[clang] Implement __builtin_stdc_rotate_left and __builtin_stdc_rotate_right

chaitanyav · chaitanyav · commit d799a6631fa1 · 2025-10-06T20:34:12.000-07:00
This patch adds type-generic rotate builtins that accept any unsigned integer type. These builtins provide: - Support for all unsigned integer types, including _BitInt - Constexpr evaluation capability - Automatic normalization of rotation counts modulo the bit-width - Proper handling of negative rotation counts (converted to equivalent positive rotations in the opposite direction) - Implicit conversion support for both arguments, allowing bool, float, and types with conversion operators (not limited to record types) The builtins follow C23 naming conventions. Resolves #122819
diff --git a/clang/docs/LanguageExtensions.rst b/clang/docs/LanguageExtensions.rst
@@ -3680,6 +3680,41 @@ the arguments. Both arguments and the result have the bitwidth specified
 by the name of the builtin. These builtins can be used within constant
 expressions.
 
+``__builtin_stdc_rotate_left`` and ``__builtin_stdc_rotate_right``
+------------------------------------------------------------------
+
+**Syntax**:
+
+.. code-block:: c
+
+    T __builtin_stdc_rotate_left(T value, count)
+    T __builtin_stdc_rotate_right(T value, count)
+
+where ``T`` is any unsigned integer type and ``count`` is any integer type.
+
+**Description**:
+
+These builtins rotate the bits in ``value`` by ``count`` positions. The
+``__builtin_stdc_rotate_left`` builtin rotates bits to the left, while
+``__builtin_stdc_rotate_right`` rotates bits to the right. The first
+argument (``value``) must be an unsigned integer type, including ``_BitInt`` types.
+The second argument (``count``) can be any integer type. The rotation count is
+normalized modulo the bit-width of the value being rotated, with negative
+counts converted to equivalent positive rotations (e.g., rotating left
+by ``-1`` is equivalent to rotating left by ``BitWidth-1``). These builtins can
+be used within constant expressions.
+
+**Example of use**:
+
+.. code-block:: c
+
+  unsigned char rotated_left = __builtin_stdc_rotate_left((unsigned char)0xB1, 3);
+  unsigned int rotated_right = __builtin_stdc_rotate_right(0x12345678U, 8);
+
+  unsigned char neg_rotate = __builtin_stdc_rotate_left((unsigned char)0xB1, -1);
+
+  unsigned _BitInt(17) rotated_odd = __builtin_stdc_rotate_left(0x1ABCD, 5);
+
 ``__builtin_unreachable``
 -------------------------
 
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
@@ -255,6 +255,11 @@ Non-comprehensive list of changes in this release
 
 - Fixed a crash when the second argument to ``__builtin_assume_aligned`` was not constant (#GH161314)
 
+- Added ``__builtin_stdc_rotate_left`` and ``__builtin_stdc_rotate_right``
+  for bit rotation of unsigned integers including ``_BitInt`` types. Rotation
+  counts are normalized modulo the bit-width and support negative values.
+  Usable in constant expressions.
+
 New Compiler Flags
 ------------------
 - New option ``-fno-sanitize-debug-trap-reasons`` added to disable emitting trap reasons into the debug info when compiling with trapping UBSan (e.g. ``-fsanitize-trap=undefined``).
diff --git a/clang/include/clang/Basic/Builtins.td b/clang/include/clang/Basic/Builtins.td
@@ -767,12 +767,24 @@ def RotateLeft : BitInt8_16_32_64BuiltinsTemplate, Builtin {
   let Prototype = "T(T, T)";
 }
 
+def StdcRotateLeft : Builtin {
+  let Spellings = ["__builtin_stdc_rotate_left"];
+  let Attributes = [NoThrow, Const, Constexpr, CustomTypeChecking];
+  let Prototype = "void(...)";
+}
+
 def RotateRight : BitInt8_16_32_64BuiltinsTemplate, Builtin {
   let Spellings = ["__builtin_rotateright"];
   let Attributes = [NoThrow, Const, Constexpr];
   let Prototype = "T(T, T)";
 }
 
