[SCCP] Merge return range attributes (#105998)

Take the intersection of the existing range attribute for the return
value and the inferred range.

Author: pedroclobo

llvm/lib/Transforms/IPO/SCCP.cpp

@@ -289,12 +289,10 @@ static bool runIPSCCP(
       if (ReturnValue.isConstantRangeIncludingUndef())
         continue;
 
-      // Do not touch existing attribute for now.
-      // TODO: We should be able to take the intersection of the existing
-      // attribute and the inferred range.
+      // Take the intersection of the existing attribute and the inferred range.
+      ConstantRange CR = ReturnValue.getConstantRange();
       if (F->hasRetAttribute(Attribute::Range))
-        continue;
-      auto &CR = ReturnValue.getConstantRange();
+        CR = CR.intersectWith(F->getRetAttribute(Attribute::Range).getRange());
       F->addRangeRetAttr(CR);
       continue;
     }

llvm/test/Transforms/SCCP/ip-constant-ranges-intersection.ll

@@ -0,0 +1,16 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -passes=ipsccp -S | FileCheck %s
+
+declare range(i32 0, 20) i32 @callee(i32)
+
+define range(i32 10, 30) i32 @caller(i32 %x) {
+; CHECK-LABEL: define range(i32 10, 20) i32 @caller(
+; CHECK-SAME: i32 [[X:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    [[CALL:%.*]] = call range(i32 0, 20) i32 @callee()
+; CHECK-NEXT:    ret i32 [[CALL]]
+;
+entry:
+  %call = call range(i32 0, 20) i32 @callee()
+  ret i32 %call
+}