http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56605
Bug #: 56605
Summary: Redundant branch introduced during loop2 phases
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: rtl-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: wschm...@gcc.gnu.org
CC: berg...@vnet.ibm.com
Host: powerpc64-unknown-linux-gnu
Target: powerpc64-unknown-linux-gnu
Build: powerpc64-unknown-linux-gnu
I've recently noticed that GCC commonly generates redundant branches prior to
vectorized loops, such as:
cmpwi 7,12,0
...
beq- 7,.L22
beq- 7,.L22
.p2align 4,,15
.L8:
... loop body ...
The branches first appear in the 186r.loop2_doloop debug dump:
(note 260 254 258 23 [bb 23] NOTE_INSN_BASIC_BLOCK)
(insn 258 260 259 23 (set (reg:CC 330)
(compare:CC (subreg:SI (reg:DI 153 [ bnd.10+-4 ]) 4)
(const_int 0 [0]))) -1
(nil))
(jump_insn 259 258 263 23 (set (pc)
(if_then_else (eq (reg:CC 330)
(const_int 0 [0]))
(label_ref 257)
(pc))) -1
(expr_list:REG_BR_PROB (const_int 0 [0])
(nil))
-> 257)
(note 263 259 261 24 [bb 24] NOTE_INSN_BASIC_BLOCK)
(insn 261 263 262 24 (set (reg:CC 331)
(compare:CC (subreg:SI (reg:DI 153 [ bnd.10+-4 ]) 4)
(const_int 0 [0]))) -1
(nil))
(jump_insn 262 261 257 24 (set (pc)
(if_then_else (eq (reg:CC 331)
(const_int 0 [0]))
(label_ref 257)
(pc))) -1
(expr_list:REG_BR_PROB (const_int 0 [0])
(nil))
-> 257)
Subsequently GCC removes the redundant compare, but does not remove the
redundant branch.
Simple test case to reproduce:
#define N 4000
void foo(short* __restrict sb, int* __restrict ia) {
int i;
for (i = 0; i < N; i++)
ia[i] = (int) sb[i];
}
$GCC_INSTALL/bin/gcc -S -O3 -mvsx example.c
(-mvsx is necessary to vectorize the loop. It may also be necessary to add
-mcpu=power7.)
