[llvm] [ValueTracking] Add subtraction support for setLimitsForBinOp (PR #143618)

[Yingwei Zheng via llvm-commits]

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@@ -9576,15 +9576,48 @@ static void setLimitsForBinOp(const BinaryOperator &BO, APInt &Lower,
   unsigned Width = Lower.getBitWidth();
   const APInt *C;
   switch (BO.getOpcode()) {
-  case Instruction::Add:
-    if (match(BO.getOperand(1), m_APInt(C)) && !C->isZero()) {
+  case Instruction::Sub:
+    if (match(BO.getOperand(0), m_APInt(C))) {
       bool HasNSW = IIQ.hasNoSignedWrap(&BO);
       bool HasNUW = IIQ.hasNoUnsignedWrap(&BO);
 
       // If the caller expects a signed compare, then try to use a signed range.
       // Otherwise if both no-wraps are set, use the unsigned range because it
       // is never larger than the signed range. Example:
-      // "add nuw nsw i8 X, -2" is unsigned [254,255] vs. signed [-128, 125].
+      // "sub nuw nsw i8 -2, x" is unsigned [0, 254] vs. signed [-128, 126].
+      // "sub nuw nsw i8 2, x" is unsigned [0, 2] vs. signed [-125, 127].
+      if (PreferSignedRange && HasNSW && HasNUW)
+        HasNUW = false;
+
+      if (HasNUW) {
+        // 'sub nuw c, x' produces [0, C].
+        Upper = *C + 1;
+      } else if (HasNSW) {
+        if (C->isNegative()) {
+          // 'sub nsw -C, x' produces [SINT_MIN, SINT_MAX - (C - 1)].
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dtcxzyw wrote:

Can you please update comments as well?


https://github.com/llvm/llvm-project/pull/143618