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POErlang compiler optimizations
 

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    <p>I'm no compiler optimization expert. I'm not even sure what's "reasonable" to expect as far as compilers or optimizations. I'm just curious and asking questions is all.</p> <p>At any rate, I was coding up some Erlang in a basic form such as this:</p> <pre><code>% TOY EXAMPLE 1 test(X) -&gt; if X-1 &gt; 0 -&gt; yes; X-1 == 0 -&gt; maybe; true -&gt; no end. </code></pre> <p>Then I then optimized it in order not to do the subtract twice:</p> <pre><code>% TOY EXAMPLE 2 test(X) -&gt; Z = X-1, if Z &gt; 0 -&gt; yes; Z == 0 -&gt; maybe; true -&gt; no end. </code></pre> <p>Then I thought "a waste of time - surely the compiler optimizes the first example into the second anyway." So I decided to check by running compile:file with 'S' option for both. Here is output:</p> <pre><code>% TOY EXAMPLE 1 {function, test, 1, 15}. {label,14}. {func_info,{atom,exchange},{atom,test},1}. {label,15}. {gc_bif,'-',{f,16},1,[{x,0},{integer,1}],{x,1}}. {test,is_lt,{f,16},[{integer,0},{x,1}]}. {move,{atom,yes},{x,0}}. return. {label,16}. {gc_bif,'-',{f,17},1,[{x,0},{integer,1}],{x,1}}. {test,is_eq,{f,17},[{x,1},{integer,0}]}. {move,{atom,maybe},{x,0}}. return. {label,17}. {move,{atom,no},{x,0}}. return. % TOY EXAMPLE 2 {function, test, 1, 15}. {label,14}. {func_info,{atom,exchange},{atom,test},1}. {label,15}. {gc_bif,'-',{f,0},1,[{x,0},{integer,1}],{x,0}}. {test,is_lt,{f,16},[{integer,0},{x,0}]}. {move,{atom,yes},{x,0}}. return. {label,16}. {test,is_eq,{f,17},[{x,0},{integer,0}]}. {move,{atom,maybe},{x,0}}. return. {label,17}. {move,{atom,no},{x,0}}. return. </code></pre> <p>They aren't the same. If I'm reading this right (maybe I'm not), the optimization isn't performed.</p> <p>I can see a few possibilities:</p> <ol> <li><p>The optimization can be performed, I'm just not enabling the optimizations because I'm using the wrong function to compile, or not using the correct flags, etc.</p></li> <li><p>The optimization just isn't performed.</p></li> <li><p>Other.</p></li> </ol> <p>Which is it?</p> <p>Note: Please don't get bogged down with talk of "if you use a case statement you can do such-and-such" or "you can avoid this by doing blah-blah." The point is simply to test which optimizations the erlang compiler does or doesn't do, and why or why not.</p> <p>Thanks.</p>
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