http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50182
--- Comment #24 from davidxl <xinliangli at gmail dot com> 2011-10-24 23:00:22
UTC ---
(In reply to comment #23)
> Here is the source preprocessed for gcc47. The test exhibits the
> slowdown mentioned in comment 11.
The problem can be reproduced with a simplified test case -- basically
depending on how the result value from the inner loop is used in the outer loop
(related to casting), the inner loop code is quite different - in the slow
case, there are two redundant sign extension and a move instructions generated.
# the fast version
gcc -O3 -DFAST_VER bug.cpp
./a.out
rv=4282167296
test description absolute operations ratio with
number time per second test0
0 "int8_t constant add" 1.05 sec 1523.81 M 1.00
Total absolute time for int8_t constant folding: 1.05 sec
# the slow version:
gcc -O3 bug.cpp
./a.out
rv=4282167296
test description absolute operations ratio with
number time per second test0
0 "int8_t constant add" 1.57 sec 1019.11 M 1.00
Total absolute time for int8_t constant folding: 1.57 sec
# however, when disabling inlining of check_shifted_sum_1 in the slow case, the
runtime is recovered:
gcc -O3 -DNOINLINE bug.cpp
./a.out
rv=4282167296
test description absolute operations ratio with
number time per second test0
0 "int8_t constant add" 1.05 sec 1523.81 M 1.00
Total absolute time for int8_t constant folding: 1.05 sec
The inner loop body in faster case:
.L60:
movzbl 0(%rbp,%rcx), %r9d
addq $1, %rcx
cmpl %ecx, %ebx
leal 10(%r8,%r9), %r8d
# SUCC: 4 [91.0%] (dfs_back,can_fallthru) 5 [9.0%]
(fallthru,can_fallthru,loop_exit)
jg .L60
while for the slow case:
.L60:
movzbl (%r12,%rcx), %eax
movsbl %r8b, %r8d
addq $1, %rcx
leal 10(%rax), %r9d
movsbl %r9b, %r9d
addl %r8d, %r9d
cmpl %ecx, %ebp
movl %r9d, %r8d
# SUCC: 4 [91.0%] (dfs_back,can_fallthru) 5 [9.0%]
(fallthru,can_fallthru,loop_exit)
jg .L60
The relevant source change:
#ifdef NOINLINE
#define INL __attribute__((noinline))
#else
#define INL inline
#endif
template <typename T, typename T2, typename Shifter>
INL void check_shifted_sum_1(T2 result) {
T temp = (T)SIZE * Shifter::do_shift((T)init_value);
if (!tolerance_equal<T>((T&)result,temp))
printf("test %i failed\n", current_test);
}
#ifdef FAST_VER
#define TYPE u_int32_t
#else
#define TYPE int8_t
#endif
template <typename T, typename Shifter>
__attribute__((noinline)) u_int32_t test_constant(T* first, int count, const
char *label)
{
int i;
u_int32_t rv = 0;
start_timer();
for (i = 0; i < iterations; ++i) {
T result = 0;
for (int n = 0; n < count; ++n) {
result += Shifter::do_shift( first[n] );
}
rv += result;
check_shifted_sum_1<T, TYPE, Shifter>(result);
}
record_result( timer(), label );
return rv;
}
