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Weight loop unrolling counts by nesting depth. Unrolling deeply nested loops tends to cause high

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register pressure and thus excess spills, which we don't currently recover from well.  This should
be re-evaluated in the future if our ability to generate good spills/splits improves.

Partial fix for <rdar://problem/7635585>.

llvm-svn: 114919
This commit is contained in:
Owen Anderson 2010-09-27 22:58:54 +00:00
parent ffbf83e49f
commit 4e6951b92f
1 changed files with 6 additions and 1 deletions
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7
lib/Transforms/Scalar/LoopUnrollPass.cpp
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@ -158,7 +158,12 @@ bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
DEBUG(dbgs() << " Not unrolling loop with inlinable calls.\n");
return false;
}
uint64_t Size = (uint64_t)LoopSize*Count;
// NOTE: We multiply by the loop depth because unrolling inner loops of
// very deep nests tends to result in high register pressure, which we don't
// currently recover from very well. When and if the register allocator/
// spiller improves to compensate, this should be re-evaluated.
uint64_t Size = (uint64_t)LoopSize*Count*L->getLoopDepth();
if (TripCount != 1 && Size > CurrentThreshold) {
DEBUG(dbgs() << " Too large to fully unroll with count: " << Count
<< " because size: " << Size << ">" << CurrentThreshold << "\n");