http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58377
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Known to work| |4.9.0
Version|unknown |4.8.1
Last reconfirmed| |2013-09-10
Component|middle-end |c++
CC| |xinliangli at gmail dot com
Ever confirmed|0 |1
Summary|spurious "may be used |spurious "may be unused"
|uninitialized" warning with |warning with -Og
|-Og |
--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed with the C++ FE, works with the C FE. Does not warn on trunk (for
no good reason I think, the reason seems to be presence of loop structure
and thus some extra BBs).
Difference:
trunk:
[WORKLIST]: add to initial list: out_2 = PHI <out_8(D)(2), out_1(8)>
[CHECK]: examining phi: out_2 = PHI <out_8(D)(2), out_1(8)>
Use in stmt out_1 = PHI <out_12(4), out_12(5), out_2(3)>
is guarded by :
if (pop_first_bucket.2_10 != 0)
[CHECK] Found def edge 0 in out_1 = PHI <out_12(4), out_12(5), out_2(3)>
[CHECK] Found def edge 1 in out_1 = PHI <out_12(4), out_12(5), out_2(3)>
Operand defs of phi out_2 = PHI <out_8(D)(2), out_1(8)>
is guarded by :
if (out_12 != 0)
[CHECK]: Found unguarded use: out_1 = PHI <out_12(4), out_12(5), out_2(3)>
[WORKLIST]: Update worklist with phi: out_1 = PHI <out_12(4), out_12(5),
out_2(3)>
[CHECK]: examining phi: out_1 = PHI <out_12(4), out_12(5), out_2(3)>
Use in stmt out_2 = PHI <out_8(D)(2), out_1(8)>
is guarded by :
(.NOT.) if (iftmp.1_3 != 0)
[CHECK] Found def edge 0 in out_1 = PHI <out_12(4), out_12(5), out_2(3)>
[CHECK] Found def edge 1 in out_1 = PHI <out_12(4), out_12(5), out_2(3)>
Operand defs of phi out_1 = PHI <out_12(4), out_12(5), out_2(3)>
is guarded by :
if (pop_first_bucket.2_10 != 0)
(.AND.)
if (out_12 != 0)
(.OR.)
if (pop_first_bucket.2_10 != 0)
(.AND.)
(.NOT.) if (out_12 != 0)
Normalized to
Operand defs of phi out_1 = PHI <out_12(4), out_12(5), out_2(3)>
is guarded by :
if (pop_first_bucket.2_10 != 0)
...
vs. 4.8 branch:
[WORKLIST]: add to initial list: out_2 = PHI <out_8(D)(2), out_1(6)>
[CHECK]: examining phi: out_2 = PHI <out_8(D)(2), out_1(6)>
Use in stmt out_1 = PHI <out_12(4), out_12(5), out_2(3)>
is guarded by :
if (pop_first_bucket.2_10 != 0)
[CHECK] Found def edge 0 in out_1 = PHI <out_12(4), out_12(5), out_2(3)>
[CHECK] Found def edge 1 in out_1 = PHI <out_12(4), out_12(5), out_2(3)>
Operand defs of phi out_2 = PHI <out_8(D)(2), out_1(6)>
is guarded by :
if (out_12 != 0)
[CHECK]: Found unguarded use: out_1 = PHI <out_12(4), out_12(5), out_2(3)>
[WORKLIST]: Update worklist with phi: out_1 = PHI <out_12(4), out_12(5),
out_2(3)>
[CHECK]: examining phi: out_1 = PHI <out_12(4), out_12(5), out_2(3)>
[CHECK]: Found unguarded use: out_2 = PHI <out_8(D)(2), out_1(6)>
[CHECK]: Found unguarded use: _4 = PHI <out_1(6), 0(7)>
[WORKLIST]: Update worklist with phi: _4 = PHI <out_1(6), 0(7)>
[CHECK]: examining phi: _4 = PHI <out_1(6), 0(7)>
[CHECK]: Found unguarded use: return _4;
The IL difference is that we have
<bb 6>:
# out_1 = PHI <out_12(4), out_12(5), out_2(3)>
# iftmp.1_3 = PHI <1(4), 0(5), 0(3)>
if (iftmp.1_3 != 0)
goto <bb 7>;
else
goto <bb 8>;
<bb 7>:
out_13 = out_1;
goto <bb 10>;
...
<bb 10>:
# _4 = PHI <out_13(7), 0(9)>
return _4;
which doesn't warn vs.
<bb 6>:
# out_1 = PHI <out_12(4), out_12(5), out_2(3)>
# iftmp.1_3 = PHI <1(4), 0(5), 0(3)>
if (iftmp.1_3 != 0)
goto <bb 8>;
else
goto <bb 7>;
...
<bb 8>:
# _4 = PHI <out_1(6), 0(7)>
return _4;
which does. The issue seems to be that the analysis doesn't consider
the PHI uses in
if (iftmp.1_3 != 0)
goto <bb 8>;
else
goto <bb 7>;
<bb 7>:
# out_2 = PHI <out_8(D)(2), out_1(6)>
guarded by anything (the out_1 use is guarded by iftmp.1_3 == 0).
David - the code does
if (gimple_code (use_stmt) == GIMPLE_PHI)
use_bb = gimple_phi_arg_edge (use_stmt,
PHI_ARG_INDEX_FROM_USE (use_p))->src;
else
use_bb = gimple_bb (use_stmt);
if (is_use_properly_guarded (use_stmt,
use_bb,
...
so it chooses the source block (as approximation?).
Splitting all edges results in us no longer warning here and:
Use in stmt out_2 = PHI <out_8(D)(15), out_1(16)>
is guarded by :
(.NOT.) if (iftmp.1_3 != 0)
Can you see to fix that please? Thanks.
