[Clang][AArch64] Fixed incorrect _BitInt alignment (#90602)

This patch makes determining alignment and width of BitInt to be target
ABI specific and makes it consistent with [Procedure Call Standard for
the Arm® 64-bit Architecture
(AArch64)](https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst)
for AArch64 targets.

Author: Lukacma

clang/include/clang/Basic/TargetInfo.h

@@ -97,6 +97,10 @@ struct TransferrableTargetInfo {
   unsigned char LongLongWidth, LongLongAlign;
   unsigned char Int128Align;
 
+  // This is an optional parameter for targets that
+  // don't use 'LongLongAlign' for '_BitInt' max alignment
+  std::optional<unsigned> BitIntMaxAlign;
+
   // Fixed point bit widths
   unsigned char ShortAccumWidth, ShortAccumAlign;
   unsigned char AccumWidth, AccumAlign;
@@ -518,6 +522,22 @@ class TargetInfo : public TransferrableTargetInfo,
   /// getInt128Align() - Returns the alignment of Int128.
   unsigned getInt128Align() const { return Int128Align; }
 
+  /// getBitIntMaxAlign() - Returns the maximum possible alignment of
+  /// '_BitInt' and 'unsigned _BitInt'.
+  unsigned getBitIntMaxAlign() const {
+    return BitIntMaxAlign.value_or(LongLongAlign);
+  }
+
+  /// getBitIntAlign/Width - Return aligned size of '_BitInt' and
+  /// 'unsigned _BitInt' for this target, in bits.
+  unsigned getBitIntWidth(unsigned NumBits) const {
+    return llvm::alignTo(NumBits, getBitIntAlign(NumBits));
+  }
+  unsigned getBitIntAlign(unsigned NumBits) const {
+    return std::clamp<unsigned>(llvm::PowerOf2Ceil(NumBits), getCharWidth(),
+                                getBitIntMaxAlign());
+  }
+
   /// getShortAccumWidth/Align - Return the size of 'signed short _Accum' and
   /// 'unsigned short _Accum' for this target, in bits.
   unsigned getShortAccumWidth() const { return ShortAccumWidth; }

clang/lib/AST/ASTContext.cpp

@@ -2258,9 +2258,8 @@ TypeInfo ASTContext::getTypeInfoImpl(const Type *T) const {
   }
   case Type::BitInt: {
     const auto *EIT = cast<BitIntType>(T);
-    Align = std::clamp<unsigned>(llvm::PowerOf2Ceil(EIT->getNumBits()),
-                                 getCharWidth(), Target->getLongLongAlign());
-    Width = llvm::alignTo(EIT->getNumBits(), Align);
+    Align = Target->getBitIntAlign(EIT->getNumBits());
+    Width = Target->getBitIntWidth(EIT->getNumBits());
     break;
   }
   case Type::Record:

clang/lib/Basic/Targets/AArch64.cpp

@@ -154,6 +154,7 @@ AArch64TargetInfo::AArch64TargetInfo(const llvm::Triple &Triple,
   else
     LongWidth = LongAlign = PointerWidth = PointerAlign = 32;
 
+  BitIntMaxAlign = 128;
   MaxVectorAlign = 128;
   MaxAtomicInlineWidth = 128;
   MaxAtomicPromoteWidth = 128;

clang/test/CodeGen/aapcs64-align.cpp

@@ -1,7 +1,7 @@
 // REQUIRES: arm-registered-target
 // RUN: %clang_cc1 -triple aarch64-none-elf \
 // RUN:   -O2 \
-// RUN:   -emit-llvm -o - %s | FileCheck %s
+// RUN:   -emit-llvm -fexperimental-max-bitint-width=1024 -o - %s | FileCheck %s
 
 extern "C" {
 
@@ -100,4 +100,66 @@ void f5m(int, int, int, int, int, P16);
 // CHECK: declare void @f5(i32 noundef, [2 x i64])
 // CHECK: declare void @f5m(i32 noundef, i32 noundef, i32 noundef, i32 noundef, i32 noundef, [2 x i64])
 
+//BitInt alignment
+struct BITINT129 {
+    char ch;
+    unsigned _BitInt(129) v;
+};
+
+int test_bitint129(){
+  return __builtin_offsetof(struct BITINT129, v);
 }
+// CHECK:  ret i32 16 
+
+struct BITINT127 {
+    char ch;
+    _BitInt(127) v;
+};
+
+int test_bitint127(){
+  return __builtin_offsetof(struct BITINT127, v);
+}
+// CHECK:  ret i32 16 
+
+struct BITINT63 {
+    char ch;
+    _BitInt(63) v;
+};
+
+int test_bitint63(){
+  return __builtin_offsetof(struct BITINT63, v);
+}
+// CHECK:  ret i32 8 
+
+struct BITINT32 {
+    char ch;
+    unsigned _BitInt(32) v;
+};
+
+int test_bitint32(){
+  return __builtin_offsetof(struct BITINT32, v);
+}
+// CHECK:  ret i32 4
+
+struct BITINT9 {
+    char ch;
+    unsigned _BitInt(9) v;
+};
+
+int test_bitint9(){
+  return __builtin_offsetof(struct BITINT9, v);
+}
+// CHECK:  ret i32 2
+
+struct BITINT8 {
+    char ch;
+    unsigned _BitInt(8) v;
+};
+
+int test_bitint8(){
+  return __builtin_offsetof(struct BITINT8, v);
+}
+// CHECK:  ret i32 1
+
+}
+