[llvm] [ValueTracking] Use `ConstantRange::toKnownBits` when computing from Range Metadata; NFC (PR #85574)

[Nikita Popov via llvm-commits]

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@@ -423,24 +423,17 @@ void llvm::computeKnownBitsFromRangeMetadata(const MDNode &Ranges,
   unsigned NumRanges = Ranges.getNumOperands() / 2;
   assert(NumRanges >= 1);
 
-  Known.Zero.setAllBits();
-  Known.One.setAllBits();
+  ConstantRange CR = ConstantRange::getFull(BitWidth);
 
   for (unsigned i = 0; i < NumRanges; ++i) {
     ConstantInt *Lower =
         mdconst::extract<ConstantInt>(Ranges.getOperand(2 * i + 0));
     ConstantInt *Upper =
         mdconst::extract<ConstantInt>(Ranges.getOperand(2 * i + 1));
     ConstantRange Range(Lower->getValue(), Upper->getValue());
-
-    // The first CommonPrefixBits of all values in Range are equal.
-    unsigned CommonPrefixBits =
-        (Range.getUnsignedMax() ^ Range.getUnsignedMin()).countl_zero();
-    APInt Mask = APInt::getHighBitsSet(BitWidth, CommonPrefixBits);
-    APInt UnsignedMax = Range.getUnsignedMax().zextOrTrunc(BitWidth);
-    Known.One &= UnsignedMax & Mask;
-    Known.Zero &= ~UnsignedMax & Mask;
+    CR = CR.intersectWith(Range.zextOrTrunc(BitWidth));
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nikic wrote:

This should be unionWith (see https://llvm.org/docs/LangRef.html#range-metadata). (And initialize to empty above.)

https://github.com/llvm/llvm-project/pull/85574