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add a compatibility note about vector builtins 

llvm-svn: 114101
This commit is contained in:
Chris Lattner 2010-09-16 18:17:55 +00:00
parent a00105be1c
commit e3c1b9317a
1 changed files with 27 additions and 0 deletions
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27
clang/www/compatibility.html
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@ -32,6 +32,7 @@
<li><a href="#c">C compatibility</a>
<ul>
<li><a href="#inline">C99 inline functions</a></li>
<li><a href="#vector_builtins">"missing" vector __builtin functions</a></li>
<li><a href="#lvalue-cast">Lvalue casts</a></li>
<li><a href="#blocks-in-protected-scope">Jumps to within <tt>__block</tt> variable scope</a></li>
</ul>
@ -114,6 +115,32 @@ Undefined symbols:
be changed.</li>
</ul>
<!-- ======================================================================= -->
<h3 id="vector_builtins">"missing" vector __builtin functions</h3>
<!-- ======================================================================= -->
<p>The Intel and AMD manuals document a number "<tt>&lt;*mmintrin.h&gt;</tt>"
header files, which define a standardized API for accessing vector operations
on X86 CPUs. These functions have names like <tt>_mm_xor_ps</tt> and
<tt>_mm256_addsub_pd</tt>. Compilers have leeway to implement these functions
however they want. Since Clang supports an excellent set of <a
href="../docs/LanguageExtensions.html#vectors">native vector operations</a>,
the Clang headers implement these interfaces in terms of the native vector
operations.
</p>
<p>In contrast, GCC implements these functions mostly as a 1-to-1 mapping to
builtin function calls, like <tt>__builtin_ia32_paddw128</tt>. These builtin
functions are an internal implementation detail of GCC, and are not portable to
the Intel compiler, the Microsoft compiler, or Clang. If you get build errors
mentioning these, the fix is simple: switch to the *mmintrin.h functions.</p>
<p>The same issue occurs for NEON and Altivec for the ARM and PowerPC
architectures respectively. For these, make sure to use the &lt;arm_neon.h&gt;
and &lt;altivec.h&gt; headers.</p>
<!-- ======================================================================= -->
<h3 id="lvalue-cast">Lvalue casts</h3>
<!-- ======================================================================= -->