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Tweak some comments that referred to the old bias computations.

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llvm-svn: 185395
This commit is contained in:
Jakob Stoklund Olesen 2013-07-01 23:36:37 +00:00
parent 90cbe0a2da
commit 7abfdbda73
1 changed files with 13 additions and 13 deletions
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26
lib/CodeGen/SpillPlacement.cpp
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@ -79,17 +79,14 @@ struct SpillPlacement::Node {
BlockFrequency BiasP;
/// Value - Output value of this node computed from the Bias and links.
/// This is always in the range [-1;1]. A positive number means the variable
/// should go in a register through this bundle.
/// This is always on of the values {-1, 0, 1}. A positive number means the
/// variable should go in a register through this bundle.
int Value;
typedef SmallVector<std::pair<BlockFrequency, unsigned>, 4> LinkVector;
/// Links - (Weight, BundleNo) for all transparent blocks connecting to other
/// bundles. The weights are all positive and add up to at most 2, weights
/// from ingoing and outgoing nodes separately add up to a most 1. The weight
/// sum can be less than 2 when the variable is not live into / out of some
/// connected basic blocks.
/// bundles. The weights are all positive block frequencies.
LinkVector Links;
/// SumLinkWeights - Cached sum of the weights of all links + ThresHold.
@ -162,12 +159,14 @@ struct SpillPlacement::Node {
SumP += I->first;
}
// The weighted sum is going to be in the range [-2;2]. Ideally, we should
// simply set Value = sign(Sum), but we will add a dead zone around 0 for
// two reasons:
// Each weighted sum is going to be less than the total frequency of the
// bundle. Ideally, we should simply set Value = sign(SumP - SumN), but we
// will add a dead zone around 0 for two reasons:
//
// 1. It avoids arbitrary bias when all links are 0 as is possible during
// initial iterations.
// 2. It helps tame rounding errors when the links nominally sum to 0.
//
bool Before = preferReg();
if (SumN >= SumP + Threshold)
Value = -1;
@ -215,10 +214,11 @@ void SpillPlacement::activate(unsigned n) {
// landing pads, or loops with many 'continue' statements. It is difficult to
// allocate registers when so many different blocks are involved.
//
// Give a small negative bias to large bundles such that 1/32 of the
// connected blocks need to be interested before we consider expanding the
// region through the bundle. This helps compile time by limiting the number
// of blocks visited and the number of links in the Hopfield network.
// Give a small negative bias to large bundles such that a substantial
// fraction of the connected blocks need to be interested before we consider
// expanding the region through the bundle. This helps compile time by
// limiting the number of blocks visited and the number of links in the
// Hopfield network.
if (bundles->getBlocks(n).size() > 100) {
nodes[n].BiasP = 0;
nodes[n].BiasN = (BlockFrequency::getEntryFrequency() / 16);