[clang] [mlir] [CIR] Add CIRABIRewriteContext for ABI function/call rewriting (PR #192119)

[Andy Kaylor via cfe-commits]

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@@ -0,0 +1,469 @@
+//===- CIRABIRewriteContext.cpp - CIR-specific ABI rewriting --------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "CIRABIRewriteContext.h"
+#include "mlir/IR/Builders.h"
+#include "mlir/IR/Types.h"
+#include "clang/CIR/Dialect/IR/CIRAttrs.h"
+#include "clang/CIR/Dialect/IR/CIRDialect.h"
+#include "clang/CIR/Dialect/IR/CIROpsEnums.h"
+
+using namespace cir;
+using namespace mlir;
+using namespace mlir::abi;
+
+/// Emit a value coercion between two types.  For scalar-to-scalar
+/// (e.g. integer sign extension), a direct cir.cast is sufficient.
+/// When one of the types is a record (struct), LLVM IR's bitcast
+/// cannot reinterpret between aggregate and scalar types, so we go
+/// through memory: alloca srcTy -> store src -> bitcast ptr -> load
+/// dstTy.
+static Value emitCoercion(OpBuilder &rewriter, Location loc, Type dstTy,
+                          Value src) {
+  Type srcTy = src.getType();
+  if (srcTy == dstTy)
+    return src;
+
+  bool needsMemory =
+      mlir::isa<cir::RecordType, cir::ComplexType>(srcTy) ||
+      mlir::isa<cir::RecordType, cir::ComplexType>(dstTy) ||
+      (mlir::isa<cir::VectorType>(srcTy) != mlir::isa<cir::VectorType>(dstTy));
+
+  if (!needsMemory)
+    return cir::CastOp::create(rewriter, loc, dstTy, cir::CastKind::bitcast,
+                               src);
+
+  auto srcPtrTy = cir::PointerType::get(srcTy);
+  auto dstPtrTy = cir::PointerType::get(dstTy);
+
+  auto alloca =
+      cir::AllocaOp::create(rewriter, loc, srcPtrTy, srcTy,
----------------
andykaylor wrote:

Is ABI lowering going to run before the HoistAllocas pass? Creating an alloca wherever we happen to be seems bad. It would probably be better to put this in the function entry block.

https://github.com/llvm/llvm-project/pull/192119