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POHow does address operand affect performance and size of machine code?
 

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  1. POHow does address operand affect performance and size of machine code?
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    <p>Starting with 32-bit CPU mode, there are extended address operands available for x86 architecture. One can specify the base address, a displacement, an index register and a scaling factor.</p> <p>For example, we would like to stride through a list of 32-bit integers (every first two from an array of 32-byte-long data structures, <code>%rdi</code> as data index, <code>%rbx</code> as base pointer).</p> <pre><code>addl $8, %rdi # skip eight values: advance index by 8 movl (%rbx, %rdi, 4), %eax # load data: pointer + scaled index movl 4(%rbx, %rdi, 4), %edx # load data: pointer + scaled index + displacement </code></pre> <p>As I know, such complex addressing fits into a single machine-code instruction. But what is the cost of such operation and how does it compare to simple addressing with independent pointer calculation:</p> <pre><code>addl $32, %rbx # skip eight values: move pointer forward by 32 bytes movl (%rbx), %eax # load data: pointer addl $4, %rbx # point next value: move pointer forward by 4 bytes movl (%rbx), %edx # load data: pointer </code></pre> <p>In the latter example, I have introduced one extra instruction and a dependency. But integer addition is very fast, I gained simpler address operands, and there are no multiplications any more. On the other hand, since the allowed scaling factors are powers of 2, the multiplication comes down to a bit shift, which is also a very fast operation. Still, two additions and a bit shift can be replaced with one addition.</p> <p>What are the performance and code size differences between these two approaches? Are there any best practices for using the extended addressing operands?</p> <p>Or, asking it from a C programmer's point of view, what is faster: array indexing or pointer arithmetic?</p> <hr> <p>Is there any assembly editor meant for size/performance tuning? I wish I could see the machine-code size of each assembly instruction, its execution time in clock cycles or a dependency graph. There are thousands of assembly freaks that would benefit from such application, so I bet that something like this already exists! </p>
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    1. USKrzysztof Abramowicz
    1. USKrzysztof Abramowicz
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      1. POHow does address operand affect performance and size of machine code?
      1. USEgor Skriptunoff
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    1. COGeneral answer #0: Optimization is voodoo, and things like adding instructions or using longer instructions can speed things up in some cases. These behaviors can vary from CPU to CPU; something true on one model may not be true on a newer model. In your case, things can go either way, and there is no good way to predict without simply measuring.
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      1. POHow does address operand affect performance and size of machine code?
      1. USNayuki
    2. COGeneral answer #1: http://www.agner.org/optimize/ ; http://www.intel.com/content/www/us/en/architecture-and-technology/64-ia-32-architectures-optimization-manual.html
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      1. USNayuki
    3. CO@NayukiMinase, some v useful links. Very worth a browse. Thanks.
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