llvm-mirror

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History

Philip Reames 6b5f658b6c Infer known bits from dominating conditions This patch adds limited support in ValueTracking for inferring known bits of a value from conditional expressions which must be true to reach the instruction we're trying to optimize. At this time, the feature is off by default. Once landed, I'm hoping for feedback from others on both profitability and compile time impact. Forms of conditional value propagation have been tried in LLVM before and have failed due to compile time problems. In an attempt to side step that, this patch only considers conditions where the edge leaving the branch dominates the context instruction. It does not attempt full dataflow. Even with that restriction, it handles many interesting cases: * Early exits from functions * Early exits from loops (for context instructions in the loop and after the check) * Conditions which control entry into loops, including multi-version loops (such as those produced during vectorization, IRCE, loop unswitch, etc..) Possible applications include optimizing using information provided by constructs such as: preconditions, assumptions, null checks, & range checks. This patch implements two approaches to the problem that need further benchmarking. Approach 1 is to directly walk the dominator tree looking for interesting conditions. Approach 2 is to inspect other uses of the value being queried for interesting comparisons. From initial benchmarking, it appears that Approach 2 is faster than Approach 1, but this needs to be further validated. Differential Revision: http://reviews.llvm.org/D7708 llvm-svn: 231879		2015-03-10 22:43:20 +00:00
..
IPA	removed function names from comments; NFC	2015-03-10 03:48:14 +00:00
AliasAnalysis.cpp	DataLayout is mandatory, update the API to reflect it with references.	2015-03-10 02:37:25 +00:00
AliasAnalysisCounter.cpp
AliasAnalysisEvaluator.cpp
AliasDebugger.cpp
AliasSetTracker.cpp
Analysis.cpp
AssumptionCache.cpp
BasicAliasAnalysis.cpp	DataLayout is mandatory, update the API to reflect it with references.	2015-03-10 02:37:25 +00:00
BlockFrequencyInfo.cpp
BlockFrequencyInfoImpl.cpp
BranchProbabilityInfo.cpp
CaptureTracking.cpp
CFG.cpp
CFGPrinter.cpp
CFLAliasAnalysis.cpp	Added ConstantExpr support to CFLAA.	2015-03-10 02:58:15 +00:00
CGSCCPassManager.cpp
CMakeLists.txt
CodeMetrics.cpp
ConstantFolding.cpp	DataLayout is mandatory, update the API to reflect it with references.	2015-03-10 02:37:25 +00:00
CostModel.cpp
Delinearization.cpp
DependenceAnalysis.cpp	Fix a memory corruption in Dependency Analysis.	2015-03-10 14:32:02 +00:00
DominanceFrontier.cpp
DomPrinter.cpp
InstCount.cpp
InstructionSimplify.cpp	DataLayout is mandatory, update the API to reflect it with references.	2015-03-10 02:37:25 +00:00
Interval.cpp
IntervalPartition.cpp
IVUsers.cpp	DataLayout is mandatory, update the API to reflect it with references.	2015-03-10 02:37:25 +00:00
LazyCallGraph.cpp
LazyValueInfo.cpp	DataLayout is mandatory, update the API to reflect it with references.	2015-03-10 02:37:25 +00:00
LibCallAliasAnalysis.cpp
LibCallSemantics.cpp
Lint.cpp	DataLayout is mandatory, update the API to reflect it with references.	2015-03-10 02:37:25 +00:00
LLVMBuild.txt
Loads.cpp	DataLayout is mandatory, update the API to reflect it with references.	2015-03-10 02:37:25 +00:00
LoopAccessAnalysis.cpp	[LoopAccesses] Add debug message to indicate the result of the analysis	2015-03-10 21:47:39 +00:00
LoopInfo.cpp
LoopPass.cpp
Makefile
MemDepPrinter.cpp
MemDerefPrinter.cpp	DataLayout is mandatory, update the API to reflect it with references.	2015-03-10 02:37:25 +00:00
MemoryBuiltins.cpp	DataLayout is mandatory, update the API to reflect it with references.	2015-03-10 02:37:25 +00:00
MemoryDependenceAnalysis.cpp	DataLayout is mandatory, update the API to reflect it with references.	2015-03-10 02:37:25 +00:00
ModuleDebugInfoPrinter.cpp
NoAliasAnalysis.cpp
PHITransAddr.cpp
PostDominators.cpp
PtrUseVisitor.cpp
README.txt
RegionInfo.cpp
RegionPass.cpp
RegionPrinter.cpp
ScalarEvolution.cpp	DataLayout is mandatory, update the API to reflect it with references.	2015-03-10 02:37:25 +00:00
ScalarEvolutionAliasAnalysis.cpp
ScalarEvolutionExpander.cpp	DataLayout is mandatory, update the API to reflect it with references.	2015-03-10 02:37:25 +00:00
ScalarEvolutionNormalization.cpp
ScopedNoAliasAA.cpp
SparsePropagation.cpp
StratifiedSets.h
TargetLibraryInfo.cpp	Make static variables const if possible. Makes them go into a read-only section.	2015-03-08 16:07:39 +00:00
TargetTransformInfo.cpp	Do not restrict interleaved unrolling to small loops, depending on the target.	2015-03-06 23:12:04 +00:00
Trace.cpp
TypeBasedAliasAnalysis.cpp
ValueTracking.cpp	Infer known bits from dominating conditions	2015-03-10 22:43:20 +00:00

README.txt

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))

//===---------------------------------------------------------------------===//