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copied!<p>That inline assembly is buggy:</p> <ol> <li>It uses multi-line strings which effectively concatenate. Without <code>\n</code> all appears on one line. Whether your assembler <em>accepts</em> statements separated by semicolons makes all the difference there ... some may not.</li> <li>It specifies the same variable as input/output constraint instead of using <code>"+r"(value)</code> as ordinarily suggested for this situation.</li> </ol> <p>Without seeing the rest of the code it's not quite clear why the inline assembly statement looks the way it does; Personally, I'd suggest to write it like:</p> <pre><code>asm("ror %%cl, %0" : "+r"(value) : "c"((((uintptr_t)address) & 3) << 3))); </code></pre> <p>because there's little need to do the calculation itself in assembly. The <code>uintptr_t</code> (from <code><stdint.h></code>) cast makes this 32/64bit agnostic as well.</p> <p><strong>Edit:</strong></p> <p>If you want it for a <em>different</em> CPU but x86 / x64, then it obviously needs to be different ... For ARM (<em>not</em> Thumb2), it'd be:</p> <pre><code>asm("ROR %0, %0, %1" : "+r"(value) : "r"((((uintptr_t)address) & 3) << 3))); </code></pre> <p>since that's how the rotate instruction there behaves.</p> <p><strong>Edit (add reference):</strong></p> <p>Regarding the operation performed here as such, <a href="http://chsc.wordpress.com/2010/01/13/compiler-optimization/" rel="nofollow">this blog post</a> gives an interesting perspective - namely, that the compiler is quite likely to create the same output for:</p> <pre><code>(a >> shift | a << (8 * sizeof(a) - shift)) </code></pre> <p>as for the x86 inline</p> <pre><code>asm("ror %%cl, %0" : "+r"(a) : "c"(shift)) </code></pre> <p>Testing this:</p> <pre><code>#include <stdint.h> int main(int argc, char **argv) { unsigned int shift = (int)((((uintptr_t)argv) & 3) << 3); unsigned int a = argc; #ifdef USE_ASM /* * Mark the assembly version with a "nop" instruction in output */ asm("nop\n\t" "ror %%cl, %0" : "+r"(a) : "c"(shift)); return a; #else return (a >> shift | a << (8 * sizeof(a) - shift)); #endif } </code></pre> <p>Compile / disassemble it:</p> <pre>$ gcc -DUSE_ASM -O8 -c tf.c; objdump -d tf.o tf.o: file format elf64-x86-64 Disassembly of section .text: 0000000000000000 : 0: 83 e6 03 and $0x3,%esi 3: 8d 0c f5 00 00 00 00 lea 0x0(,%rsi,8),%ecx a: 90 nop b: d3 cf ror %cl,%edi d: 89 f8 mov %edi,%eax f: c3 retq $ gcc -O8 -c tf.c; objdump -d tf.o tf.o: file format elf64-x86-64 Disassembly of section .text: 0000000000000000 : 0: 83 e6 03 and $0x3,%esi 3: 8d 0c f5 00 00 00 00 lea 0x0(,%rsi,8),%ecx a: d3 cf ror %cl,%edi c: 89 f8 mov %edi,%eax e: c3 retq</pre> <p>Ergo, this inline assembly is unnecessary.</p>

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