Hal Finkel c9890f4fe1 [BDCE] Add a bit-tracking DCE pass ... BDCE is a bit-tracking dead code elimination pass. It is based on ADCE (the "aggressive DCE" pass), with the added capability to track dead bits of integer valued instructions and remove those instructions when all of the bits are dead. Currently, it does not actually do this all-bits-dead removal, but rather replaces the instruction's uses with a constant zero, and lets instcombine (and the later run of ADCE) do the rest. Because we essentially get a run of ADCE "for free" while tracking the dead bits, we also do what ADCE does and removes actually-dead instructions as well (this includes instructions newly trivially dead because all bits were dead, but not all such instructions can be removed). The motivation for this is a case like: int __attribute__((const)) foo(int i); int bar(int x) { x |= (4 & foo(5)); x |= (8 & foo(3)); x |= (16 & foo(2)); x |= (32 & foo(1)); x |= (64 & foo(0)); x |= (128& foo(4)); return x >> 4; } As it turns out, if you order the bit-field insertions so that all of the dead ones come last, then instcombine will remove them. However, if you pick some other order (such as the one above), the fact that some of the calls to foo() are useless is not locally obvious, and we don't remove them (without this pass). I did a quick compile-time overhead check using sqlite from the test suite (Release+Asserts). BDCE took ~0.4% of the compilation time (making it about twice as expensive as ADCE). I've not looked at why yet, but we eliminate instructions due to having all-dead bits in: External/SPEC/CFP2006/447.dealII/447.dealII External/SPEC/CINT2006/400.perlbench/400.perlbench External/SPEC/CINT2006/403.gcc/403.gcc MultiSource/Applications/ClamAV/clamscan MultiSource/Benchmarks/7zip/7zip-benchmark llvm-svn: 229462		2015-02-17 01:36:59 +00:00
..
ArgumentPromotion.cpp	Add range adapters predecessors() and successors() for BBs	2015-02-04 19:14:57 +00:00
BarrierNoopPass.cpp
CMakeLists.txt	Use ADDITIONAL_HEADER_DIRS in all LLVM CMake projects.	2015-02-11 03:28:02 +00:00
ConstantMerge.cpp	Repace SmallPtrSet with SmallPtrSetImpl in function arguments to avoid needing to mention the size.	2014-08-21 05:55:13 +00:00
DeadArgumentElimination.cpp	DeadArgElim: aggregate Return assessment properly.	2015-02-11 23:13:11 +00:00
ExtractGV.cpp	Reformat partially, where I touched for whitespace changes.	2014-10-28 11:54:52 +00:00
FunctionAttrs.cpp	[PM] Separate the TargetLibraryInfo object from the immutable pass.	2015-01-15 10:41:28 +00:00
GlobalDCE.cpp	Prologue support	2014-12-03 02:08:38 +00:00
GlobalOpt.cpp	[PM] Separate the TargetLibraryInfo object from the immutable pass.	2015-01-15 10:41:28 +00:00
InlineAlways.cpp	[PM] Split the AssumptionTracker immutable pass into two separate APIs:	2015-01-04 12:03:27 +00:00
Inliner.cpp	Transforms: Canonicalize access to function attributes, NFC	2015-02-14 01:11:29 +00:00
InlineSimple.cpp	[PM] Split the AssumptionTracker immutable pass into two separate APIs:	2015-01-04 12:03:27 +00:00
Internalize.cpp	Use range based for loops to avoid needing to re-mention SmallPtrSet size.	2014-08-24 23:23:06 +00:00
IPConstantPropagation.cpp
IPO.cpp	[PM] Remove the old 'PassManager.h' header file at the top level of	2015-02-13 10:01:29 +00:00
LLVMBuild.txt	Update libdeps since TLI was moved from Target to Analysis in r226078.	2015-01-15 05:21:00 +00:00
LoopExtractor.cpp	[PM] Lift the analyses into the interface for	2015-01-19 03:03:39 +00:00
Makefile
MergeFunctions.cpp	Update SetVector to rely on the underlying set's insert to return a pair<iterator, bool>	2014-11-19 07:49:26 +00:00
PartialInlining.cpp	Add range adapters predecessors() and successors() for BBs	2015-02-04 19:14:57 +00:00
PassManagerBuilder.cpp	[BDCE] Add a bit-tracking DCE pass	2015-02-17 01:36:59 +00:00
PruneEH.cpp	Don't promote asynch EH invokes of nounwind functions to calls	2015-02-11 01:23:16 +00:00
StripDeadPrototypes.cpp
StripSymbols.cpp	IR: Split Metadata from Value	2014-12-09 18:38:53 +00:00