[clang] [clang] Implement __builtin_stdc_rotate_left, __builtin_stdc_rotate_right (PR #160259)

[NagaChaitanya Vellanki via cfe-commits]

================
@@ -2492,12 +2492,36 @@ RValue CodeGenFunction::emitRotate(const CallExpr *E, bool IsRotateRight) {
   // The builtin's shift arg may have a different type than the source arg and
   // result, but the LLVM intrinsic uses the same type for all values.
   llvm::Type *Ty = Src->getType();
+
+  unsigned BitWidth = Ty->getIntegerBitWidth();
+  llvm::Value *BitWidthVal = ConstantInt::get(Ty, BitWidth);
+
+  bool IsShiftAmtSigned = E->getArg(1)->getType()->isSignedIntegerType();
+
+  if (IsShiftAmtSigned) {
+    // converting BitWidth to the type of ShiftAmt
+    llvm::Type *ShiftTy = ShiftAmt->getType();
+    llvm::Value *BitWidthInShiftTy = ConstantInt::get(ShiftTy, BitWidth);
+
+    // if ShiftAmt is negative, normalize ShiftAmt to [0, BitWidth - 1]
+    llvm::Value *RemResult = Builder.CreateSRem(ShiftAmt, BitWidthInShiftTy);
+    llvm::Value *PositiveShift =
+        Builder.CreateAdd(RemResult, BitWidthInShiftTy);
+
+    llvm::Value *Zero = ConstantInt::get(ShiftTy, 0);
+    llvm::Value *IsRemNegative = Builder.CreateICmpSLT(RemResult, Zero);
+
+    ShiftAmt = Builder.CreateSelect(IsRemNegative, PositiveShift, RemResult);
+  }
+
+  // converting ShiftAmt to Src type, ShiftAmt is positive
   ShiftAmt = Builder.CreateIntCast(ShiftAmt, Ty, false);
+  ShiftAmt = Builder.CreateURem(ShiftAmt, BitWidthVal);
----------------
chaitanyav wrote:

i am able to reproduce the issue and working on the fix

```bash
nagac at macbook-pro  ~/scratch/llvm_test  ./test_rotate_runtime
Static assertions passed - compile-time evaluation works!
Runtime: 341 rotate left 1 = 171 (expected 171)
Runtime: 341 rotate right 1 = 426 (expected 426)
Runtime: 1 rotate left 36 (_BitInt(37)) = 68719476736 (expected 68719476736)
All tests passed! Both compile-time and runtime paths work correctly.
```

https://github.com/llvm/llvm-project/pull/160259