Spill large statics from constant to global memory

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

An argument can be made to just _always_ use global
memory and we don't have to worry about getting the
packing right.

Fixes #208.

See also the debugging and discussion in
#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
         }