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Make LoopUnswitch's cost estimation count Instructions, rather than

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BasicBlocks, so that it doesn't blindly procede in the presence of
large individual BasicBlocks. This addresses a class of code-size
expansion problems.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@83992 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman 2009-10-13 17:50:43 +00:00
parent 61358ab843
commit b24f6c7495
1 changed files with 7 additions and 19 deletions
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26
lib/Transforms/Scalar/LoopUnswitch.cpp
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@ -56,9 +56,11 @@ STATISTIC(NumSelects , "Number of selects unswitched");
STATISTIC(NumTrivial , "Number of unswitches that are trivial"); STATISTIC(NumTrivial , "Number of unswitches that are trivial");
STATISTIC(NumSimplify, "Number of simplifications of unswitched code"); STATISTIC(NumSimplify, "Number of simplifications of unswitched code");
// The specific value of 50 here was chosen based only on intuition and a
// few specific examples.
static cl::opt<unsigned> static cl::opt<unsigned>
Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"), Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
cl::init(10), cl::Hidden); cl::init(50), cl::Hidden);
namespace { namespace {
class LoopUnswitch : public LoopPass { class LoopUnswitch : public LoopPass {
@ -406,27 +408,13 @@ unsigned LoopUnswitch::getLoopUnswitchCost(Value *LIC) {
if (IsTrivialUnswitchCondition(LIC)) if (IsTrivialUnswitchCondition(LIC))
return 0; return 0;
// FIXME: This is really overly conservative. However, more liberal // FIXME: This is really overly conservative and brain dead.
// estimations have thus far resulted in excessive unswitching, which is bad
// both in compile time and in code size. This should be replaced once
// someone figures out how a good estimation.
return currentLoop->getBlocks().size();
unsigned Cost = 0; unsigned Cost = 0;
// FIXME: this is brain dead. It should take into consideration code
// shrinkage.
for (Loop::block_iterator I = currentLoop->block_begin(), for (Loop::block_iterator I = currentLoop->block_begin(),
E = currentLoop->block_end(); E = currentLoop->block_end();
I != E; ++I) { I != E; ++I)
BasicBlock *BB = *I; // Count instructions.
// Do not include empty blocks in the cost calculation. This happen due to Cost += (*I)->size();
// loop canonicalization and will be removed.
if (BB->begin() == BasicBlock::iterator(BB->getTerminator()))
continue;
// Count basic blocks.
++Cost;
}
return Cost; return Cost;
} }