[ValueTracking] Enabling ValueTracking patch by default (recommit). P…

…art 2.

 
The original patch was an improvement to IR ValueTracking on non-negative
integers. It has been checked in to trunk (D18777, r284022). But was disabled by
default due to performance regressions.
Perf impact has improved. The patch would be enabled by default.
 
Reviewers: reames, hfinkel
 
Differential Revision: https://reviews.llvm.org/D34101
 
Patch by: Olga Chupina <olga.chupina@intel.com>

llvm-svn: 310583

llvm/lib/Analysis/ValueTracking.cpp

@@ -54,12 +54,6 @@ const unsigned MaxDepth = 6;
 static cl::opt<unsigned> DomConditionsMaxUses("dom-conditions-max-uses",
                                               cl::Hidden, cl::init(20));
 
-// This optimization is known to cause performance regressions is some cases,
-// keep it under a temporary flag for now.
-static cl::opt<bool>
-DontImproveNonNegativePhiBits("dont-improve-non-negative-phi-bits",
-                              cl::Hidden, cl::init(true));
-
 /// Returns the bitwidth of the given scalar or pointer type. For vector types,
 /// returns the element type's bitwidth.
 static unsigned getBitWidth(Type *Ty, const DataLayout &DL) {
@@ -1258,9 +1252,6 @@ static void computeKnownBitsFromOperator(const Operator *I, KnownBits &Known,
           Known.Zero.setLowBits(std::min(Known2.countMinTrailingZeros(),
                                          Known3.countMinTrailingZeros()));
 
-          if (DontImproveNonNegativePhiBits)
-            break;
-
           auto *OverflowOp = dyn_cast<OverflowingBinaryOperator>(LU);
           if (OverflowOp && OverflowOp->hasNoSignedWrap()) {
             // If initial value of recurrence is nonnegative, and we are adding

llvm/test/Analysis/ValueTracking/non-negative-phi-bits.ll

@@ -0,0 +1,27 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -instcombine < %s -S | FileCheck %s
+
+define void @test() #0 {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[INDVARS_IV:%.*]] = phi i64 [ 0, [[ENTRY:%.*]] ], [ [[INDVARS_IV_NEXT:%.*]], [[FOR_BODY]] ]
+; CHECK-NEXT:    [[INDVARS_IV_NEXT]] = add nuw nsw i64 [[INDVARS_IV]], 1
+; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ult i64 [[INDVARS_IV_NEXT]], 40
+; CHECK-NEXT:    br i1 [[EXITCOND]], label [[FOR_END:%.*]], label [[FOR_BODY]]
+; CHECK:       for.end:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %indvars.iv.next = add nsw i64 %indvars.iv, 1
+  %exitcond = icmp slt i64 %indvars.iv.next, 40
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body
+  ret void
+}