Polly - Memory access optimizations
+ +WARNING: This project is part of the Google Summer of Code 2011. Hence, +it is currently not finished, but it is in the design and implementation stage. +The Ideas/Plans described here may not yet be implemented in Polly and may be +changed during the actual implementation.
+ +The project which adds memory access transformations to Polly. In many cases +changing the memory access pattern yields to better data locality or removes +dependences that would otherwise block transformations. + +An examples which uses this feature is given below.
+ Consider the following loop --
-
-
-
for
-(i = 0; i < 8; i++)
-sum += A[i];
-
--sum += A[i];
-
-With support for memory access transformation this loop can be executed
-in parallel. It can be -transformed to
-
- -
<create
-and
-initialize an array 'tmp'
-with size 4>
-for (i = 0; i < 8; i++) {
-tmp[i % 4] += A[i];
-}
-sum = tmp[0] + tmp[1] + tmp[2] -+ tmp[3];
-
--for (i = 0; i < 8; i++) {
-tmp[i % 4] += A[i];
-}
-sum = tmp[0] + tmp[1] + tmp[2] -+ tmp[3];
-
-With the help of some optimizer (like -PluTo) the following code can be
-generated, where the outer loop is -parallel. -
parfor (ii =
-0; ii < 4; ii++) {
- tmp[ii] = 0;
- for (i = ii * 2; i < (ii+1) * 2; i++)
- tmp[ii] += A[i];
}
-sum = tmp[0] + tmp[1] + tmp[2] +
-tmp[3];
-
TODO
-
Step 1
-
-layout transformations are going to be expressed in Polly and -in -the JSCOP file.
-A simple example is given below.
-
-Consider the following loop.
-
-
for
-(i
-= 0; i < 12; i++)
- A[i] = 0;
-
-- A[i] = 0;
-
-In the JSCOP file the memory is represented as follows.
-
-
-"accesses":
-[{
- -"kind": -"write",
- -"relation": -"{ -Stmt[i] -> A[i] -}"
- }]
-
-- -"kind": -"write",
- -"relation": -"{ -Stmt[i] -> A[i] -}"
- }]
-
-Suppose -we want to perform a transformation such that the following
-code is generated
-
-
for
-(i
-= 0; i < 12; i++)
- - A[0] = i;
-
-- - A[0] = i;
-
-The corresponding JSCOP file represenation would be
-
-
-"accesses":
-[{
- -"kind": -"read",
- -"relation": -"{ -Stmt[i] -> A[0] -}"
- }]
-
-- -"kind": -"read",
- -"relation": -"{ -Stmt[i] -> A[0] -}"
- }]
-
-We need to detect this access function change.
+
+for (i = 0; i < 8; i++) + sum += A[i]; ++Through memory access transformations this loop can be executed in parallel. +It can be transformed to +
+// Create and initialize an array 'tmp' with size 4 +for (i = 0; i < 8; i++) + tmp[i % 4] += A[i]; +sum = tmp[0] + tmp[1] + tmp[2] + tmp[3]; ++ +Optimizers like PluTo can schedule the code such that an outer, parallel +loop is created: +
+parfor (ii = 0; ii < 4; ii++) {
+ tmp[ii] = 0;
+ for (i = ii * 2; i < (ii+1) * 2; i++)
+ tmp[ii] += A[i];
+ }
+sum = tmp[0] + tmp[1] + tmp[2] + tmp[3];
+
+
+TODO
+Step 1
+Polly exports its polyhedral description in a JSCoP file. Define how memory +layout transformations are expressed in Polly and in the JSCOP file. +Example: + +Consider the following loop.
++for (i = 0; i < 12; i++) + A[i] = 0; ++In the JSCOP file the memory accesses are represented as follows. +
+"accesses": [{
+ "kind": "write",
+ "relation": "{ Stmt[i] -> A[i] }"
+}]
+
+To perform a transformation we generate the following code:
++for (i = 0; i < 12; i++) + A[0] = i; ++The representation in the JSCoP file is: +
+"accesses": [{
+ "kind": "read",
+ "relation": "{ Stmt[i] -> A[0] }"
+}]
+
+We need to detect this access function change.