TPC: place shared constants in the Metal constant address space (#15802)

* TPC: place shared constants in the Metal constant address space

MSL requires namespace-scope constexpr to live in the constant address space.
Generic pointers do not help here: constant is explicitly not among the
address spaces MSL 4.1 makes generic, so the annotation is required whatever
the language version. Guarded on __METAL__, so no other build changes.

* Please consider the following formatting changes

---------

Co-authored-by: ALICE Action Bot <alibuild@cern.ch>

Author: ktf

DataFormats/Detectors/TPC/include/DataFormatsTPC/Constants.h

@@ -17,6 +17,8 @@
 #ifndef AliceO2_TPC_Constants_H
 #define AliceO2_TPC_Constants_H
 
+#include "GPUCommonDef.h"
+
 namespace o2
 {
 namespace tpc
@@ -25,17 +27,17 @@ namespace constants
 {
 
 // the number of sectors
-constexpr int MAXSECTOR = 36;
+GPUglobalconstexpr() int MAXSECTOR = 36;
 
 // the number of global pad rows
 #if defined(GPUCA_STANDALONE) && defined(GPUCA_RUN2)
-constexpr int MAXGLOBALPADROW = 159; // Number of pad rows in Run 2, used for GPU TPC tests with Run 2 data
+GPUglobalconstexpr() int MAXGLOBALPADROW = 159; // Number of pad rows in Run 2, used for GPU TPC tests with Run 2 data
 #else
-constexpr int MAXGLOBALPADROW = 152; // Correct number of pad rows in Run 3
+GPUglobalconstexpr() int MAXGLOBALPADROW = 152; // Correct number of pad rows in Run 3
 #endif
 
 // number of LHC bunch crossings per TPC time bin (40 MHz / 5 MHz)
-constexpr int LHCBCPERTIMEBIN = 8;
+GPUglobalconstexpr() int LHCBCPERTIMEBIN = 8;
 } // namespace constants
 } // namespace tpc
 } // namespace o2

GPU/Common/GPUCommonDefAPI.h

@@ -48,6 +48,7 @@
   #define GPUglobal()                               // global memory variable declaration (only used for kernel input pointers)
   #define GPUconstant()                             // constant memory variable declaraion
   #define GPUconstexpr() static constexpr           // constexpr on GPU that needs to be instantiated for dynamic access (e.g. arrays), becomes __constant on GPU
+  #define GPUglobalconstexpr() constexpr            // constexpr variable at program scope, needs the constant address space in MSL
   #define GPUprivate()                              // private memory variable declaration
   #define GPUgeneric()                              // reference / ptr to generic address space
   #define GPUbarrier()                              // synchronize all GPU threads in block
@@ -160,6 +161,7 @@
   #define GPUglobal() device
   #define GPUconstant() constant // TODO: possibly add const __restrict where possible later!
   #define GPUconstexpr() constant
+  #define GPUglobalconstexpr() constant constexpr
   #define GPUprivate() thread
   #define GPUgeneric()
   #define GPUglobalref() device
@@ -255,6 +257,9 @@
 #ifndef GPUconstexprref
 #define GPUconstexprref()
 #endif
+#ifndef GPUglobalconstexpr
+#define GPUglobalconstexpr() constexpr
+#endif
 
 #define GPUrestrict() __restrict__
 
@@ -277,5 +282,5 @@
   #define get_group_id(dim) iBlock
 #endif
 
-// clang-format on
+    // clang-format on
 #endif