Jim Grosbach dc2dc390f6 LowerSwitch: track bounding range for the condition tree. ... When LowerSwitch transforms a switch instruction into a tree of ifs it is actually performing a binary search into the various case ranges, to see if the current value falls into one cases range of values. So, if we have a program with something like this: switch (a) { case 0: do0(); break; case 1: do1(); break; case 2: do2(); break; default: break; } the code produced is something like this: if (a < 1) { if (a == 0) { do0(); } } else { if (a < 2) { if (a == 1) { do1(); } } else { if (a == 2) { do2(); } } } This code is inefficient because the check (a == 1) to execute do1() is not needed. The reason is that because we already checked that (a >= 1) initially by checking that also (a < 2) we basically already inferred that (a == 1) without the need of an extra basic block spawned to check if actually (a == 1). The patch addresses this problem by keeping track of already checked bounds in the LowerSwitch algorithm, so that when the time arrives to produce a Leaf Block that checks the equality with the case value / range the algorithm can decide if that block is really needed depending on the already checked bounds . For example, the above with "a = 1" would work like this: the bounds start as LB: NONE , UB: NONE as (a < 1) is emitted the bounds for the else path become LB: 1 UB: NONE. This happens because by failing the test (a < 1) we know that the value "a" cannot be smaller than 1 if we enter the else branch. After the emitting the check (a < 2) the bounds in the if branch become LB: 1 UB: 1. This is because by checking that "a" is smaller than 2 then the upper bound becomes 2 - 1 = 1. When it is time to emit the leaf block for "case 1:" we notice that 1 can be squeezed exactly in between the LB and UB, which means that if we arrived to that block there is no need to emit a block that checks if (a == 1). Patch by: Marcello Maggioni <hayarms@gmail.com> git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211038 91177308-0d34-0410-b5e6-96231b3b80d8		2014-06-16 16:55:20 +00:00
..
AddDiscriminators.cpp
ASanStackFrameLayout.cpp
BasicBlockUtils.cpp
BreakCriticalEdges.cpp
BuildLibCalls.cpp
BypassSlowDivision.cpp
CloneFunction.cpp	Revert "Patch for function cloning to inline all blocks whose address is taken"	2014-05-19 16:04:10 +00:00
CloneModule.cpp	Allow alias to point to an arbitrary ConstantExpr.	2014-06-03 02:41:57 +00:00
CMakeLists.txt
CmpInstAnalysis.cpp
CodeExtractor.cpp
CtorUtils.cpp
DemoteRegToStack.cpp
FlattenCFG.cpp
GlobalStatus.cpp
InlineFunction.cpp	Fix line numbers for code inlined from __nodebug__ functions.	2014-06-09 09:09:19 +00:00
InstructionNamer.cpp
IntegerDivision.cpp
LCSSA.cpp
LLVMBuild.txt
Local.cpp
LoopSimplify.cpp
LoopUnroll.cpp	LCSSA should be performed on the outermost affected loop while unrolling loop.	2014-05-29 06:47:23 +00:00
LoopUnrollRuntime.cpp
LowerExpectIntrinsic.cpp
LowerInvoke.cpp
LowerSwitch.cpp	LowerSwitch: track bounding range for the condition tree.	2014-06-16 16:55:20 +00:00
Makefile
Mem2Reg.cpp
MetaRenamer.cpp
ModuleUtils.cpp
PromoteMemoryToRegister.cpp
SimplifyCFG.cpp	Make bitcast, extractelement, and insertelement considered cheap for speculation.	2014-05-30 18:34:43 +00:00
SimplifyIndVar.cpp
SimplifyInstructions.cpp
SimplifyLibCalls.cpp	Add support for missed and analysis optimization remarks.	2014-05-22 14:19:46 +00:00
SpecialCaseList.cpp	Remove 'using std::errro_code' from lib.	2014-06-13 02:24:39 +00:00
SSAUpdater.cpp
UnifyFunctionExitNodes.cpp
Utils.cpp
ValueMapper.cpp