+def StdcRotateRight : Builtin {
+  let Spellings = ["__builtin_stdc_rotate_right"];
+  let Attributes = [NoThrow, Const, Constexpr, CustomTypeChecking];
+  let Prototype = "void(...)";
+}
+
 // Random GCC builtins
 // FIXME: The builtins marked FunctionWithBuiltinPrefix below should be
 //        merged with the library definitions. They are currently not because
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
@@ -14346,23 +14346,17 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
   case Builtin::BI__builtin_rotateleft16:
   case Builtin::BI__builtin_rotateleft32:
   case Builtin::BI__builtin_rotateleft64:
-  case Builtin::BI_rotl8: // Microsoft variants of rotate right
-  case Builtin::BI_rotl16:
-  case Builtin::BI_rotl:
-  case Builtin::BI_lrotl:
-  case Builtin::BI_rotl64: {
-    APSInt Val, Amt;
-    if (!EvaluateInteger(E->getArg(0), Val, Info) ||
-        !EvaluateInteger(E->getArg(1), Amt, Info))
-      return false;
-
-    return Success(Val.rotl(Amt), E);
-  }
-
   case Builtin::BI__builtin_rotateright8:
   case Builtin::BI__builtin_rotateright16:
   case Builtin::BI__builtin_rotateright32:
   case Builtin::BI__builtin_rotateright64:
+  case Builtin::BI__builtin_stdc_rotate_left:
+  case Builtin::BI__builtin_stdc_rotate_right:
+  case Builtin::BI_rotl8: // Microsoft variants of rotate left
+  case Builtin::BI_rotl16:
+  case Builtin::BI_rotl:
+  case Builtin::BI_lrotl:
+  case Builtin::BI_rotl64:
   case Builtin::BI_rotr8: // Microsoft variants of rotate right
   case Builtin::BI_rotr16:
   case Builtin::BI_rotr:
@@ -14373,7 +14367,45 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
         !EvaluateInteger(E->getArg(1), Amt, Info))
       return false;
 
-    return Success(Val.rotr(Amt), E);
+    // Normalize shift amount to [0, BitWidth) range to match runtime behavior
+    unsigned BitWidth = Val.getBitWidth();
+    unsigned AmtBitWidth = Amt.getBitWidth();
+    if (BitWidth == 1) {
+      // if BitWidth is 1, Amt % Divisor = 0
+      // No need for rotation
+      Amt = APSInt(APInt(AmtBitWidth, 0), Amt.isUnsigned());
+    } else {
+      APSInt Divisor;
+      if (AmtBitWidth > BitWidth) {
+        Divisor = APSInt(llvm::APInt(AmtBitWidth, BitWidth), Amt.isUnsigned());
+      } else {
+        Divisor = APSInt(llvm::APInt(BitWidth, BitWidth), Amt.isUnsigned());
+        if (AmtBitWidth < BitWidth) {
+          Amt = Amt.extend(BitWidth);
+        }
+      }
+
+      Amt = Amt % Divisor;
+      if (Amt.isNegative()) {
+        Amt += Divisor;
+      }
+    }
+
+    switch (BuiltinOp) {
+    case Builtin::BI__builtin_rotateright8:
+    case Builtin::BI__builtin_rotateright16:
+    case Builtin::BI__builtin_rotateright32:
+    case Builtin::BI__builtin_rotateright64:
+    case Builtin::BI__builtin_stdc_rotate_right:
+    case Builtin::BI_rotr8:
+    case Builtin::BI_rotr16:
+    case Builtin::BI_rotr:
+    case Builtin::BI_lrotr:
+    case Builtin::BI_rotr64:
+      return Success(APSInt(Val.rotr(Amt.getZExtValue()), Val.isUnsigned()), E);
+    default:
+      return Success(APSInt(Val.rotl(Amt.getZExtValue()), Val.isUnsigned()), E);
+    }
   }
 
   case Builtin::BI__builtin_elementwise_add_sat: {
diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -2492,12 +2492,41 @@ 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();
-  ShiftAmt = Builder.CreateIntCast(ShiftAmt, Ty, false);
+  llvm::Type *ShiftTy = ShiftAmt->getType();
+
+  unsigned BitWidth = Ty->getIntegerBitWidth();
+
+  if (!llvm::isPowerOf2_32(BitWidth) &&
+      E->getArg(1)->getType()->isSignedIntegerType()) {
+    // converting BitWidth to the type of ShiftAmt
+    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);
+  }
+
+  unsigned ShiftAmtBitWidth = ShiftTy->getIntegerBitWidth();
+  if (ShiftAmtBitWidth > BitWidth) {
+    llvm::Value *BitWidthInShiftTy = ConstantInt::get(ShiftTy, BitWidth);
+    ShiftAmt = Builder.CreateURem(ShiftAmt, BitWidthInShiftTy);
+    ShiftAmt = Builder.CreateIntCast(ShiftAmt, Ty, false);
+  } else {
+    llvm::Value *BitWidthInShiftTy = ConstantInt::get(Ty, BitWidth);
+    ShiftAmt = Builder.CreateIntCast(ShiftAmt, Ty, false);
+    ShiftAmt = Builder.CreateURem(ShiftAmt, BitWidthInShiftTy);
+  }
 
   // Rotate is a special case of LLVM funnel shift - 1st 2 args are the same.
   unsigned IID = IsRotateRight ? Intrinsic::fshr : Intrinsic::fshl;
   Function *F = CGM.getIntrinsic(IID, Ty);
-  return RValue::get(Builder.CreateCall(F, { Src, Src, ShiftAmt }));
+  return RValue::get(Builder.CreateCall(F, {Src, Src, ShiftAmt}));
 }
 
