[InstSimplify] Simplify saturating add/sub + icmp

If a saturating add/sub has one constant operand, then we can
determine the possible range of outputs it can produce, and simplify
an icmp comparison based on that.

The implementation is based on a similar existing mechanism for
simplifying binary operator + icmps.

Differential Revision: https://reviews.llvm.org/D55735

llvm-svn: 349369

1 files changed, 66 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 120a024ed60..ccf907c144f 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -2630,6 +2630,70 @@ static void setLimitsForBinOp(BinaryOperator &BO, APInt &Lower, APInt &Upper,
   }
 }
 
+/// Some intrinsics with a constant operand have an easy-to-compute range of
+/// outputs. This can be used to fold a comparison to always true or always
+/// false.
+static void setLimitsForIntrinsic(IntrinsicInst &II, APInt &Lower,
+                                  APInt &Upper) {
+  unsigned Width = Lower.getBitWidth();
+  const APInt *C;
+  switch (II.getIntrinsicID()) {
+  case Intrinsic::uadd_sat:
+    // uadd.sat(x, C) produces [C, UINT_MAX].
+    if (match(II.getOperand(0), m_APInt(C)) ||
+        match(II.getOperand(1), m_APInt(C)))
+      Lower = *C;
+    break;
+  case Intrinsic::sadd_sat:
+    if (match(II.getOperand(0), m_APInt(C)) ||
+        match(II.getOperand(1), m_APInt(C))) {
+      if (C->isNegative()) {
+        // sadd.sat(x, -C) produces [SINT_MIN, SINT_MAX + (-C)].
+        Lower = APInt::getSignedMinValue(Width);
+        Upper = APInt::getSignedMaxValue(Width) + *C + 1;
+      } else {
+        // sadd.sat(x, +C) produces [SINT_MIN + C, SINT_MAX].
+        Lower = APInt::getSignedMinValue(Width) + *C;
+        Upper = APInt::getSignedMaxValue(Width) + 1;
+      }
+    }
+    break;
+  case Intrinsic::usub_sat:
+    // usub.sat(C, x) produces [0, C].
+    if (match(II.getOperand(0), m_APInt(C)))
+      Upper = *C + 1;
+    // usub.sat(x, C) produces [0, UINT_MAX - C].
+    else if (match(II.getOperand(1), m_APInt(C)))
+      Upper = APInt::getMaxValue(Width) - *C + 1;
+    break;
+  case Intrinsic::ssub_sat:
+    if (match(II.getOperand(0), m_APInt(C))) {
+      if (C->isNegative()) {
+        // ssub.sat(-C, x) produces [SINT_MIN, -SINT_MIN + (-C)].
+        Lower = APInt::getSignedMinValue(Width);
+        Upper = *C - APInt::getSignedMinValue(Width) + 1;
+      } else {
+        // ssub.sat(+C, x) produces [-SINT_MAX + C, SINT_MAX].
+        Lower = *C - APInt::getSignedMaxValue(Width);
+        Upper = APInt::getSignedMaxValue(Width) + 1;
+      }
+    } else if (match(II.getOperand(1), m_APInt(C))) {
+      if (C->isNegative()) {
+        // ssub.sat(x, -C) produces [SINT_MIN - (-C), SINT_MAX]:
+        Lower = APInt::getSignedMinValue(Width) - *C;
+        Upper = APInt::getSignedMaxValue(Width) + 1;
+      } else {
+        // ssub.sat(x, +C) produces [SINT_MIN, SINT_MAX - C].
+        Lower = APInt::getSignedMinValue(Width);
+        Upper = APInt::getSignedMaxValue(Width) - *C + 1;
+      }
+    }
+    break;
+  default:
+    break;
+  }
+}
+
 static Value *simplifyICmpWithConstant(CmpInst::Predicate Pred, Value *LHS,
                                        Value *RHS, const InstrInfoQuery &IIQ) {
   Type *ITy = GetCompareTy(RHS); // The return type.
@@ -2663,6 +2727,8 @@ static Value *simplifyICmpWithConstant(CmpInst::Predicate Pred, Value *LHS,
   APInt Upper = APInt(Width, 0);
   if (auto *BO = dyn_cast<BinaryOperator>(LHS))
     setLimitsForBinOp(*BO, Lower, Upper, IIQ);
+  else if (auto *II = dyn_cast<IntrinsicInst>(LHS))
+    setLimitsForIntrinsic(*II, Lower, Upper);
 
   ConstantRange LHS_CR =
       Lower != Upper ? ConstantRange(Lower, Upper) : ConstantRange(Width, true);