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https://issues.apache.org/jira/browse/STDCXX-723?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12567931#action_12567931
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Martin Sebor commented on STDCXX-723:
-------------------------------------
The gcc
[i386-prefetch.exp|http://gcc.gnu.org/viewcvs/trunk/gcc/testsuite/gcc.misc-tests/i386-prefetch.exp?view=markup]
test suite file contains some info on when {{__builtin_prefetch}} is useful.
> [gcc] use __builtin_prefetch to optimize string
> -----------------------------------------------
>
> 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: 4.1.2, 4.1.3, 4.1.4, 4.2.0
> Reporter: Martin Sebor
> Priority: Minor
> Fix For: 4.2.1
>
> Original Estimate: 4h
> Remaining Estimate: 4h
>
> 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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