Add a new pass "inductive range check elimination"

IRCE eliminates range checks of the form

  0 <= A * I + B < Length

by splitting a loop's iteration space into three segments in a way
that the check is completely redundant in the middle segment.  As an
example, IRCE will convert

  len = < known positive >
  for (i = 0; i < n; i++) {
    if (0 <= i && i < len) {
      do_something();
    } else {
      throw_out_of_bounds();
    }
  }

to

  len = < known positive >
  limit = smin(n, len)
  // no first segment
  for (i = 0; i < limit; i++) {
    if (0 <= i && i < len) { // this check is fully redundant
      do_something();
    } else {
      throw_out_of_bounds();
    }
  }
  for (i = limit; i < n; i++) {
    if (0 <= i && i < len) {
      do_something();
    } else {
      throw_out_of_bounds();
    }
  }


IRCE can deal with multiple range checks in the same loop (it takes
the intersection of the ranges that will make each of them redundant
individually).

Currently IRCE does not do any profitability analysis.  That is a
TODO.

Please note that the status of this pass is *experimental*, and it is
not part of any default pass pipeline.  Having said that, I will love
to get feedback and general input from people interested in trying
this out.

This pass was originally r226201.  It was reverted because it used C++
features not supported by MSVC 2012.

Differential Revision: http://reviews.llvm.org/D6693

llvm-svn: 226238

llvm/include/llvm/InitializePasses.h

@@ -142,6 +142,7 @@ void initializeIPCPPass(PassRegistry&);
 void initializeIPSCCPPass(PassRegistry&);
 void initializeIVUsersPass(PassRegistry&);
 void initializeIfConverterPass(PassRegistry&);
+void initializeInductiveRangeCheckEliminationPass(PassRegistry&);
 void initializeIndVarSimplifyPass(PassRegistry&);
 void initializeInlineCostAnalysisPass(PassRegistry&);
 void initializeInstCombinerPass(PassRegistry&);

llvm/include/llvm/LinkAllPasses.h

@@ -86,6 +86,7 @@ namespace {
       (void) llvm::createGlobalsModRefPass();
       (void) llvm::createIPConstantPropagationPass();
       (void) llvm::createIPSCCPPass();
+      (void) llvm::createInductiveRangeCheckEliminationPass();
       (void) llvm::createIndVarSimplifyPass();
       (void) llvm::createInstructionCombiningPass();
       (void) llvm::createInternalizePass();

llvm/include/llvm/Transforms/Scalar.h

@@ -96,6 +96,13 @@ FunctionPass *createScalarReplAggregatesPass(signed Threshold = -1,
                                              signed ArrayElementThreshold = -1,
                                              signed ScalarLoadThreshold = -1);
 
+//===----------------------------------------------------------------------===//
+//
+// InductiveRangeCheckElimination - Transform loops to elide range checks on
+// linear functions of the induction variable.
+//
+Pass *createInductiveRangeCheckEliminationPass();
+
 //===----------------------------------------------------------------------===//
 //
 // InductionVariableSimplify - Transform induction variables in a program to all

llvm/lib/Transforms/Scalar/CMakeLists.txt

@@ -9,6 +9,7 @@ add_llvm_library(LLVMScalarOpts
   EarlyCSE.cpp
   FlattenCFGPass.cpp
   GVN.cpp
+  InductiveRangeCheckElimination.cpp
   IndVarSimplify.cpp
   JumpThreading.cpp
   LICM.cpp