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POFast Division on GCC/ARM
 

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    <p>As far as I know most compilers will do fast division by multiplying and then bit shifting to the right. For instance, if you check <a href="https://stackoverflow.com/questions/5558492/divide-by-10-using-bit-shifts">this SO thread</a> it says that when you ask the Microsoft compiler to do division by 10 it will multiply the dividend by 0x1999999A (which is 2^32/10) and then divide the result by 2^32 (using 32 shifts to the right). </p> <p>So far so good.</p> <p>Once I tested the same division by 10 on ARM using GCC, though, the compiler did something slightly different. First it multiplied the dividend by 0x66666667 (2^34/10), then it divided the result by 2^34. Thus far it's the same as Microsoft, except using a higher multiplier. After that, however, it subtracted (dividend/2^31) from the result. </p> <p><strong>My question</strong>: why on the ARM version there's that extra subtraction? Can you give me an numeric example where without that subtraction the result will be wrong? </p> <p>If you want to check the generated code, it's below (with my comments):</p> <pre><code> ldr r2, [r7, #4] @--this loads the dividend from memory into r2 movw r3, #:lower16:1717986919 @--moves the lower 16 bits of the constant movt r3, #:upper16:1717986919 @--moves the upper 16 bits of the constant smull r1, r3, r3, r2 @--multiply long, put lower 32 bits in r1, higher 32 in r3 asr r1, r3, #2 @--r3&gt;&gt;2, then store in r1 (effectively &gt;&gt;34, since r3 was higher 32 bits of multiplication) asr r3, r2, #31 @--dividend&gt;&gt;31, then store in r3 rsb r3, r3, r1 @--r1 - r3, store in r3 str r3, [r7, #0] @--this stores the result in memory (from r3) </code></pre>
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    1. USDaniel Scocco
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    1. COIt's for negative values, integer division truncates, and just multiplication and shift produce `x/10 - 1` for negative `x`. (Assuming arithmetic right-shift, of course.)
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      1. USDaniel Fischer
    2. COI can see that if I make -99 / 10 with the multiplication/shift method I'll get -10 as a result. But if I subtract 1 from that I'll get -11, when what I want is -9 ain't it?
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    3. COYou subtract `-1`, i.e. you add 1.
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