Spill large statics from constant to global memory

This isn't fuly correct, as ideally we keep track
of what we have put into constant memory and when
it is filled up spill insead of only spilling when
a static is big. But, this is materially better than
what is there.

Fixes Rust-GPU#208.

See also the debugging and discussion in
Rust-GPU#216

Author: LegNeato

crates/rustc_codegen_nvvm/src/context.rs

@@ -22,7 +22,7 @@ use rustc_errors::DiagMessage;
 use rustc_hash::FxHashMap;
 use rustc_middle::dep_graph::DepContext;
 use rustc_middle::ty::layout::{
-    FnAbiError, FnAbiOf, FnAbiRequest, HasTyCtxt, HasTypingEnv, LayoutError,
+    FnAbiError, FnAbiOf, FnAbiRequest, HasTyCtxt, HasTypingEnv, LayoutError, LayoutOf,
 };
 use rustc_middle::ty::layout::{FnAbiOfHelpers, LayoutOfHelpers};
 use rustc_middle::ty::{Ty, TypeVisitableExt};
@@ -40,6 +40,10 @@ use rustc_target::callconv::FnAbi;
 use rustc_target::spec::{HasTargetSpec, Target};
 use tracing::{debug, trace};
 
+/// "There is a total of 64 KB constant memory on a device."
+/// <https://docs.nvidia.com/cuda/archive/12.8.1/pdf/CUDA_C_Best_Practices_Guide.pdf>
+const CONSTANT_MEMORY_SIZE_LIMIT_BYTES: u64 = 64 * 1024;
+
 pub(crate) struct CodegenCx<'ll, 'tcx> {
     pub tcx: TyCtxt<'tcx>,
 
@@ -267,7 +271,18 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
         }
 
         if !is_mutable && self.type_is_freeze(ty) {
-            AddressSpace(4)
+            let layout = self.layout_of(ty);
+            if layout.size.bytes() > CONSTANT_MEMORY_SIZE_LIMIT_BYTES {
+                self.tcx.sess.dcx().warn(format!(
+                    "static `{}` exceeds the constant-memory limit; placing in global memory (performance may be reduced)",
+                    instance
+                ));
+                // Global memory
+                AddressSpace(1)
+            } else {
+                // Constant memory
+                AddressSpace(4)
+            }
         } else {
             AddressSpace::DATA
         }