[gcc] use __builtin_prefetch to optimize string
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Key: STDCXX-723
URL: https://issues.apache.org/jira/browse/STDCXX-723
Project: C++ Standard Library
Issue Type: Sub-task
Components: 21. Strings
Affects Versions: 4.2.0, 4.1.4, 4.1.3, 4.1.2
Reporter: Martin Sebor
Priority: Minor
Fix For: 4.2.1
We might be able to use the gcc {{__builtin_prefetch}} function in
{{basic_string}} to give the hardware a hint when a string object's data is
about to be accessed (e.g., the reference count which is stored at a negative
offset from the {{basic_string::_C_data}} member pointer. This could improve
performance on modern processors that implement prefetching (e.g., IA-64,
x86_64, or PowerPC).
Quoting from section [5.46 Other built-in functions provided by
GCC|http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Other-Builtins.html#Other-Builtins]
of the gcc manual:
{quote}
Built-in Function: {{void __builtin_prefetch (const void *addr, ...)}}
This function is used to minimize cache-miss latency by moving data into a
cache before it is accessed. You can insert calls to {{__builtin_prefetch}}
into code for which you know addresses of data in memory that is likely to be
accessed soon. If the target supports them, data prefetch instructions will be
generated. If the prefetch is done early enough before the access then the data
will be in the cache by the time it is accessed.
The value of {{addr}} is the address of the memory to prefetch. There are
two optional arguments, {{rw}} and {{locality}}. The value of {{rw}} is a
compile-time constant one or zero; one means that the prefetch is preparing for
a write to the memory address and zero, the default, means that the prefetch is
preparing for a read. The value {{locality}} must be a compile-time constant
integer between zero and three. A value of zero means that the data has no
temporal locality, so it need not be left in the cache after the access. A
value of three means that the data has a high degree of temporal locality and
should be left in all levels of cache possible. Values of one and two mean,
respectively, a low or moderate degree of temporal locality. The default is
three.
{code}
for (i = 0; i < n; i++)
{
a[i] = a[i] + b[i];
__builtin_prefetch (&a[i+j], 1, 1);
__builtin_prefetch (&b[i+j], 0, 1);
/* ... */
}
{code}
Data prefetch does not generate faults if {{addr}} is invalid, but the
address expression itself must be valid. For example, a prefetch of {{p->next}}
will not fault if {{p->next}} is not a valid address, but evaluation will fault
if {{p}} is not a valid address.
If the target does not support data prefetch, the address expression is
evaluated if it includes side effects but no other code is generated and GCC
does not issue a warning.
{quote}
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