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d04f838629
introduce no-return or unreachable heuristics. The return heuristics from the Ball and Larus paper don't work well in practice as they pessimize early return paths. The only good hitrate return heuristics are those for: - NULL return - Constant return - negative integer return Only the last of these three can possibly require significant code for the returning block, and even the last is fairly rare and usually also a constant. As a consequence, even for the cold return paths, there is little code on that return path, and so little code density to be gained by sinking it. The places where sinking these blocks is valuable (inner loops) will already be weighted appropriately as the edge is a loop-exit branch. All of this aside, early returns are nearly as common as all three of these return categories, and should actually be predicted as taken! Rather than muddy the waters of the static predictions, just remain silent on returns and let the CFG itself dictate any layout or other issues. However, the return heuristic was flagging one very important case: unreachable. Unfortunately it still gave a 1/4 chance of the branch-to-unreachable occuring. It also didn't do a rigorous job of finding those blocks which post-dominate an unreachable block. This patch builds a more powerful analysis that should flag all branches to blocks known to then reach unreachable. It also has better worst-case runtime complexity by not looping through successors for each block. The previous code would perform an N^2 walk in the event of a single entry block branching to N successors with a switch where each successor falls through to the next and they finally fall through to a return. Test case added for noreturn heuristics. Also doxygen comments improved along the way. llvm-svn: 142793 |
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|---|---|---|
| .. | ||
| IPA | ||
| AliasAnalysis.cpp | ||
| AliasAnalysisCounter.cpp | ||
| AliasAnalysisEvaluator.cpp | ||
| AliasDebugger.cpp | ||
| AliasSetTracker.cpp | ||
| Analysis.cpp | ||
| BasicAliasAnalysis.cpp | ||
| BlockFrequencyInfo.cpp | ||
| BranchProbabilityInfo.cpp | ||
| CaptureTracking.cpp | ||
| CFGPrinter.cpp | ||
| CMakeLists.txt | ||
| ConstantFolding.cpp | ||
| DbgInfoPrinter.cpp | ||
| DebugInfo.cpp | ||
| DIBuilder.cpp | ||
| DominanceFrontier.cpp | ||
| DomPrinter.cpp | ||
| InlineCost.cpp | ||
| InstCount.cpp | ||
| InstructionSimplify.cpp | ||
| Interval.cpp | ||
| IntervalPartition.cpp | ||
| IVUsers.cpp | ||
| LazyValueInfo.cpp | ||
| LibCallAliasAnalysis.cpp | ||
| LibCallSemantics.cpp | ||
| Lint.cpp | ||
| Loads.cpp | ||
| LoopDependenceAnalysis.cpp | ||
| LoopInfo.cpp | ||
| LoopPass.cpp | ||
| Makefile | ||
| MemDepPrinter.cpp | ||
| MemoryBuiltins.cpp | ||
| MemoryDependenceAnalysis.cpp | ||
| ModuleDebugInfoPrinter.cpp | ||
| NoAliasAnalysis.cpp | ||
| PathNumbering.cpp | ||
| PathProfileInfo.cpp | ||
| PathProfileVerifier.cpp | ||
| PHITransAddr.cpp | ||
| PostDominators.cpp | ||
| ProfileEstimatorPass.cpp | ||
| ProfileInfo.cpp | ||
| ProfileInfoLoader.cpp | ||
| ProfileInfoLoaderPass.cpp | ||
| ProfileVerifierPass.cpp | ||
| README.txt | ||
| RegionInfo.cpp | ||
| RegionPass.cpp | ||
| RegionPrinter.cpp | ||
| ScalarEvolution.cpp | ||
| ScalarEvolutionAliasAnalysis.cpp | ||
| ScalarEvolutionExpander.cpp | ||
| ScalarEvolutionNormalization.cpp | ||
| SparsePropagation.cpp | ||
| Trace.cpp | ||
| TypeBasedAliasAnalysis.cpp | ||
| ValueTracking.cpp | ||
Analysis Opportunities:
//===---------------------------------------------------------------------===//
In test/Transforms/LoopStrengthReduce/quadradic-exit-value.ll, the
ScalarEvolution expression for %r is this:
{1,+,3,+,2}<loop>
Outside the loop, this could be evaluated simply as (%n * %n), however
ScalarEvolution currently evaluates it as
(-2 + (2 * (trunc i65 (((zext i64 (-2 + %n) to i65) * (zext i64 (-1 + %n) to i65)) /u 2) to i64)) + (3 * %n))
In addition to being much more complicated, it involves i65 arithmetic,
which is very inefficient when expanded into code.
//===---------------------------------------------------------------------===//
In formatValue in test/CodeGen/X86/lsr-delayed-fold.ll,
ScalarEvolution is forming this expression:
((trunc i64 (-1 * %arg5) to i32) + (trunc i64 %arg5 to i32) + (-1 * (trunc i64 undef to i32)))
This could be folded to
(-1 * (trunc i64 undef to i32))
//===---------------------------------------------------------------------===//