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Re: [wwwdocs] Document Runtime CPU detection builtins
- From: Gerald Pfeifer <gerald at pfeifer dot com>
- To: Sriraman Tallam <tmsriram at google dot com>
- Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>, "H.J. Lu" <hjl dot tools at gmail dot com>
- Date: Mon, 20 Aug 2012 22:31:01 +0200 (CEST)
- Subject: Re: [wwwdocs] Document Runtime CPU detection builtins
- References: <CAAs8HmwO=jycJSgMS2LOfBNJrdUxyDDNDxWDX=kcABCZQEEMnw@mail.gmail.com>
Hi Sriraman,
On Fri, 10 Aug 2012, Sriraman Tallam wrote:
> I have added a release note for x86 builtins __builtin_cpu_is and
> __builtin_cpu_supports. They were checked in to trunk in rev. 186789.
I had hoped one of the x86 maintainers would review this from his
perspective given that they have more background. For the lack of
that, let me give it a try.
Index: changes.html
===================================================================
+ <li> New builtin functions to detect run-time CPU type and ISA:<br>
"built-in", cf. http://gcc.gnu.org/codingconventions.html; here and
in the following.
No <br> here; <ul> should just do that.
+ <ul>
+ <li>Builtin <code>__builtin_cpu_is</code> has been added to detect if
+ the run-time CPU is of a particular type. The builtin returns a postive
+ integer on a match and zero otherwise. The builtin accepts one string
+ literal argument, the CPU name. For example,
"A built-in function..."
"positive"
"It accepts one string" (to make this shorter)
+ <code>__builtin_cpu_is("westmere")</code> returns a postive integer if
"positive"
+ the run-time CPU is an Intel Corei7 Westmere processor. The following
I don't work for Intel, but should there be a space before "i7"?
+ are the CPU names recognized by <code>__builtin_cpu_is:</code>
How about making this "The following are the CPU names recognized for
now", which avoids another reference to the name of the built-in and
makes it clear that this is subject to change.
+ <li>Builtin <code>__builtin_cpu_supports</code> has been added to detect
"A built-in function..."
+ returns a postive integer on a match and zero otherwise. The builtin
"positive"
+ following are the ISA features recognized by
+ <code>__builtin_cpu_supports:</code>
Same is above?
+ <p>Caveat: If the above builtins are called before any constructors are
+ invoked, like during IFUNC initialization, then the CPU detection
+ initialization must be explicity run using this newly provided
+ builtin, <code>__builtin_cpu_init</code>.
"...using the new built-in function <code>__builtin_cpu_init</code>."
What is a constructor in this context, by the way? Will this be clear
to all the users?
+ <code>
+ static void (*some_ifunc_resolver(void))(void)<br>
+ {<br>
+    __builtin_cpu_init();<br>
+    if (__builtin_cpu_is("amdfam10h") ...<br>
+    if (__builtin_cpu_supports("popcnt") ...<br>
+ }
+ </code>
How about using <pre> here? That avoids the <br/>s which will cause
problems with the web page validator, by the way.
Nice job for documenting this so well. Thanks for taking the time
and your patience!
The patch is fine modulo the changes I pointed out (though some of
them are more suggestions and you do not need to slavishly follow
those).
Gerald