Consider the following functions:
// g++ -mtune=core2 -O3 -S -dp
void loop(int* dest, int* src, int count) {
for(int i=0; i < count; i++)
dest[i] = src[i];
}
void loop_few(int* dest, int* src) { loop(dest, src, 8); }
void loop_many(int* dest, int* src) { loop(dest, src, 64); }
loop() unrolls 8x, as expected. loop_few() peels completely, as expected.
However, loop_many() neither peels nor unrolls.
_Z9loop_manyPiS_:
xorl %edx, %edx # 34 *movdi_xor_rex64 [length = 2]
.L47:
movl (%rsi,%rdx,4), %eax # 11 *movsi_1/1 [length = 3]
movl %eax, (%rdi,%rdx,4) # 12 *movsi_1/2 [length = 3]
incq %rdx # 13 *adddi_1_rex64/1 [length = 3]
cmpq $64, %rdx # 15 cmpdi_1_insn_rex64/1 [length = 4]
jne .L47 # 16 *jcc_1 [length = 2]
rep ; ret # 35 return_internal_long [length = 1]
Ideally the optimizer would unroll 8x, then notice that (count%8==0) and
eliminate the partial unroll code. However, even a stock unroll would be better
than nothing.
--
Summary: Loop unrolling not performed with large constant loop
bound
Product: gcc
Version: 4.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: scovich at gmail dot com
GCC target triplet: x86_64-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32729
