From c51f86cd13bff130db14a767417d100b4b5434c1 Mon Sep 17 00:00:00 2001
From: scott snyder <snyder@bnl.gov>
Date: Tue, 19 Jan 2021 19:58:46 +0100
Subject: [PATCH] AthenaKernel: Fix Units for clang.
The Units header provides units objects for which division is automatically changed at compile-time
to multiplication by a reciprocal of the unit constant. This relied on the compiler changing
(1./x) to multiplication by a reciprocal, if x is constant. However, this does not work
with clang if the -ffp-exception-behavior=maytrap option is used.
Rewrite to fix this. The price is that we need to have a separate class for each unit
(which can then return the value as a constexpr).
---
Control/AthenaKernel/AthenaKernel/Units.h | 52 +++++++++++------------
1 file changed, 26 insertions(+), 26 deletions(-)
diff --git a/Control/AthenaKernel/AthenaKernel/Units.h b/Control/AthenaKernel/AthenaKernel/Units.h
index 264a2f7a7069..72c0f9625403 100644
--- a/Control/AthenaKernel/AthenaKernel/Units.h
+++ b/Control/AthenaKernel/AthenaKernel/Units.h
@@ -1,10 +1,9 @@
// This file's extension implies that it's C, but it's really -*- C++ -*-.
/*
- Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+ Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
*/
-// $Id$
/**
* @file AthenaKernel/Units.h
* @author scott snyder <snyder@bnl.gov>
@@ -43,42 +42,43 @@
namespace Athena {
+namespace Units {
/**
* @brief Wrapper to avoid constant divisions when using units.
+ * The Unit class defined here is a placeholder.
+ * A specialization will be generated for each unit by the UNIT
+ * macro below. TAG is used to distinguish these specializations.
+ *
+ * (We used to do this with a single, non-template, class,
+ * but that didn't work with clang when -ffp-exception-behavior=maytrap
+ * is used.)
*/
-class Unit
-{
-public:
- constexpr Unit(double val) : m_val(val) {}
- constexpr operator double() const { return m_val; }
-
-private:
- double m_val;
-};
-
-
-constexpr double operator/ (double x, const Unit& u)
+template <int TAG> class Unit {};
+template <int TAG>
+constexpr double operator/ (double x, const Unit<TAG> u)
{
- return x * (1./static_cast<double>(u));
+ constexpr double recip = 1. / static_cast<double>(u);
+ return x * recip;
}
-
-
-constexpr float operator/ (float x, const Unit& u)
+template <int TAG>
+constexpr float operator/ (float x, const Unit<TAG> u)
{
- return x * (1./static_cast<float>(u));
+ constexpr float recip = 1. / static_cast<float>(u);
+ return x * recip;
}
-namespace Units {
-
-
-// Would be nice to declare this constexpr, but can't in general
-// because the symbols from SystemOfUnits aren't constexpr.
-//
// Include a selection of units from SystemOfUnits.h
-#define UNIT(NAME) /*constexpr*/ static const Unit NAME (Gaudi::Units::NAME)
+#define UNIT(NAME) \
+template <> class Unit<__LINE__> \
+{ \
+public: \
+ constexpr operator double() const { return Gaudi::Units::NAME; } \
+}; \
+constexpr static const Unit<__LINE__> NAME
+
UNIT (millimeter);
UNIT (millimeter2);
UNIT (millimeter3);
--
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