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POHow do I implement an efficient 32 bit DivMod in 64 bit code
 

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  1. POHow do I implement an efficient 32 bit DivMod in 64 bit code
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    <p>I want to use a <code>DivMod</code> function that operates exclusively on 32 bit operands. The <a href="http://docwiki.embarcadero.com/Libraries/en/System.Math.DivMod" rel="nofollow noreferrer">implementation in the RTL</a> returns values in 16 bit variables. Its declaration is:</p> <pre><code>procedure DivMod(Dividend: Cardinal; Divisor: Word; var Result, Remainder: Word); </code></pre> <p>So, I cannot use that since my inputs may overflow the return values.</p> <p>The naive Pascal implementation looks like this:</p> <pre><code>procedure DivMod(Dividend, Divisor: Cardinal; out Quotient, Remainder: Cardinal); begin Quotient := Dividend div Divisor; Remainder := Dividend mod Divisor; end; </code></pre> <p>This works splendidly but performs the division twice. Since the function is called by part of my code that is in a performance bottleneck, I would like to perform the division once only. To that end I am using Serg's 32 bit DivMod from this question: <a href="https://stackoverflow.com/questions/2397656/is-there-a-divmod-that-is-not-limited-to-words-65535">Is there a DivMod that is *not* Limited to Words (&lt;=65535)?</a></p> <pre><code>procedure DivMod(Dividend, Divisor: Cardinal; out Quotient, Remainder: Cardinal); asm PUSH EBX MOV EBX,EDX XOR EDX,EDX DIV EBX MOV [ECX],EAX MOV EBX,Remainder MOV [EBX],EDX POP EBX end; </code></pre> <p>This works perfectly.</p> <p>But now I would like a version of the function for 64 bit code. Note that I still want to operate on 32 bit operands, and return 32 bit values. </p> <p>Should I re-write the function using 64 bit assembler, or is it sufficient to use the <code>DivMod</code> overload from the RTL that operates on, and returns, 64 bit values? </p> <p>Specifically I would like to know if there is a performance benefit in writing 64 bit code that does 32 bit operations. Is that even possible? Or would I simply end up re-implementing the <code>DivMod</code> overload with <code>UInt64</code> parameters? If it is worth implementing a bespoke 64 bit asm version, how would I go about doing it, noting that the operands and operations are 32 bit.</p> <p>I think that it would look like this, but I am no expert and likely have got something wrong:</p> <pre><code>procedure DivMod(Dividend, Divisor: Cardinal; out Quotient, Remainder: Cardinal); asm MOV EAX,ECX // move Dividend to EAX MOV ECX,EDX // move Divisor to ECX XOR EDX,EDX // zeroise EDX DIV ECX // divide EDX:EAX by ECX MOV [R8],EAX // save quotient MOV [R9],EDX // save remainder end; </code></pre>
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    1. USDavid Heffernan
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      1. POHow do I implement an efficient 32 bit DivMod in 64 bit code
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    1. COSeems ok. About 33% faster than the purepascal version.
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      1. POHow do I implement an efficient 32 bit DivMod in 64 bit code
      1. USLU RD
    2. CO@LURD Do you know if `DIV ECX` in the code above performs a 32 bit operation? Did I understand that right? Does it do unsigned divide of the 64 bit value EDX:EXA by ECX? And is it worth doing the 32 bit operation rather than the full 64 bit operation?
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      1. POHow do I implement an efficient 32 bit DivMod in 64 bit code
      1. USDavid Heffernan
    3. COReplaced `64-bit` tag with `x86-64` tag, as the question is specific to x86-64, not to 64-bit architectures in general.
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