[llvm] [TableGen][DecoderEmitter] Rework table construction/emission (PR #155889)

[Rahul Joshi via llvm-commits]

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@@ -0,0 +1,342 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 "DecoderTableEmitter.h"
+#include "Common/CodeGenInstruction.h"
+#include "Common/InstructionEncoding.h"
+#include "llvm/Support/Format.h"
+#include "llvm/Support/LEB128.h"
+
+using namespace llvm;
+
+void DecoderTableEmitter::analyzeNode(const DecoderTreeNode *Node) const {
+  switch (Node->getKind()) {
+  case DecoderTreeNode::CheckAny: {
+    const auto *N = static_cast<const CheckAnyNode *>(Node);
+    for (const DecoderTreeNode *Child : N->children())
+      analyzeNode(Child);
+    break;
+  }
+  case DecoderTreeNode::CheckAll: {
+    const auto *N = static_cast<const CheckAllNode *>(Node);
+    for (const DecoderTreeNode *Child : N->children())
+      analyzeNode(Child);
+    break;
+  }
+  case DecoderTreeNode::CheckField:
+    break;
+  case DecoderTreeNode::SwitchField: {
+    const auto *N = static_cast<const SwitchFieldNode *>(Node);
+    for (const DecoderTreeNode *Child : make_second_range(N->cases()))
+      analyzeNode(Child);
+    break;
+  }
+  case DecoderTreeNode::CheckPredicate: {
+    const auto *N = static_cast<const CheckPredicateNode *>(Node);
+    TableInfo.insertPredicate(N->getPredicateString());
+    break;
+  }
+  case DecoderTreeNode::SoftFail:
+    break;
+  case DecoderTreeNode::Decode: {
+    const auto *N = static_cast<const DecodeNode *>(Node);
+    TableInfo.insertDecoder(N->getDecoderString());
+    break;
+  }
+  }
+}
+
+unsigned
+DecoderTableEmitter::computeNodeSize(const DecoderTreeNode *Node) const {
+  // To make the arithmetic below clearer.
+  static constexpr unsigned OpcodeSize = 1;
+  static constexpr unsigned FieldWidthSize = 1;
+
+  switch (Node->getKind()) {
+  case DecoderTreeNode::CheckAny: {
+    const auto *N = static_cast<const CheckAnyNode *>(Node);
+    // Pretend the node was optimized. See the comment in emitCheckAnyNode.
+    if (range_size(N->children()) == 1)
----------------
jurahul wrote:

nit: Is this needed? It seems we would get the right result if we remove this if.

https://github.com/llvm/llvm-project/pull/155889