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author | Hal Finkel <hfinkel@anl.gov> | 2013-11-16 23:59:05 +0000 |
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committer | Hal Finkel <hfinkel@anl.gov> | 2013-11-16 23:59:05 +0000 |
commit | bebe48dbfe00078329341945bfb11f778ace6d12 (patch) | |
tree | 5efe3810ad3db10985cd43351ed4a7bd83727908 /include/llvm | |
parent | 0ccb37a7339883e1fd090beadc2deb1ce40ea7d4 (diff) | |
download | external_llvm-bebe48dbfe00078329341945bfb11f778ace6d12.zip external_llvm-bebe48dbfe00078329341945bfb11f778ace6d12.tar.gz external_llvm-bebe48dbfe00078329341945bfb11f778ace6d12.tar.bz2 |
Add a loop rerolling pass
This adds a loop rerolling pass: the opposite of (partial) loop unrolling. The
transformation aims to take loops like this:
for (int i = 0; i < 3200; i += 5) {
a[i] += alpha * b[i];
a[i + 1] += alpha * b[i + 1];
a[i + 2] += alpha * b[i + 2];
a[i + 3] += alpha * b[i + 3];
a[i + 4] += alpha * b[i + 4];
}
and turn them into this:
for (int i = 0; i < 3200; ++i) {
a[i] += alpha * b[i];
}
and loops like this:
for (int i = 0; i < 500; ++i) {
x[3*i] = foo(0);
x[3*i+1] = foo(0);
x[3*i+2] = foo(0);
}
and turn them into this:
for (int i = 0; i < 1500; ++i) {
x[i] = foo(0);
}
There are two motivations for this transformation:
1. Code-size reduction (especially relevant, obviously, when compiling for
code size).
2. Providing greater choice to the loop vectorizer (and generic unroller) to
choose the unrolling factor (and a better ability to vectorize). The loop
vectorizer can take vector lengths and register pressure into account when
choosing an unrolling factor, for example, and a pre-unrolled loop limits that
choice. This is especially problematic if the manual unrolling was optimized
for a machine different from the current target.
The current implementation is limited to single basic-block loops only. The
rerolling recognition should work regardless of how the loop iterations are
intermixed within the loop body (subject to dependency and side-effect
constraints), but the significant restriction is that the order of the
instructions in each iteration must be identical. This seems sufficient to
capture all current use cases.
This pass is not currently enabled by default at any optimization level.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194939 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'include/llvm')
-rw-r--r-- | include/llvm/InitializePasses.h | 1 | ||||
-rw-r--r-- | include/llvm/LinkAllPasses.h | 1 | ||||
-rw-r--r-- | include/llvm/Transforms/Scalar.h | 6 |
3 files changed, 8 insertions, 0 deletions
diff --git a/include/llvm/InitializePasses.h b/include/llvm/InitializePasses.h index acfe3bd..aefb3c0 100644 --- a/include/llvm/InitializePasses.h +++ b/include/llvm/InitializePasses.h @@ -162,6 +162,7 @@ void initializeLoopRotatePass(PassRegistry&); void initializeLoopSimplifyPass(PassRegistry&); void initializeLoopStrengthReducePass(PassRegistry&); void initializeGlobalMergePass(PassRegistry&); +void initializeLoopRerollPass(PassRegistry&); void initializeLoopUnrollPass(PassRegistry&); void initializeLoopUnswitchPass(PassRegistry&); void initializeLoopIdiomRecognizePass(PassRegistry&); diff --git a/include/llvm/LinkAllPasses.h b/include/llvm/LinkAllPasses.h index d97e54c..8183fa2 100644 --- a/include/llvm/LinkAllPasses.h +++ b/include/llvm/LinkAllPasses.h @@ -91,6 +91,7 @@ namespace { (void) llvm::createLoopExtractorPass(); (void) llvm::createLoopSimplifyPass(); (void) llvm::createLoopStrengthReducePass(); + (void) llvm::createLoopRerollPass(); (void) llvm::createLoopUnrollPass(); (void) llvm::createLoopUnswitchPass(); (void) llvm::createLoopIdiomPass(); diff --git a/include/llvm/Transforms/Scalar.h b/include/llvm/Transforms/Scalar.h index 5d06157..1521c4c 100644 --- a/include/llvm/Transforms/Scalar.h +++ b/include/llvm/Transforms/Scalar.h @@ -145,6 +145,12 @@ Pass *createLoopUnrollPass(int Threshold = -1, int Count = -1, //===----------------------------------------------------------------------===// // +// LoopReroll - This pass is a simple loop rerolling pass. +// +Pass *createLoopRerollPass(); + +//===----------------------------------------------------------------------===// +// // LoopRotate - This pass is a simple loop rotating pass. // Pass *createLoopRotatePass(); |