http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49095
Richard Guenther <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target| |x86_64-*-*, i?86-*-*
Status|UNCONFIRMED |NEW
Keywords| |missed-optimization
Last reconfirmed| |2011.05.21 09:52:49
Component|other |rtl-optimization
Ever Confirmed|0 |1
--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-05-21
09:52:49 UTC ---
Confirmed (not using decq is because it is slower for some archs). On the
tree level we cannot do better than
D.2722_2 = *argv_1(D);
D.2723_3 = D.2722_2 + -1;
*argv_1(D) = D.2723_3;
if (D.2723_3 == 0B)
because we lack anything like direct operations on memory (and that's good).
On the RTL side combine tries to do
Trying 7, 8 -> 9:
Failed to match this instruction:
(parallel [
(set (mem/f:DI (reg/v/f:DI 63 [ argv ]) [2 *argv_1(D)+0 S8 A64])
(plus:DI (mem/f:DI (reg/v/f:DI 63 [ argv ]) [2 *argv_1(D)+0 S8
A64])
(const_int -1 [0xffffffffffffffff])))
(set (reg/f:DI 60 [ D.2723 ])
(plus:DI (mem/f:DI (reg/v/f:DI 63 [ argv ]) [2 *argv_1(D)+0 S8
A64])
(const_int -1 [0xffffffffffffffff])))
])
because we have a use of the decrement result in the comparison. It doesn't
try to combine this with the comparison though.
So this case is really special ;) Without the use of the decremented
value we get the desired subq $1, (%rsi).
Manually sinking the store to *argv into the if and the else yields
movq (%rsi), %rbx
subq $1, %rbx
je L4
L2:
movq %rbx, (%rsi)
...
L4:
LCFI4:
movq $0, (%rsi)
movq %rsi, %rdi
movq %rsi, 8(%rsp)
call _fncall
so at least we then can combine the decrement with the test ...
As usual combine doesn't like stores.
