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add a note about inline asm compatibility. 

llvm-svn: 118740
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Chris Lattner 2010-11-10 23:51:50 +00:00
parent 5d6c464f46
commit 3bb7aab0b0
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clang/www/compatibility.html
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@ -36,6 +36,7 @@
<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>
<li><a href="#block-variable-initialization">Non-initialization of <tt>__block</tt> variables</a></li>
<li><a href="#inline-asm">Inline assembly</a></li>
</ul>
</li>
<li><a href="#objective-c">Objective-C compatibility</a>
@ -247,6 +248,49 @@ other local variables.</p>
<p>Clang does not zero initialize local block variables, and programs which rely
on such behavior will most likely break when built with Clang.</p>
<!-- ======================================================================= -->
<h3 id="inline-asm">Inline assembly</h3>
<!-- ======================================================================= -->
<p>In general, Clang is highly compatible with the GCC inline assembly
extensions, allowing the same set of constraints, modifiers and operands as GCC
inline assembly.</p>
<p>On targets that use the integrated assembler (such as most X86 targets),
inline assembly is run through the integrated assembler instead of your system
assembler (which is most commonly "gas", the GNU assembler). The LLVM
integrated assembler is extremely compatible with GAS, but there are a couple of
minor places where it is more picky, particularly due to outright GAS bugs.</p>
<p>One specific example is that the assembler rejects ambiguous X86 instructions
that don't have suffixes. For example:</p>
<pre>
asm("add %al, (%rax)");
asm("addw $4, (%rax)");
asm("add $4, (%rax)");
</pre>
<p>Both clang and GAS accept the first instruction: because the first
instruction uses the 8-bit <tt>%al</tt> register as an operand, it is clear that
it is an 8-bit add. The second instruction is accepted by both because the "w"
suffix indicates that it is a 16-bit add. The last instruction is accepted by
GAS even though there is nothing that specifies the size of the instruction (and
the assembler randomly picks a 32-bit add). Because it is ambiguous, Clang
rejects the instruction with this error message:
</p>
<pre>
&lt;inline asm&gt;:3:1: error: ambiguous instructions require an explicit suffix (could be 'addb', 'addw', 'addl', or 'addq')
add $4, (%rax)
^
1 error generated.
</pre>
<p>To fix this compatibility issue, add an explicit suffix to the instruction:
this makes your code more clear and is compatible with both GCC and Clang.</p>
<!-- ======================================================================= -->
<h2 id="objective-c">Objective-C compatibility</h3>
<!-- ======================================================================= -->