[clang] [clang] Lower _BitInt(129+) to a different type in LLVM IR (PR #91364)

[Mariya Podchishchaeva via cfe-commits]

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@@ -118,6 +124,37 @@ llvm::Type *CodeGenTypes::ConvertTypeForMem(QualType T, bool ForBitField) {
   return R;
 }
 
+bool CodeGenTypes::LLVMTypeLayoutMatchesAST(QualType ASTTy,
+                                            llvm::Type *LLVMTy) {
+  CharUnits ASTSize = Context.getTypeSizeInChars(ASTTy);
+  CharUnits LLVMSize =
+      CharUnits::fromQuantity(getDataLayout().getTypeAllocSize(LLVMTy));
+  return ASTSize == LLVMSize;
+}
+
+llvm::Type *CodeGenTypes::convertTypeForLoadStore(QualType T,
+                                                  llvm::Type *LLVMTy) {
+  if (!LLVMTy)
+    LLVMTy = ConvertType(T);
+
+  if (!T->isBitIntType() && LLVMTy->isIntegerTy(1))
+    return llvm::IntegerType::get(getLLVMContext(),
+                                  (unsigned)Context.getTypeSize(T));
+
+  if (T->isBitIntType()) {
+    llvm::Type *R = ConvertType(T);
+    if (!LLVMTypeLayoutMatchesAST(T, R))
+      return llvm::Type::getIntNTy(
+          getLLVMContext(), Context.getTypeSizeInChars(T).getQuantity() * 8);
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Fznamznon wrote:

The choice is conditional to keep changes less invasive.

I wonder if that requires to have different in-memory type for all _BitInt types?
But in case when the "layout" matches we could use `iNBytes ` for both memory type and load-store type.
Though that makes me wonder what will be the whole point of small `_BitInt` then...

https://github.com/llvm/llvm-project/pull/91364