[llvm] [ValueTracking] Handle range attributes (PR #85143)

[Nikita Popov via llvm-commits]

================
@@ -1467,14 +1467,21 @@ static void computeKnownBitsFromOperator(const Operator *I,
     break;
   }
   case Instruction::Call:
-  case Instruction::Invoke:
+  case Instruction::Invoke: {
     // If range metadata is attached to this call, set known bits from that,
     // and then intersect with known bits based on other properties of the
     // function.
     if (MDNode *MD =
             Q.IIQ.getMetadata(cast<Instruction>(I), LLVMContext::MD_range))
       computeKnownBitsFromRangeMetadata(*MD, Known);
-    if (const Value *RV = cast<CallBase>(I)->getReturnedArgOperand()) {
+
+    const CallBase *CB = cast<CallBase>(I);
+
+    const Attribute RangeAttr = CB->getRetAttr(llvm::Attribute::Range);
+    if (RangeAttr.isValid())
+      Known = RangeAttr.getRange().toKnownBits();
----------------
nikic wrote:

```suggestion
    const Attribute RangeAttr = CB->getRetAttr(llvm::Attribute::Range);
    if (std::optional<ConstantRange> Range = CB->getRange())
      Known = Range->toKnownBits();
```
I'd suggest adding a small wrapper for this, given how often we're going to repeat it...

https://github.com/llvm/llvm-project/pull/85143