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POWhy the ror op in this inline assembly not working properly?
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6285707
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6286349
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3
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2011-06-08T21:51:21.383
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2011-06-09T00:04:16.087
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I have written a program to process some data written to disk in big-endian format, so the program needs to swap bytes in order to do anything else. After profiling the code I found that my byte swapping function was taking 30% of the execution time. So I thought to myself, how can I speed this up? So I decided to write a little piece inline assembly. I would up replacing this: <pre><code>void swapTwoByte(char* array, int numChunks) { for(int i= (2*numChunks-1); i>=0; i-=2) { char temp=array[i]; array[i]=array[i-1]; array[i-1]=temp; } } </code></pre> with this: <pre><code>void swapTwoByte(int16* array, int numChunks) { for(int i= (numChunks-1); i>=0; --i) { asm("movw %1, %%ax;" "rorw %%ax;" "rorw %%ax;" "rorw %%ax;" "rorw %%ax;" "rorw %%ax;" "rorw %%ax;" "rorw %%ax;" "rorw %%ax;" "movw %%ax, %0;" : "=r" ( array[i] ) : "r" (array[i]) :"%ax" ); } } </code></pre> Which does the intended job, but that is a lot of rotate operations. So here is my question: According to <a href="http://en.wikibooks.org/wiki/X86_Assembly/Shift_and_Rotate" rel="nofollow">this source</a> rorw can take two operands, and in the gas sytax the source operand should be the number of bits to rotate by, but every time I try to replace that list of 8 rotate rights with something like <pre><code>".set rotate, 0x0008" "rorw rotate, %%ax" </code></pre> I get an assembler error stating: <pre><code>"Error: number of operands mismatch for `ror'" </code></pre> Why is this? What am I missing?
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<c++><assembly><inline-assembly>
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Why the ror op in this inline assembly not working properly?
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