llvm-6502

mirror of https://github.com/c64scene-ar/llvm-6502.git synced 2024-11-08 04:07:07 +00:00

History

Chandler Carruth de1c9bb450 Remove return heuristics from the static branch probabilities, and 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. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142793 91177308-0d34-0410-b5e6-96231b3b80d8		2011-10-24 12:01:08 +00:00
..
IPA	Simplify; no intended functional change.	2011-10-20 03:23:14 +00:00
AliasAnalysis.cpp
AliasAnalysisCounter.cpp
AliasAnalysisEvaluator.cpp
AliasDebugger.cpp
AliasSetTracker.cpp
Analysis.cpp
BasicAliasAnalysis.cpp	Remove the old atomic instrinsics. autoupgrade functionality is included with this patch.	2011-10-06 23:20:49 +00:00
BlockFrequencyInfo.cpp	Add pass printing support to BlockFrequencyInfo pass. The implementation	2011-10-19 10:12:41 +00:00
BranchProbabilityInfo.cpp	Remove return heuristics from the static branch probabilities, and	2011-10-24 12:01:08 +00:00
CaptureTracking.cpp
CFGPrinter.cpp
CMakeLists.txt
ConstantFolding.cpp
DbgInfoPrinter.cpp
DebugInfo.cpp	Update DebugInfoFinder to match recent debug info encoding changes.	2011-10-17 22:30:34 +00:00
DIBuilder.cpp	Add a new wrapper node for a DILexicalBlock that encapsulates it and a	2011-10-11 22:59:11 +00:00
DominanceFrontier.cpp
DomPrinter.cpp
InlineCost.cpp	A FIXME about block addresses and indirectbr.	2011-10-20 04:05:33 +00:00
InstCount.cpp
InstructionSimplify.cpp
Interval.cpp
IntervalPartition.cpp
IVUsers.cpp	Slightly more useful tracing.	2011-10-13 17:06:38 +00:00
LazyValueInfo.cpp
LibCallAliasAnalysis.cpp
LibCallSemantics.cpp
Lint.cpp
Loads.cpp
LoopDependenceAnalysis.cpp
LoopInfo.cpp
LoopPass.cpp
Makefile
MemDepPrinter.cpp	Enhance the memdep interface so that users can tell the difference between a dependency which cannot be calculated and a path reaching the entry point of the function. This patch introduces isNonFuncLocal, which replaces isUnknown in some cases.	2011-10-13 22:14:57 +00:00
MemoryBuiltins.cpp
MemoryDependenceAnalysis.cpp	Enhance the memdep interface so that users can tell the difference between a dependency which cannot be calculated and a path reaching the entry point of the function. This patch introduces isNonFuncLocal, which replaces isUnknown in some cases.	2011-10-13 22:14:57 +00:00
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	Reapply r142781 with fix. Original message:	2011-10-24 06:57:05 +00:00
ScalarEvolutionAliasAnalysis.cpp
ScalarEvolutionExpander.cpp	Fix SCEVExpander assert during LSR: "argument of incompatible type".	2011-10-15 06:19:55 +00:00
ScalarEvolutionNormalization.cpp	Fix memory corruption I introduced a few checkins ago.	2011-10-13 18:49:23 +00:00
SparsePropagation.cpp
Trace.cpp
TypeBasedAliasAnalysis.cpp
ValueTracking.cpp	"@string = constant i8 0" is a value i8* string of length zero. Analyze that	2011-10-20 00:34:35 +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))

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