http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51964
Bug #: 51964
Summary: Missed tail merging opportunity
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: tree-optimization
AssignedTo: [email protected]
ReportedBy: [email protected]
pr51879-5.c:
...
int bar (int);
void baz (int);
void foo2 (void);
void
foo (int y, int z)
{
int a;
if (y == 6)
{
if (z)
foo2 ();
a = bar (7);
}
else
a = bar (7);
baz (a);
}
...
compile:
...
gcc -O2 pr51879-5.c -S -fdump-tree-all-all
...
pr51879-5.c.094t.pre:
...
# BLOCK 5 freq:4877
# PRED: 8 [100.0%] (fallthru) 4 [100.0%] (fallthru,exec)
# .MEMD.1719_6 = PHI <.MEMD.1719_8(D)(8), .MEMD.1719_9(4)>
# .MEMD.1719_10 = VDEF <.MEMD.1719_6>
# USE = nonlocal
# CLB = nonlocal
aD.1712_4 = barD.1703 (7);
goto <bb 7>;
# SUCC: 7 [100.0%] (fallthru,exec)
# BLOCK 6 freq:5123
# PRED: 2 [51.2%] (false,exec)
# .MEMD.1719_11 = VDEF <.MEMD.1719_8(D)>
# USE = nonlocal
# CLB = nonlocal
aD.1712_5 = barD.1703 (7);
# SUCC: 7 [100.0%] (fallthru,exec)
# BLOCK 7 freq:10000
# PRED: 5 [100.0%] (fallthru,exec) 6 [100.0%] (fallthru,exec)
# aD.1712_1 = PHI <aD.1712_4(5), aD.1712_5(6)>
# .MEMD.1719_7 = PHI <.MEMD.1719_10(5), .MEMD.1719_11(6)>
# .MEMD.1719_12 = VDEF <.MEMD.1719_7>
# USE = nonlocal
# CLB = nonlocal
bazD.1705 (aD.1712_1);
# VUSE <.MEMD.1719_12>
return;
# SUCC: EXIT [100.0%]
...
Blocks 5 and 6 are not merged by tail_merge_optimize (they are merged by rtl
cross-jumping though).
The reason the blocks are not merged by tail_merge_optimize is that
tail_merge_optimize uses value numbering to determine equivalence of blocks.
And since the calls have a different vuse (.MEMD.1719_6 and .MEMD.1719_8(D))
the results of the calls won't have the same value number (even after fixing
PR51879).
However, the reason we can merge the calls is not because the calls have the
same result. It's because the results are used in the same way. To detect this
we should use a different comparison mechanism than the current.
