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POlong double (80 bits) twice as fast as double with -funsafe-math-optimizations
 

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  1. POlong double (80 bits) twice as fast as double with -funsafe-math-optimizations
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    <p>I'm getting consistently that using <code>long double</code> datatype is about twice as fast as using <code>double</code> for my calculations when using <code>-funsafe-math-optimizations</code>. I would like to have an insight on this, because the 80 bit format is deprecated since long, or I might be doing something really dumb with the <code>double</code> datatype. The compiler is g++ 4.8.2, target is x86_64 (so gcc would prefer SSE2 if I don't use <code>long double</code>).</p> <p>My code is more or less like this (pseudocode):</p> <pre><code>//x is an array of floating point numbers for i -&gt; x.size accumulator = 0 for k -&gt; kmax accumulator += A[k]*(B[k]*cos(C*k*x[i]) - D[k]*sin(C*k*x[i])); x[i] += F*accumulator; if(x[i] &gt;= 1/2) x[i] -= integer(x[i]+1/2); else if(x[i] &lt; -1/2) x[i] -= integer(x[i]-1/2); </code></pre> <p><code>A</code>, <code>B</code>, .. are some precomputed arrays/constants.</p> <p>The speedup seems unrelated to cacheline problems, because I get the same relative speedup if I parallelize the outer for loop with OpenMP.</p> <p>EDIT: I corrected the pseudocode: notice the <code>cos</code> and <code>sin</code> have the same argument, that is in the end the reason for the speedup (see gsg's answer and the comments).</p>
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    1. USLorenzo Pistone
    1. USLorenzo Pistone
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    1. COIs there any change in the branch history of the `if` and `for` statements?
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      1. USPatricia Shanahan
    2. COwhat is the assembler output for the two versions?
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      1. USJeff
    3. COWhen using `long double`, are you using `cosl()` or `cos()`?
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