Fix overflow in ParallelFor(Long n, ...)

Src/Base/AMReX_GpuLaunch.H

@@ -157,7 +157,12 @@ namespace Gpu {
     makeExecutionConfig (Long N) noexcept
     {
         ExecutionConfig ec(dim3{}, dim3{});
-        ec.numBlocks.x = (std::max(N,Long(1)) + MT - 1) / MT;
+        Long numBlocks = (std::max(N,Long(1)) + MT - 1) / MT;
+        // ensure that blockDim.x*gridDim.x does not overflow
+        numBlocks = std::min(numBlocks, Long(std::numeric_limits<unsigned int>::max()/MT));
+        // ensure that the maximum grid size of 2^31-1 won't be exceeded
+        numBlocks = std::min(numBlocks, Long(std::numeric_limits<int>::max()));
+        ec.numBlocks.x = numBlocks;
         ec.numThreads.x = MT;
         AMREX_ASSERT(MT % Gpu::Device::warp_size == 0);
         return ec;
@@ -167,11 +172,7 @@ namespace Gpu {
     ExecutionConfig
     makeExecutionConfig (const Box& box) noexcept
     {
-        ExecutionConfig ec(dim3{}, dim3{});
-        ec.numBlocks.x = (std::max(box.numPts(),Long(1)) + MT - 1) / MT;
-        ec.numThreads.x = MT;
-        AMREX_ASSERT(MT % Gpu::Device::warp_size == 0);
-        return ec;
+        return makeExecutionConfig<MT>(box.numPts());
     }
 #endif