 // Map math builtins for long-double to f128 version.
@@ -3642,6 +3671,7 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
   case Builtin::BI__builtin_rotateleft16:
   case Builtin::BI__builtin_rotateleft32:
   case Builtin::BI__builtin_rotateleft64:
+  case Builtin::BI__builtin_stdc_rotate_left:
   case Builtin::BI_rotl8: // Microsoft variants of rotate left
   case Builtin::BI_rotl16:
   case Builtin::BI_rotl:
@@ -3653,6 +3683,7 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
   case Builtin::BI__builtin_rotateright16:
   case Builtin::BI__builtin_rotateright32:
   case Builtin::BI__builtin_rotateright64:
+  case Builtin::BI__builtin_stdc_rotate_right:
   case Builtin::BI_rotr8: // Microsoft variants of rotate right
   case Builtin::BI_rotr16:
   case Builtin::BI_rotr:
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
@@ -2267,6 +2267,67 @@ static bool BuiltinCountZeroBitsGeneric(Sema &S, CallExpr *TheCall) {
   return false;
 }
 
+/// Checks that __builtin_stdc_rotate_{left,right} was called with two
+/// arguments, that the first argument is an unsigned integer type, and that
+/// the second argument is an integer type.
+static bool BuiltinRotateGeneric(Sema &S, CallExpr *TheCall) {
+  if (S.checkArgCount(TheCall, 2))
+    return true;
+
+  ExprResult Arg0Res = S.DefaultLvalueConversion(TheCall->getArg(0));
+  if (Arg0Res.isInvalid())
+    return true;
+
+  Expr *Arg0 = Arg0Res.get();
+  TheCall->setArg(0, Arg0);
+
+  QualType Arg0Ty = Arg0->getType();
+  if (!Arg0Ty->isUnsignedIntegerType()) {
+    ExprResult ConvArg0Res = S.PerformImplicitConversion(
+        TheCall->getArg(0), S.Context.UnsignedIntTy, AssignmentAction::Passing);
+    if (ConvArg0Res.isUsable()) {
+      Arg0 = ConvArg0Res.get();
+      Arg0Ty = Arg0->getType();
+      TheCall->setArg(0, Arg0);
+    }
+  }
+
+  if (!Arg0Ty->isUnsignedIntegerType()) {
+    S.Diag(Arg0->getBeginLoc(), diag::err_builtin_invalid_arg_type)
+        << 1 << /* scalar */ 1 << /* unsigned integer ty */ 3 << /* no fp */ 0
+        << Arg0Ty;
+    return true;
+  }
+
+  ExprResult Arg1Res = S.DefaultLvalueConversion(TheCall->getArg(1));
+  if (Arg1Res.isInvalid())
+    return true;
+
+  Expr *Arg1 = Arg1Res.get();
+  TheCall->setArg(1, Arg1);
+
+  QualType Arg1Ty = Arg1->getType();
+
+  if (!Arg1Ty->isIntegerType()) {
+    ExprResult ConvArg1Res = S.PerformImplicitConversion(
+        TheCall->getArg(1), S.Context.IntTy, AssignmentAction::Passing);
+    if (ConvArg1Res.isUsable()) {
+      Arg1 = ConvArg1Res.get();
+      Arg1Ty = Arg1->getType();
+      TheCall->setArg(1, Arg1);
+    }
+  }
+
+  if (!Arg1Ty->isIntegerType()) {
+    S.Diag(Arg1->getBeginLoc(), diag::err_builtin_invalid_arg_type)
+        << 2 << /* scalar */ 1 << /* integer ty */ 2 << /* no fp */ 0 << Arg1Ty;
+    return true;
+  }
+
+  TheCall->setType(Arg0Ty);
+  return false;
+}
+
 static bool CheckMaskedBuiltinArgs(Sema &S, Expr *MaskArg, Expr *PtrArg,
                                    unsigned Pos, bool AllowConst,
                                    bool AllowAS) {
@@ -3458,6 +3519,12 @@ Sema::CheckBuiltinFunctionCall(FunctionDecl *FDecl, unsigned BuiltinID,
       return ExprError();
     break;
 
+  case Builtin::BI__builtin_stdc_rotate_left:
+  case Builtin::BI__builtin_stdc_rotate_right:
+    if (BuiltinRotateGeneric(*this, TheCall))
+      return ExprError();
+    break;
+
   case Builtin::BI__builtin_allow_runtime_check: {
     Expr *Arg = TheCall->getArg(0);
     // Check if the argument is a string literal.
diff --git a/clang/test/CodeGen/builtin-rotate.c b/clang/test/CodeGen/builtin-rotate.c
diff --git a/clang/test/Sema/builtin-stdc-rotate.c b/clang/test/Sema/builtin-stdc-rotate.c
diff --git a/clang/test/SemaCXX/constexpr-builtin-stdc-rotate.cpp b/clang/test/SemaCXX/constexpr-builtin-stdc-rotate.cpp
