Add intrinsic for dynamic shared memory

Shared memory is a memory region that is shared between all threads in a
thread block/workgroup on GPUs. Dynamic shared memory is in that memory
region, though the allocated size is specified late, when launching a
kernel.

Shared memory in amdgpu and nvptx lives in address space 3. Dynamic
shared memory is implemented by creating an external global variable in
address space 3. The global is declared with size 0, as the actual size
is only known at runtime. It is defined behavior in LLVM to access an
external global outside the defined size.

As far as I know, there is no similar way to get the allocated size of
dynamic shared memory on amdgpu an nvptx, so users have to pass this
out-of-band or rely on target specific ways.

Author: Flakebi

compiler/rustc_abi/src/lib.rs

@@ -1716,6 +1716,8 @@ pub struct AddressSpace(pub u32);
 impl AddressSpace {
     /// LLVM's `0` address space.
     pub const ZERO: Self = AddressSpace(0);
+    /// The address space for shared memory on nvptx and amdgpu.
+    pub const SHARED: Self = AddressSpace(3);
 }
 
 /// The way we represent values to the backend

compiler/rustc_codegen_llvm/src/declare.rs

@@ -14,6 +14,7 @@
 use std::borrow::Borrow;
 
 use itertools::Itertools;
+use rustc_abi::AddressSpace;
 use rustc_codegen_ssa::traits::TypeMembershipCodegenMethods;
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_middle::ty::{Instance, Ty};
@@ -99,6 +100,28 @@ impl<'ll, CX: Borrow<SCx<'ll>>> GenericCx<'ll, CX> {
             )
         }
     }
+
+    /// Declare a global value in a specific address space.
+    ///
+    /// If there’s a value with the same name already declared, the function will
+    /// return its Value instead.
+    pub(crate) fn declare_global_in_addrspace(
+        &self,
+        name: &str,
+        ty: &'ll Type,
+        addr_space: AddressSpace,
+    ) -> &'ll Value {
+        debug!("declare_global(name={name:?}, addrspace={addr_space:?})");
+        unsafe {
+            llvm::LLVMRustGetOrInsertGlobalInAddrspace(
+                (**self).borrow().llmod,
+                name.as_c_char_ptr(),
+                name.len(),
+                ty,
+                addr_space.0,
+            )
+        }
+    }
 }
 
 impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -1,7 +1,9 @@
 use std::assert_matches::assert_matches;
 use std::cmp::Ordering;
 
-use rustc_abi::{Align, BackendRepr, ExternAbi, Float, HasDataLayout, Primitive, Size};
+use rustc_abi::{
+    AddressSpace, Align, BackendRepr, ExternAbi, Float, HasDataLayout, Primitive, Size,
+};
 use rustc_codegen_ssa::base::{compare_simd_types, wants_msvc_seh, wants_wasm_eh};
 use rustc_codegen_ssa::codegen_attrs::autodiff_attrs;
 use rustc_codegen_ssa::common::{IntPredicate, TypeKind};
@@ -532,6 +534,22 @@ impl<'ll, 'tcx> IntrinsicCallBuilderMethods<'tcx> for Builder<'_, 'll, 'tcx> {
                 return Ok(());
             }
 
+            sym::dynamic_shared_memory => {
+                let global = self.declare_global_in_addrspace(
+                    "dynamic_shared_memory",
+                    self.type_array(self.type_i8(), 0),
+                    AddressSpace::SHARED,
+                );
+                let ty::RawPtr(inner_ty, _) = result.layout.ty.kind() else { unreachable!() };
+                let alignment = self.align_of(*inner_ty).bytes() as u32;
+                unsafe {
+                    if alignment > llvm::LLVMGetAlignment(global) {
+                        llvm::LLVMSetAlignment(global, alignment);
+                    }
+                }
+                self.cx().const_pointercast(global, self.type_ptr())
+            }
+
             _ if name.as_str().starts_with("simd_") => {
                 // Unpack non-power-of-2 #[repr(packed, simd)] arguments.
                 // This gives them the expected layout of a regular #[repr(simd)] vector.

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1907,6 +1907,13 @@ unsafe extern "C" {
         NameLen: size_t,
         T: &'a Type,
     ) -> &'a Value;
+    pub(crate) fn LLVMRustGetOrInsertGlobalInAddrspace<'a>(
+        M: &'a Module,
+        Name: *const c_char,
+        NameLen: size_t,
+        T: &'a Type,
+        AddressSpace: c_uint,
+    ) -> &'a Value;
     pub(crate) fn LLVMRustInsertPrivateGlobal<'a>(M: &'a Module, T: &'a Type) -> &'a Value;
     pub(crate) fn LLVMRustGetNamedValue(
         M: &Module,

compiler/rustc_codegen_ssa/src/mir/intrinsic.rs

@@ -110,6 +110,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 sym::abort
                 | sym::unreachable
                 | sym::cold_path
+                | sym::dynamic_shared_memory
                 | sym::breakpoint
                 | sym::assert_zero_valid
                 | sym::assert_mem_uninitialized_valid

