[llvm] [SelectionDAG] Add a flag to sort DAGs before showing them in debug prints (PR #149732)

[Min-Yih Hsu via llvm-commits]

https://github.com/mshockwave created https://github.com/llvm/llvm-project/pull/149732

There are many cases where SDNodes shown in the `-debug-only=isel/isel-dump` of a DAG -- especially a big DAG -- just randomly spread everywhere, which can be really really painful to read sometimes.
This patch adds a flag to sort DAGs before printing them in those debug prints. Note that sorted DAGs might have different schedulings and hence different results compared to the original one, but I thought it's still useful for debugging many of the (codegen) cases.

>From 92d72c42d98bd97d1d22aeac5e9690017a9a276a Mon Sep 17 00:00:00 2001
From: Min Hsu <min at myhsu.dev>
Date: Sun, 20 Jul 2025 12:17:40 -0700
Subject: [PATCH] [SelectionDAG] Add a flag to sort DAGs before showing in
 debug prints

---
 .../CodeGen/SelectionDAG/SelectionDAGISel.cpp | 34 +++++++++++++------
 1 file changed, 24 insertions(+), 10 deletions(-)

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
index 26071ed70c9db..4964edd620e47 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
@@ -144,6 +144,12 @@ UseMBPI("use-mbpi",
         cl::init(true), cl::Hidden);
 
 #ifndef NDEBUG
+static cl::opt<bool>
+    SortDAGBeforeDump("sort-dags-before-isel-dump", cl::Hidden,
+                      cl::desc("Sort SelectionDAGs before showing them in"
+                               " debug prints during SelectionDAGISel."),
+                      cl::init(false));
+
 static cl::opt<std::string>
 FilterDAGBasicBlockName("filter-view-dags", cl::Hidden,
                         cl::desc("Only display the basic block whose name "
@@ -903,6 +909,14 @@ void SelectionDAGISel::ComputeLiveOutVRegInfo() {
   } while (!Worklist.empty());
 }
 
+#ifndef NDEBUG
+static void dumpSelectionDAG(SelectionDAG *DAG) {
+  if (SortDAGBeforeDump)
+    DAG->AssignTopologicalOrder();
+  DAG->dump();
+}
+#endif
+
 void SelectionDAGISel::CodeGenAndEmitDAG() {
   StringRef GroupName = "sdag";
   StringRef GroupDescription = "Instruction Selection and Scheduling";
@@ -930,7 +944,7 @@ void SelectionDAGISel::CodeGenAndEmitDAG() {
   ISEL_DUMP(dbgs() << "\nInitial selection DAG: "
                    << printMBBReference(*FuncInfo->MBB) << " '" << BlockName
                    << "'\n";
-            CurDAG->dump());
+            dumpSelectionDAG(CurDAG));
 
 #if !defined(NDEBUG) && LLVM_ENABLE_ABI_BREAKING_CHECKS
   if (TTI->hasBranchDivergence())
@@ -950,7 +964,7 @@ void SelectionDAGISel::CodeGenAndEmitDAG() {
   ISEL_DUMP(dbgs() << "\nOptimized lowered selection DAG: "
                    << printMBBReference(*FuncInfo->MBB) << " '" << BlockName
                    << "'\n";
-            CurDAG->dump());
+            dumpSelectionDAG(CurDAG));
 
 #if !defined(NDEBUG) && LLVM_ENABLE_ABI_BREAKING_CHECKS
   if (TTI->hasBranchDivergence())
@@ -972,7 +986,7 @@ void SelectionDAGISel::CodeGenAndEmitDAG() {
   ISEL_DUMP(dbgs() << "\nType-legalized selection DAG: "
                    << printMBBReference(*FuncInfo->MBB) << " '" << BlockName
                    << "'\n";
-            CurDAG->dump());
+            dumpSelectionDAG(CurDAG));
 
