@gmail.com
The test case require optimization level "-O2" and it passes on ARM/cortex-m0
with "-O2", but the failure with "-Os" does reveal potential bug in
tree-ssa-uninit.c
Test command line:
arm-none-eabi-gcc ./uninit-pred-8_b.c -fno-diagnostics-show-caret
-Wuninitialized -fno-tree-dominator-opts -S -mthumb -mcpu=cortex-m0 -Os -o
uninit-pred-8_b.s
The warning info:
.../trunk-orig/gcc/gcc/testsuite/gcc.dg/uninit-pred-8_b.c: In function 'foo':
.../trunk-orig/gcc/gcc/testsuite/gcc.dg/uninit-pred-8_b.c:23:11: warning: 'v'
may be used uninitialized in this function [-Wmaybe-uninitialized]
.../trunk-orig/gcc/gcc/testsuite/gcc.dg/uninit-pred-8_b.c: In function 'foo_2':
.../trunk-orig/gcc/gcc/testsuite/gcc.dg/uninit-pred-8_b.c:42:11: warning: 'v'
may be used uninitialized in this function [-Wmaybe-uninitialized]
This failure occurs after checking in r193687. The patch prefers to generate
branches on ARM/cortex-m0.
After investigating tree dump of tree-ssa-uninit.c, I think:
tree-ssa-uninit.c computes control dependent chain for uses/def of variable and
checks whether each use is guarded by def. It has a upper bound on the number
of control dependent chains(MAX_NUM_CHAINS==8) and just retreat to false
warning if the number of chains exceeds MAX_NUM_CHAINS. In our scenario, the
number of chains exceeds MAX_NUM_CHAINS because we prefer short circuit now,
resulting in false warning information. These false warning cannot be fully
removed if the MAX_NUM_CHAINS exists, but we can improve it in following way:
There are lots of invalid control dependent chains computed in
tree-ssa-uninit.c now and should be pruned. I have already implemented a quick
fix and it works for our scenario.
I am not sure it should be fixed in this way, so please comments if you have
any opinions.
Thanks
Dump of tree-ssa-uninit.c:
;; Function foo (foo, funcdef_no=0, decl_uid=4065, cgraph_uid=0)
Use in stmt v_24 = PHI <v_25(8), v_1(9)>
is guarded by :
(.NOT.) if (m_6(D) != 0)
Operand defs of phi v_1 = PHI <v_9(D)(6), r_7(D)(4), r_7(D)(2), r_7(D)(5)>
is guarded by :
(.NOT.) if (n_5(D) <= 9)
(.AND.)
(.NOT.) if (m_6(D) > 100)
(.AND.)
if (r_7(D) <= 19)
(.OR.)
if (n_5(D) <= 9)
(.OR.)
(.NOT.) if (n_5(D) <= 9)
(.AND.)
(.NOT.) if (m_6(D) > 100)
(.AND.)
(.NOT.) if (r_7(D) <= 19)
(.AND.)
if (l_8(D) != 0)
foo (int n, int l, int m, int r)
{
int v;
int g.1;
int g.0;
<bb 2>:
if (n_5(D) <= 9)
goto <bb 7>;
else
goto <bb 3>;
<bb 3>:
if (m_6(D) > 100)
goto <bb 18>;
else
goto <bb 4>;
<bb 4>:
if (r_7(D) <= 19)
goto <bb 7>;
else
goto <bb 5>;
<bb 5>:
if (l_8(D) != 0)
goto <bb 7>;
else
goto <bb 6>;
<bb 6>:
<bb 7>:
# v_1 = PHI <v_9(D)(6), r_7(D)(4), r_7(D)(2), r_7(D)(5)>
if (m_6(D) != 0)
goto <bb 8>;
else
goto <bb 9>;
<bb 8>:
# v_25 = PHI <v_1(7), v_22(18)>
g.0_11 = g;
g.1_12 = g.0_11 + 1;
g = g.1_12;
goto <bb 10>;
<bb 9>:
bar ();
<bb 10>:
# v_24 = PHI <v_25(8), v_1(9)>
if (n_5(D) <= 9)
goto <bb 14>;
else
goto <bb 11>;
<bb 11>:
if (m_6(D) > 100)
goto <bb 14>;
else
goto <bb 12>;
<bb 12>:
if (r_7(D) <= 19)
goto <bb 14>;
else
goto <bb 16>;
<bb 13>:
if (m_6(D) > 100)
goto <bb 15>;
else
goto <bb 16>;
<bb 14>:
blah (v_24);
if (n_5(D) <= 9)
goto <bb 15>;
else
goto <bb 13>;
<bb 15>:
blah (v_24);
goto <bb 17>;
<bb 16>:
if (r_7(D) <= 9)
goto <bb 15>;
else
goto <bb 17>;
<bb 17>:
return 0;
<bb 18>:
# v_22 = PHI <r_7(D)(3)>
goto <bb 8>;
}
