[llvm] [DebugCounter] Add -print-debug-counter-queries option (PR #162827)

[Nikita Popov via llvm-commits]

https://github.com/nikic created https://github.com/llvm/llvm-project/pull/162827

Add a `-print-debug-counter-queries` option which prints the current value of the counter and whether it is executed/skipped each time it is queried. This is useful when interleaving the output with the usual transform debug output, in order to find the correct counter value to use to hit a specific point in the transform.

>From 2d55115aae165c16027e6c563de9e0486d3980de Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Fri, 10 Oct 2025 12:55:01 +0200
Subject: [PATCH] [DebugCounter] Add -print-debug-counter-queries option

Add a `-print-debug-counter-queries` option which prints the
current value of the counter and whether it is executed/skipped
each time it is queried. This is useful when interleaving the output
with the usual transform debug output, in order to find the correct
counter value to use to hit a specific point in the transform.
---
 llvm/include/llvm/Support/DebugCounter.h |  3 ++
 llvm/lib/Support/DebugCounter.cpp        | 56 +++++++++++++++---------
 llvm/test/Other/debugcounter-dce.ll      | 10 ++++-
 3 files changed, 48 insertions(+), 21 deletions(-)

diff --git a/llvm/include/llvm/Support/DebugCounter.h b/llvm/include/llvm/Support/DebugCounter.h
index 48fc60035b189..1b30efa740298 100644
--- a/llvm/include/llvm/Support/DebugCounter.h
+++ b/llvm/include/llvm/Support/DebugCounter.h
@@ -178,6 +178,7 @@ class DebugCounter {
     std::string Desc;
     SmallVector<Chunk> Chunks;
   };
+  LLVM_ABI bool handleCounterIncrement(CounterInfo &Info);
 
   DenseMap<unsigned, CounterInfo> Counters;
   CounterVector RegisteredCounters;
@@ -188,6 +189,8 @@ class DebugCounter {
 
   bool ShouldPrintCounter = false;
 
+  bool ShouldPrintCounterQueries = false;
+
   bool BreakOnLast = false;
 };
 
diff --git a/llvm/lib/Support/DebugCounter.cpp b/llvm/lib/Support/DebugCounter.cpp
index 6b65720440f30..5ab1def43313b 100644
--- a/llvm/lib/Support/DebugCounter.cpp
+++ b/llvm/lib/Support/DebugCounter.cpp
@@ -136,6 +136,13 @@ struct DebugCounterOwner : DebugCounter {
       cl::location(this->ShouldPrintCounter),
       cl::init(false),
       cl::desc("Print out debug counter info after all counters accumulated")};
+  cl::opt<bool, true> PrintDebugCounterQueries{
+      "print-debug-counter-queries",
+      cl::Hidden,
+      cl::Optional,
+      cl::location(this->ShouldPrintCounterQueries),
+      cl::init(false),
+      cl::desc("Print out each query of an enabled debug counter")};
   cl::opt<bool, true> BreakOnLastCount{
       "debug-counter-break-on-last",
       cl::Hidden,
@@ -221,31 +228,40 @@ void DebugCounter::print(raw_ostream &OS) const {
   }
 }
 
+bool DebugCounter::handleCounterIncrement(CounterInfo &Info) {
+  int64_t CurrCount = Info.Count++;
+  uint64_t CurrIdx = Info.CurrChunkIdx;
+
+  if (Info.Chunks.empty())
+    return true;
+  if (CurrIdx >= Info.Chunks.size())
+    return false;
+
+  bool Res = Info.Chunks[CurrIdx].contains(CurrCount);
+  if (BreakOnLast && CurrIdx == (Info.Chunks.size() - 1) &&
+      CurrCount == Info.Chunks[CurrIdx].End) {
+    LLVM_BUILTIN_DEBUGTRAP;
+  }
+  if (CurrCount > Info.Chunks[CurrIdx].End) {
+    Info.CurrChunkIdx++;
+
+    /// Handle consecutive blocks.
+    if (Info.CurrChunkIdx < Info.Chunks.size() &&
+        CurrCount == Info.Chunks[Info.CurrChunkIdx].Begin)
+      return true;
+  }
+  return Res;
+}
+
 bool DebugCounter::shouldExecuteImpl(unsigned CounterName) {
   auto &Us = instance();
   auto Result = Us.Counters.find(CounterName);
   if (Result != Us.Counters.end()) {
     auto &CounterInfo = Result->second;
-    int64_t CurrCount = CounterInfo.Count++;
-    uint64_t CurrIdx = CounterInfo.CurrChunkIdx;
-
-    if (CounterInfo.Chunks.empty())
-      return true;
-    if (CurrIdx >= CounterInfo.Chunks.size())
-      return false;
-
-    bool Res = CounterInfo.Chunks[CurrIdx].contains(CurrCount);
-    if (Us.BreakOnLast && CurrIdx == (CounterInfo.Chunks.size() - 1) &&
-        CurrCount == CounterInfo.Chunks[CurrIdx].End) {
-      LLVM_BUILTIN_DEBUGTRAP;
-    }
-    if (CurrCount > CounterInfo.Chunks[CurrIdx].End) {
-      CounterInfo.CurrChunkIdx++;
-
-      /// Handle consecutive blocks.
-      if (CounterInfo.CurrChunkIdx < CounterInfo.Chunks.size() &&
-          CurrCount == CounterInfo.Chunks[CounterInfo.CurrChunkIdx].Begin)
-        return true;
+    bool Res = Us.handleCounterIncrement(CounterInfo);
+    if (Us.ShouldPrintCounterQueries && CounterInfo.IsSet) {
+      dbgs() << "DebugCounter " << Us.RegisteredCounters[CounterName] << "="
+             << (CounterInfo.Count - 1) << (Res ? " execute" : " skip") << "\n";
     }
     return Res;
   }
diff --git a/llvm/test/Other/debugcounter-dce.ll b/llvm/test/Other/debugcounter-dce.ll
index 54d929f219aef..3b1dfb453593a 100644
--- a/llvm/test/Other/debugcounter-dce.ll
+++ b/llvm/test/Other/debugcounter-dce.ll
@@ -1,8 +1,16 @@
 ; REQUIRES: asserts
-; RUN: opt -passes=dce -S -debug-counter=dce-transform=1-2  < %s | FileCheck %s
+; RUN: opt -passes=dce -S -debug-counter=dce-transform=1-2  < %s | FileCheck %s --check-prefixes=CHECK,NO-PRINT
+; RUN: opt -passes=dce -S -debug-counter=dce-transform=1-2 -print-debug-counter-queries < %s 2>&1 | FileCheck %s --check-prefixes=CHECK,PRINT
 ;; Test that, with debug counters on, we will skip the first DCE opportunity, perform next 2,
 ;; and ignore all the others left.
 
+; NO-PRINT-NOT: DebugCounter
+; PRINT: DebugCounter dce-transform=0 skip
+; PRINT-NEXT: DebugCounter dce-transform=1 execute
+; PRINT-NEXT: DebugCounter dce-transform=2 execute
+; PRINT-NEXT: DebugCounter dce-transform=3 skip
+; PRINT-NEXT: DebugCounter dce-transform=4 skip
+
 ; CHECK-LABEL: @test
 ; CHECK-NEXT: %add1 = add i32 1, 2
 ; CHECK-NEXT: %sub1 = sub i32 %add1, 1