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127 lines
3.7 KiB
HTML
127 lines
3.7 KiB
HTML
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<title>Polly - Memory access optimizations</title>
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<!--*********************************************************************-->
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<h1>Polly - Memory access optimizations</h1>
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<!--*********************************************************************-->
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<p><em>WARNING: This project was part of the Google Summer of Code 2011.
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Tt is currently not finished, but it is in the design and implementation stage.
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The Ideas/Plans described here may not yet be implemented in Polly and may be
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changed during the actual implementation.</em></p>
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This project adds memory access transformations to Polly. In many cases
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changing the memory access pattern yields to better data locality or removes
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dependences that would otherwise block transformations.
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<p>An examples which uses this feature is given below.</p>
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Consider the following loop
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<pre>
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for (i = 0; i < 8; i++)
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sum += A[i];
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</pre>
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Through memory access transformations this loop can be executed in parallel.
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It can be transformed to
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<pre>
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<em>// Create and initialize an array 'tmp' with size 4</em>
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for (i = 0; i < 8; i++)
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tmp[i % 4] += A[i];
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sum = tmp[0] + tmp[1] + tmp[2] + tmp[3];
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</pre>
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Optimizers like PluTo can schedule the code such that an outer, parallel
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loop is created:
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<pre>
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parfor (ii = 0; ii < 4; ii++) {
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tmp[ii] = 0;
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for (i = ii * 2; i < (ii+1) * 2; i++)
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tmp[ii] += A[i];
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}
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sum = tmp[0] + tmp[1] + tmp[2] + tmp[3];
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</pre>
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<h2>TODO</h2>
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<h3>Step 1</h3>
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Polly exports its polyhedral description in a JSCoP file. Define how memory
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layout transformations are expressed in Polly and in the JSCOP file.
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Example:
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<p>Consider the following loop.</p>
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<pre>
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for (i = 0; i < 12; i++)
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A[i] = 0;
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</pre>
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In the JSCOP file the memory accesses are represented as follows.
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<pre>
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"accesses": [{
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"kind": "write",
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"relation": "{ Stmt[i] -> A[i] }"
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}]
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</pre>
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To perform a transformation we generate the following code:
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<pre>
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for (i = 0; i < 12; i++)
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A[0] = i;
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</pre>
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The representation in the JSCoP file is:
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<pre>
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"accesses": [{
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"kind": "read",
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"relation": "{ Stmt[i] -> A[0] }"
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}]
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</pre>
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We need to detect this access function change.
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<h3>Step 2</h3>
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Update the code generation module to reflect the access function change made
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in Step 1.
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<h3>Step 2.1 Code generation for a constant</h3>
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In the JSCOP file an access function which has variables is changed to a
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constant. Code is generated to reflect this change. Let the content of original
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JSCOP file be:
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<pre>
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"accesses" : [{
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"kind" : "read",
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"relation" : "{ Stmt_for_body[i0] -> MemRef_A[i0] }"
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}]
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</pre>
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The transformed JSCOP file is:
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<pre>
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"accesses" : [{
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"kind" : "read",
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"relation" : "{ Stmt_for_body[i0] -> MemRef_A[10] }"
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}]
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</pre>
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Code is generated for this change.
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<h3>Step 2.2 Code generation for coefficients</h3>
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The coefficients of induction variables are changed here. Let the original
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JSCOP file be:
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<pre>
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"accesses" : [{
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"kind" : "read",
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"relation" : "{ Stmt_for_body3[i0, i1] -> MemRef_A[32i0+i1] }"
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}]
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</pre>
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The transformed JSCOP file is:
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<pre>
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"accesses" : [{
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"kind" : "read",
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"relation" : "{ Stmt_for_body3[i0, i1] -> MemRef_A[16i0+2i1+5] }"
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}]
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</pre>
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Code is generated for this change.
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</div>
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</body>
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</html>
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