 #if !defined(NDEBUG) && LLVM_ENABLE_ABI_BREAKING_CHECKS
   if (TTI->hasBranchDivergence())
@@ -996,7 +1010,7 @@ void SelectionDAGISel::CodeGenAndEmitDAG() {
     ISEL_DUMP(dbgs() << "\nOptimized type-legalized selection DAG: "
                      << printMBBReference(*FuncInfo->MBB) << " '" << BlockName
                      << "'\n";
-              CurDAG->dump());
+              dumpSelectionDAG(CurDAG));
 
 #if !defined(NDEBUG) && LLVM_ENABLE_ABI_BREAKING_CHECKS
     if (TTI->hasBranchDivergence())
@@ -1014,7 +1028,7 @@ void SelectionDAGISel::CodeGenAndEmitDAG() {
     ISEL_DUMP(dbgs() << "\nVector-legalized selection DAG: "
                      << printMBBReference(*FuncInfo->MBB) << " '" << BlockName
                      << "'\n";
-              CurDAG->dump());
+              dumpSelectionDAG(CurDAG));
 
 #if !defined(NDEBUG) && LLVM_ENABLE_ABI_BREAKING_CHECKS
     if (TTI->hasBranchDivergence())
@@ -1030,7 +1044,7 @@ void SelectionDAGISel::CodeGenAndEmitDAG() {
     ISEL_DUMP(dbgs() << "\nVector/type-legalized selection DAG: "
                      << printMBBReference(*FuncInfo->MBB) << " '" << BlockName
                      << "'\n";
-              CurDAG->dump());
+              dumpSelectionDAG(CurDAG));
 
 #if !defined(NDEBUG) && LLVM_ENABLE_ABI_BREAKING_CHECKS
     if (TTI->hasBranchDivergence())
@@ -1050,7 +1064,7 @@ void SelectionDAGISel::CodeGenAndEmitDAG() {
     ISEL_DUMP(dbgs() << "\nOptimized vector-legalized selection DAG: "
                      << printMBBReference(*FuncInfo->MBB) << " '" << BlockName
                      << "'\n";
-              CurDAG->dump());
+              dumpSelectionDAG(CurDAG));
 
 #if !defined(NDEBUG) && LLVM_ENABLE_ABI_BREAKING_CHECKS
     if (TTI->hasBranchDivergence())
@@ -1070,7 +1084,7 @@ void SelectionDAGISel::CodeGenAndEmitDAG() {
   ISEL_DUMP(dbgs() << "\nLegalized selection DAG: "
                    << printMBBReference(*FuncInfo->MBB) << " '" << BlockName
                    << "'\n";
-            CurDAG->dump());
+            dumpSelectionDAG(CurDAG));
 
 #if !defined(NDEBUG) && LLVM_ENABLE_ABI_BREAKING_CHECKS
   if (TTI->hasBranchDivergence())
@@ -1090,7 +1104,7 @@ void SelectionDAGISel::CodeGenAndEmitDAG() {
   ISEL_DUMP(dbgs() << "\nOptimized legalized selection DAG: "
                    << printMBBReference(*FuncInfo->MBB) << " '" << BlockName
                    << "'\n";
-            CurDAG->dump());
+            dumpSelectionDAG(CurDAG));
 
 #if !defined(NDEBUG) && LLVM_ENABLE_ABI_BREAKING_CHECKS
   if (TTI->hasBranchDivergence())
@@ -1114,7 +1128,7 @@ void SelectionDAGISel::CodeGenAndEmitDAG() {
   ISEL_DUMP(dbgs() << "\nSelected selection DAG: "
                    << printMBBReference(*FuncInfo->MBB) << " '" << BlockName
                    << "'\n";
-            CurDAG->dump());
+            dumpSelectionDAG(CurDAG));
 
   if (ViewSchedDAGs && MatchFilterBB)
     CurDAG->viewGraph("scheduler input for " + BlockName);