From 7619bce552d5a379c06dde0e2cbdaa0ace712ebe Mon Sep 17 00:00:00 2001
From: Giulio Eulisse <10544+ktf@users.noreply.github.com>
Date: Fri, 16 Oct 2026 13:09:12 +0200
Subject: [PATCH] DPL Analysis: out-of-line HistogramSpec related construction

The price we pay in compile time is not worth the speedup in registering the histograms, so we move the related methods out of line.
---
 .../Core/include/Framework/HistogramSpec.h    | 53 +++++--------------
 Framework/Core/src/HistogramSpec.cxx          | 43 +++++++++++++++
 2 files changed, 57 insertions(+), 39 deletions(-)

diff --git a/Framework/Core/include/Framework/HistogramSpec.h b/Framework/Core/include/Framework/HistogramSpec.h
index bfd9ba702496d..365b2255eeb99 100644
--- a/Framework/Core/include/Framework/HistogramSpec.h
+++ b/Framework/Core/include/Framework/HistogramSpec.h
@@ -84,41 +84,20 @@ using HistPtr = std::variant<std::shared_ptr<THn>, std::shared_ptr<THnSparse>, s
 constexpr double VARIABLE_WIDTH = 0.;
 
 struct AxisSpec {
-  AxisSpec(std::vector<double> binEdges_, std::optional<std::string> title_ = std::nullopt, std::optional<std::string> name_ = std::nullopt)
-    : nBins(std::nullopt),
-      binEdges(std::move(binEdges_)),
-      title(std::move(title_)),
-      name(std::move(name_))
-  {
-  }
-
-  AxisSpec(int nBins_, double binMin_, double binMax_, std::optional<std::string> title_ = std::nullopt, std::optional<std::string> name_ = std::nullopt)
-    : nBins(nBins_),
-      binEdges({binMin_, binMax_}),
-      title(std::move(title_)),
-      name(std::move(name_))
-  {
-    if (binMin_ > binMax_) {
-      LOG(fatal) << "Defined ill-defined axis";
-    }
-  }
+  // Defined out of line in HistogramSpec.cxx on purpose. These run once per
+  // histogram at init() time, so inlining them buys nothing at runtime, while
+  // a task that books a few hundred histograms in one init() pays for every
+  // expansion: each one materialises a std::vector<double> and two
+  // std::optional<std::string> at the call site, with the matching destructor
+  // and exception-unwind paths. Under -g that also means a debug record per
+  // temporary, which is what makes the register allocator's rewrite pass go
+  // superlinear on such functions.
+  AxisSpec(std::vector<double> binEdges_, std::optional<std::string> title_ = std::nullopt, std::optional<std::string> name_ = std::nullopt);
+
+  AxisSpec(int nBins_, double binMin_, double binMax_, std::optional<std::string> title_ = std::nullopt, std::optional<std::string> name_ = std::nullopt);
 
   // first entry is assumed to be the number of bins; in case of variable size binning it must be set to zero
-  AxisSpec(ConfigurableAxis binEdges_, std::optional<std::string> title_ = std::nullopt, std::optional<std::string> name_ = std::nullopt)
-    : nBins(std::nullopt),
-      binEdges(std::vector<double>(binEdges_)),
-      title(std::move(title_)),
-      name(std::move(name_))
-  {
-    if (binEdges.empty()) {
-      return;
-    }
-    if (binEdges[0] != VARIABLE_WIDTH) {
-      nBins = static_cast<int>(binEdges[0]);
-      binEdges.resize(3); // nBins, lowerBound, upperBound, disregard whatever else is stored in vecotr
-    }
-    binEdges.erase(binEdges.begin()); // remove first entry that we assume to be number of bins
-  }
+  AxisSpec(ConfigurableAxis binEdges_, std::optional<std::string> title_ = std::nullopt, std::optional<std::string> name_ = std::nullopt);
 
   [[nodiscard]] long getNbins() const;
 
@@ -138,12 +117,8 @@ struct AxisSpec {
  */
 //**************************************************************************************************
 struct HistogramConfigSpec {
-  HistogramConfigSpec(HistType type_, std::vector<AxisSpec> axes_, uint8_t nSteps_ = 1)
-    : type(type_),
-      axes(std::move(axes_)),
-      nSteps(nSteps_)
-  {
-  }
+  // Out of line for the same reason as the AxisSpec constructors above.
+  HistogramConfigSpec(HistType type_, std::vector<AxisSpec> axes_, uint8_t nSteps_ = 1);
   HistogramConfigSpec() = default;
   HistogramConfigSpec(HistogramConfigSpec const& other) = default;
   HistogramConfigSpec(HistogramConfigSpec&& other) = default;
diff --git a/Framework/Core/src/HistogramSpec.cxx b/Framework/Core/src/HistogramSpec.cxx
index ef23a29c828f7..496a07a6dcba4 100644
--- a/Framework/Core/src/HistogramSpec.cxx
+++ b/Framework/Core/src/HistogramSpec.cxx
@@ -15,6 +15,49 @@
 namespace o2::framework
 {
 
+// Out of line: see the comment on the declarations in HistogramSpec.h.
+AxisSpec::AxisSpec(std::vector<double> binEdges_, std::optional<std::string> title_, std::optional<std::string> name_)
+  : nBins(std::nullopt),
+    binEdges(std::move(binEdges_)),
+    title(std::move(title_)),
+    name(std::move(name_))
+{
+}
+
+AxisSpec::AxisSpec(int nBins_, double binMin_, double binMax_, std::optional<std::string> title_, std::optional<std::string> name_)
+  : nBins(nBins_),
+    binEdges({binMin_, binMax_}),
+    title(std::move(title_)),
+    name(std::move(name_))
+{
+  if (binMin_ > binMax_) {
+    LOG(fatal) << "Defined ill-defined axis";
+  }
+}
+
+AxisSpec::AxisSpec(ConfigurableAxis binEdges_, std::optional<std::string> title_, std::optional<std::string> name_)
+  : nBins(std::nullopt),
+    binEdges(std::vector<double>(binEdges_)),
+    title(std::move(title_)),
+    name(std::move(name_))
+{
+  if (binEdges.empty()) {
+    return;
+  }
+  if (binEdges[0] != VARIABLE_WIDTH) {
+    nBins = static_cast<int>(binEdges[0]);
+    binEdges.resize(3); // nBins, lowerBound, upperBound, disregard whatever else is stored in vecotr
+  }
+  binEdges.erase(binEdges.begin()); // remove first entry that we assume to be number of bins
+}
+
+HistogramConfigSpec::HistogramConfigSpec(HistType type_, std::vector<AxisSpec> axes_, uint8_t nSteps_)
+  : type(type_),
+    axes(std::move(axes_)),
+    nSteps(nSteps_)
+{
+}
+
 void AxisSpec::makeLogarithmic()
 {
   if (binEdges.size() > 2) {