GPU: let the constant-address-space constants be built on Metal

A constructor's implicit `this` is generic in MSL 4.1, and a generic pointer does
not reach the constant address space, so a class-type constant at program scope
could not be constructed at all: gpustd::bitset for the DetID and GlobalTrackID
masks, CfChargePos for INVALID_CHARGE_POS.

MSL lets a member function be qualified with the address space of its `this`, so
each of these gains a constant-qualified overload next to the existing one,
alongside the copy constructor and the OpenCL __constant one that are already
there for the same reason. Only the members actually called on a constant object
need it.

Author: ktf

GPU/GPUTracking/TPCClusterFinder/CfChargePos.h

@@ -34,6 +34,14 @@ struct CfChargePos {
     : gpad(tpcGlobalPadIdx(row, pad)), timePadded(t + GPUCF_PADDING_TIME)
   {
   }
+#ifdef __METAL__
+  // INVALID_CHARGE_POS below lives in the constant address space, which a
+  // generic `this` does not reach in MSL.
+  constexpr GPUhdi() CfChargePos(tpccf::Row row, tpccf::Pad pad, tpccf::TPCFragmentTime t) constant
+    : gpad(tpcGlobalPadIdx(row, pad)), timePadded(t + GPUCF_PADDING_TIME)
+  {
+  }
+#endif
 
   GPUdi() CfChargePos(const tpccf::GlobalPad& p, const tpccf::TPCFragmentTime& t) : gpad(p), timePadded(t) {}
 

GPU/Utils/GPUCommonBitSet.h

@@ -47,6 +47,11 @@ class bitset
   GPUdDefault() constexpr bitset(const __constant bitset&) = default;
 #endif // __OPENCL__
   GPUd() constexpr bitset(uint32_t vv) : v(vv) {};
+#ifdef __METAL__
+  // Objects in the constant address space are built and read through their own
+  // overloads: a generic `this` does not reach constant memory in MSL.
+  GPUd() constexpr bitset(uint32_t vv) constant : v(vv) {};
+#endif
   static GPUglobalconstexpr() uint32_t full_set = ((1ul << N) - 1ul);
 
   GPUd() constexpr bool all() const { return (v & full_set) == full_set; }
@@ -83,6 +88,13 @@ class bitset
   GPUd() constexpr bool operator!=(const bitset b) const { return v != b.v; }
 
   GPUd() constexpr bool operator[](uint32_t i) const { return (v >> i) & 1u; }
+#ifdef __METAL__
+  GPUd() constexpr bitset operator|(const bitset b) constant { return v | b.v; }
+  GPUd() constexpr bitset operator&(const bitset b) constant { return v & b.v; }
+  GPUd() constexpr bool operator[](uint32_t i) constant { return (v >> i) & 1u; }
+  GPUd() constexpr bool any() constant { return v & full_set; }
+  GPUd() constexpr uint32_t to_ulong() constant { return v; }
+#endif
 
   GPUd() constexpr uint32_t to_ulong() const { return v; }