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -74,6 +74,7 @@ fn intrinsic_operation_unsafety(tcx: TyCtxt<'_>, intrinsic_id: LocalDefId) -> hi
         | sym::align_of
         | sym::needs_drop
         | sym::caller_location
+        | sym::dynamic_shared_memory
         | sym::add_with_overflow
         | sym::sub_with_overflow
         | sym::mul_with_overflow
@@ -213,6 +214,7 @@ pub(crate) fn check_intrinsic_type(
         }
         sym::rustc_peek => (1, 0, vec![param(0)], param(0)),
         sym::caller_location => (0, 0, vec![], tcx.caller_location_ty()),
+        sym::dynamic_shared_memory => (1, 0, vec![], Ty::new_mut_ptr(tcx, param(0))),
         sym::assert_inhabited | sym::assert_zero_valid | sym::assert_mem_uninitialized_valid => {
             (1, 0, vec![], tcx.types.unit)
         }

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -209,10 +209,11 @@ extern "C" LLVMValueRef LLVMRustGetOrInsertFunction(LLVMModuleRef M,
                   .getCallee());
 }
 
-extern "C" LLVMValueRef LLVMRustGetOrInsertGlobal(LLVMModuleRef M,
-                                                  const char *Name,
-                                                  size_t NameLen,
-                                                  LLVMTypeRef Ty) {
+extern "C" LLVMValueRef LLVMRustGetOrInsertGlobalInAddrspace(LLVMModuleRef M,
+                                                             const char *Name,
+                                                             size_t NameLen,
+                                                             LLVMTypeRef Ty,
+                                                             unsigned AddressSpace) {
   Module *Mod = unwrap(M);
   auto NameRef = StringRef(Name, NameLen);
 
@@ -223,10 +224,20 @@ extern "C" LLVMValueRef LLVMRustGetOrInsertGlobal(LLVMModuleRef M,
   GlobalVariable *GV = Mod->getGlobalVariable(NameRef, true);
   if (!GV)
     GV = new GlobalVariable(*Mod, unwrap(Ty), false,
-                            GlobalValue::ExternalLinkage, nullptr, NameRef);
+                            GlobalValue::ExternalLinkage, nullptr, NameRef,
+                            nullptr, GlobalValue::NotThreadLocal, AddressSpace);
   return wrap(GV);
 }
 
+extern "C" LLVMValueRef LLVMRustGetOrInsertGlobal(LLVMModuleRef M,
+                                                  const char *Name,
+                                                  size_t NameLen,
+                                                  LLVMTypeRef Ty) {
+  Module *Mod = unwrap(M);
+  unsigned AddressSpace = Mod->getDataLayout().getDefaultGlobalsAddressSpace();
+  return LLVMRustGetOrInsertGlobalInAddrspace(M, Name, NameLen, Ty, AddressSpace);
+}
+
 extern "C" LLVMValueRef LLVMRustInsertPrivateGlobal(LLVMModuleRef M,
                                                     LLVMTypeRef Ty) {
   return wrap(new GlobalVariable(*unwrap(M), unwrap(Ty), false,

compiler/rustc_span/src/symbol.rs

@@ -903,6 +903,7 @@ symbols! {
         dyn_star,
         dyn_trait,
         dynamic_no_pic: "dynamic-no-pic",
+        dynamic_shared_memory,
         e,
         edition_panic,
         effective_target_features,

library/core/src/intrinsics/mod.rs

@@ -3238,6 +3238,23 @@ pub(crate) const fn miri_promise_symbolic_alignment(ptr: *const (), align: usize
     )
 }
 
+/// Returns a pointer to dynamic shared memory.
+///
+/// The returned pointer is the start of the dynamic shared memory region.
+/// All pointers returned by `dynamic_shared_memory` point to the same address,
+/// so alias the same memory.
+/// The returned pointer is aligned by at least the alignment of `T`.
+///
+/// # Other APIs
+///
+/// CUDA and HIP call this shared memory.
+/// OpenCL and SYCL call this local memory.
+#[rustc_intrinsic]
+#[rustc_nounwind]
+#[unstable(feature = "dynamic_shared_memory", issue = "135513")]
+#[cfg(any(target_arch = "amdgpu", target_arch = "nvptx64"))]
+pub fn dynamic_shared_memory<T: ?Sized>() -> *mut T;
+
 /// Copies the current location of arglist `src` to the arglist `dst`.
 ///
 /// FIXME: document safety requirements

src/bootstrap/src/core/build_steps/compile.rs

@@ -668,6 +668,14 @@ pub fn std_cargo(builder: &Builder<'_>, target: TargetSelection, cargo: &mut Car
         cargo.rustflag("-Cforce-unwind-tables=yes");
     }
 
+    // amdgcn must have a cpu specified, otherwise it refuses to compile.
+    // We want to be able to run tests for amdgcn that depend on core, therefore
+    // we need to be able to compiler core.
+    // The cpu used here must match in tests that use the standard library.
+    if target.contains("amdgcn") && target.file.is_none() {
+        cargo.rustflag("-Ctarget-cpu=gfx900");
+    }
+
     // Enable frame pointers by default for the library. Note that they are still controlled by a
     // separate setting for the compiler.
     cargo.rustflag("-Zunstable-options");