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POHow can I prove or disprove the efficiency of compilation?
 

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  1. POHow can I prove or disprove the efficiency of compilation?
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    <p>This is an unusual question, but I do hope there's a definitive answer.</p> <p>There's a longstanding debate in our office about how efficiently compilers generate code, specifically number of instructions. We write code for low power embedded systems with virtually no loops. Therefore, the number of instructions emitted is directly proportional to power consumed.</p> <p>Much of our code looks like this (notice, no dynamic memory allocation, no system calls, very few function calls, very few loops).</p> <pre><code>foo += 3 * (77 + bar); if (baz &gt; 18 - qux) bar -= 19 + 7 &gt;&gt; spam; </code></pre> <p>I can compile the above snippet with <code>-O3</code> and read the assembly, but I couldn't write it myself.</p> <p>The claim I would like to prove or disprove is that compilers generate code that is 2-4X "fatter" (and therefore consume 2-4X times as much power) compared with hand written assembly code.</p> <p>I'm interested in any compiler with which you have experience.</p> <p>From <a href="https://stackoverflow.com/questions/840321/how-can-i-see-the-assembly-code-for-a-c-program">this answer</a> I know that GCC and clang can emit assembly interleaved with the C code with</p> <pre><code>gcc -g -c -Wa,-alh foo.cc </code></pre> <p>These answers provide solid basis:</p> <p><a href="https://stackoverflow.com/questions/577554/when-is-assembler-faster-than-c?rq=1">When is assembly faster?</a></p> <p><a href="https://stackoverflow.com/questions/791533/why-do-you-program-in-assembly?lq=1">Why do you program in assembly?</a></p> <p>How can I measure the efficiency with which a compiler generates code?</p>
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    1. COI think you would have to prove that "number of instructions" == "power consumed". What does your processor do when it is "not doing anything"?
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      1. POHow can I prove or disprove the efficiency of compilation?
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    2. COSimple answer: benchmarking. You can argue the merits of speed or size all day long. But at the end of the day there is something satisfying in seeing an actual result. Build your exe optimized for speed or for size, then test it on a machine. Build it again, using the methods being debated, then test it against the first method, and so on. You will have more definitive results than you will by simple debate.
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      1. POHow can I prove or disprove the efficiency of compilation?
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    3. COAlso, it would be better to compare gcc with `-Os` not `-O3` if you are really concerned about code size: http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
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      1. POHow can I prove or disprove the efficiency of compilation?
      1. USDrew MacInnis
 

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