[InstSimplify] fold icmp based on range of abs/nabs

This is a fix for PR39475:
https://bugs.llvm.org/show_bug.cgi?id=39475

We managed to get some of these patterns using computeKnownBits in D47041, but that 
can't be used for nabs(). Instead, put in some range-based logic, so we can fold 
both abs/nabs with icmp with a constant value.

Alive proofs:
https://rise4fun.com/Alive/21r

Name: abs_nsw_is_positive
  %cmp = icmp slt i32 %x, 0
  %negx = sub nsw i32 0, %x
  %abs = select i1 %cmp, i32 %negx, i32 %x
  %r = icmp sgt i32 %abs, -1
    =>
  %r = i1 true
 
Name: abs_nsw_is_not_negative
  %cmp = icmp slt i32 %x, 0
  %negx = sub nsw i32 0, %x
  %abs = select i1 %cmp, i32 %negx, i32 %x
  %r = icmp slt i32 %abs, 0
    =>
  %r = i1 false
 
Name: nabs_is_negative_or_0
  %cmp = icmp slt i32 %x, 0
  %negx = sub i32 0, %x
  %nabs = select i1 %cmp, i32 %x, i32 %negx
  %r = icmp slt i32 %nabs, 1
    =>
  %r = i1 true

Name: nabs_is_not_over_0
  %cmp = icmp slt i32 %x, 0
  %negx = sub i32 0, %x
  %nabs = select i1 %cmp, i32 %x, i32 %negx
  %r = icmp sgt i32 %nabs, 0
    =>
  %r = i1 false

Differential Revision: https://reviews.llvm.org/D53844

llvm-svn: 345717

1 files changed, 42 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 6ff72638512..b1381932e7f 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -2996,6 +2996,45 @@ static Value *simplifyICmpWithBinOp(CmpInst::Predicate Pred, Value *LHS,
   return nullptr;
 }
 
+static Value *simplifyICmpWithAbsNabs(CmpInst::Predicate Pred, Value *Op0,
+                                      Value *Op1) {
+  // We need a comparison with a constant.
+  const APInt *C;
+  if (!match(Op1, m_APInt(C)))
+    return nullptr;
+
+  // matchSelectPattern returns the negation part of an abs pattern in SP1.
+  // If the negate has an NSW flag, abs(INT_MIN) is undefined. Without that
+  // constraint, we can't make a contiguous range for the result of abs.
+  ICmpInst::Predicate AbsPred = ICmpInst::BAD_ICMP_PREDICATE;
+  Value *SP0, *SP1;
+  SelectPatternFlavor SPF = matchSelectPattern(Op0, SP0, SP1).Flavor;
+  if (SPF == SelectPatternFlavor::SPF_ABS &&
+      cast<Instruction>(SP1)->hasNoSignedWrap())
+    // The result of abs(X) is >= 0 (with nsw).
+    AbsPred = ICmpInst::ICMP_SGE;
+  if (SPF == SelectPatternFlavor::SPF_NABS)
+    // The result of -abs(X) is <= 0.
+    AbsPred = ICmpInst::ICMP_SLE;
+
+  if (AbsPred == ICmpInst::BAD_ICMP_PREDICATE)
+    return nullptr;
+
+  // Intersect the range of abs/nabs with the range of this icmp.
+  // If there is no intersection, the icmp must be false.
+  // If the intersection equals the range of abs/nabs, the icmp must be true.
+  APInt Zero = APInt::getNullValue(C->getBitWidth());
+  ConstantRange AbsRange = ConstantRange::makeExactICmpRegion(AbsPred, Zero);
+  ConstantRange CmpRange = ConstantRange::makeExactICmpRegion(Pred, *C);
+  ConstantRange Intersection = AbsRange.intersectWith(CmpRange);
+  if (Intersection.isEmptySet())
+    return getFalse(GetCompareTy(Op0));
+  if (Intersection == AbsRange)
+    return getTrue(GetCompareTy(Op0));
+
+  return nullptr;
+}
+
 /// Simplify integer comparisons where at least one operand of the compare
 /// matches an integer min/max idiom.
 static Value *simplifyICmpWithMinMax(CmpInst::Predicate Pred, Value *LHS,
@@ -3427,6 +3466,9 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
   if (Value *V = simplifyICmpWithMinMax(Pred, LHS, RHS, Q, MaxRecurse))
     return V;
 
+  if (Value *V = simplifyICmpWithAbsNabs(Pred, LHS, RHS))
+    return V;
+
   // Simplify comparisons of related pointers using a powerful, recursive
   // GEP-walk when we have target data available..
   if (LHS->getType()->isPointerTy())