[flang][cuda] Add support for NV_CUDAFOR_DEVICE_IS_MANAGED (#133778)

Add support for the environment variable `NV_CUDAFOR_DEVICE_IS_MANAGED`
as described in the documentation:
https://docs.nvidia.com/hpc-sdk/compilers/cuda-fortran-prog-guide/index.html#controlling-device-data-is-managed.

This mainly switch device allocation to managed allocation.

flang-rt/include/flang-rt/runtime/environment.h

@@ -59,6 +59,7 @@ struct ExecutionEnvironment {
 
   // CUDA related variables
   std::size_t cudaStackLimit{0}; // ACC_OFFLOAD_STACK_SIZE
+  bool cudaDeviceIsManaged{false}; // NV_CUDAFOR_DEVICE_IS_MANAGED
 };
 
 RT_OFFLOAD_VAR_GROUP_BEGIN

flang-rt/lib/cuda/allocator.cpp

@@ -9,6 +9,7 @@
 #include "flang/Runtime/CUDA/allocator.h"
 #include "flang-rt/runtime/allocator-registry.h"
 #include "flang-rt/runtime/derived.h"
+#include "flang-rt/runtime/environment.h"
 #include "flang-rt/runtime/stat.h"
 #include "flang-rt/runtime/terminator.h"
 #include "flang-rt/runtime/type-info.h"
@@ -43,7 +44,12 @@ void CUFFreePinned(void *p) { CUDA_REPORT_IF_ERROR(cudaFreeHost(p)); }
 
 void *CUFAllocDevice(std::size_t sizeInBytes) {
   void *p;
-  CUDA_REPORT_IF_ERROR(cudaMalloc(&p, sizeInBytes));
+  if (Fortran::runtime::executionEnvironment.cudaDeviceIsManaged) {
+    CUDA_REPORT_IF_ERROR(
+        cudaMallocManaged((void **)&p, sizeInBytes, cudaMemAttachGlobal));
+  } else {
+    CUDA_REPORT_IF_ERROR(cudaMalloc(&p, sizeInBytes));
+  }
   return p;
 }
 

flang-rt/lib/cuda/memory.cpp

@@ -9,6 +9,7 @@
 #include "flang/Runtime/CUDA/memory.h"
 #include "flang-rt/runtime/assign-impl.h"
 #include "flang-rt/runtime/descriptor.h"
+#include "flang-rt/runtime/environment.h"
 #include "flang-rt/runtime/terminator.h"
 #include "flang/Runtime/CUDA/common.h"
 #include "flang/Runtime/CUDA/descriptor.h"
@@ -26,7 +27,12 @@ void *RTDEF(CUFMemAlloc)(
   void *ptr = nullptr;
   if (bytes != 0) {
     if (type == kMemTypeDevice) {
-      CUDA_REPORT_IF_ERROR(cudaMalloc((void **)&ptr, bytes));
+      if (Fortran::runtime::executionEnvironment.cudaDeviceIsManaged) {
+        CUDA_REPORT_IF_ERROR(
+            cudaMallocManaged((void **)&ptr, bytes, cudaMemAttachGlobal));
+      } else {
+        CUDA_REPORT_IF_ERROR(cudaMalloc((void **)&ptr, bytes));
+      }
     } else if (type == kMemTypeManaged || type == kMemTypeUnified) {
       CUDA_REPORT_IF_ERROR(
           cudaMallocManaged((void **)&ptr, bytes, cudaMemAttachGlobal));

flang-rt/lib/runtime/environment.cpp

@@ -155,6 +155,19 @@ void ExecutionEnvironment::Configure(int ac, const char *av[],
     }
   }
 
+  if (auto *x{std::getenv("NV_CUDAFOR_DEVICE_IS_MANAGED")}) {
+    char *end;
+    auto n{std::strtol(x, &end, 10)};
+    if (n >= 0 && n <= 1 && *end == '\0') {
+      cudaDeviceIsManaged = n != 0;
+    } else {
+      std::fprintf(stderr,
+          "Fortran runtime: NV_CUDAFOR_DEVICE_IS_MANAGED=%s is invalid; "
+          "ignored\n",
+          x);
+    }
+  }
+
   // TODO: Set RP/ROUND='PROCESSOR_DEFINED' from environment
 }