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diff --git a/www/compatibility.html b/www/compatibility.html index d33686d151..b068ec5827 100644 --- a/www/compatibility.html +++ b/www/compatibility.html @@ -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> +<inline asm>: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> <!-- ======================================================================= --> |