Add intrinsic for launch-sized workgroup memory on GPUs

Workgroup memory is a memory region that is shared between all
threads in a workgroup on GPUs. Workgroup memory can be allocated
statically or after compilation, when launching a gpu-kernel.
The intrinsic added here returns the pointer to the memory that is
allocated at launch-time.

# Interface

With this change, workgroup memory can be accessed in Rust by
calling the new `gpu_launch_sized_workgroup_mem<T>() -> *mut T`
intrinsic.

It returns the pointer to workgroup memory guaranteeing that it is
aligned to at least the alignment of `T`.
The pointer is dereferencable for the size specified when launching the
current gpu-kernel (which may be the size of `T` but can also be larger
or smaller or zero).

All calls to this intrinsic return a pointer to the same address.

See the intrinsic documentation for more details.

## Alternative Interfaces

It was also considered to expose dynamic workgroup memory as extern
static variables in Rust, like they are represented in LLVM IR.
However, due to the pointer not being guaranteed to be dereferencable
(that depends on the allocated size at runtime), such a global must be
zero-sized, which makes global variables a bad fit.

# Implementation Details

Workgroup memory in amdgpu and nvptx lives in address space 3.
Workgroup memory from a launch 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.

There is no similar way to get the allocated size of launch-sized
workgroup memory on amdgpu an nvptx, so users have to pass this
out-of-band or rely on target specific ways for now.

Author: Flakebi

compiler/rustc_abi/src/lib.rs

@@ -1719,6 +1719,9 @@ pub struct AddressSpace(pub u32);
 impl AddressSpace {
     /// LLVM's `0` address space.
     pub const ZERO: Self = AddressSpace(0);
+    /// The address space for workgroup memory on nvptx and amdgpu.
+    /// See e.g. the `gpu_launch_sized_workgroup_mem` intrinsic for details.
+    pub const GPU_WORKGROUP: 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};
@@ -97,6 +98,28 @@ 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};
@@ -20,7 +22,7 @@ use rustc_session::config::CrateType;
 use rustc_span::{Span, Symbol, sym};
 use rustc_symbol_mangling::{mangle_internal_symbol, symbol_name_for_instance_in_crate};
 use rustc_target::callconv::PassMode;
-use rustc_target::spec::Os;
+use rustc_target::spec::{Arch, Os};
 use tracing::debug;
 
 use crate::abi::FnAbiLlvmExt;
@@ -553,6 +555,44 @@ impl<'ll, 'tcx> IntrinsicCallBuilderMethods<'tcx> for Builder<'_, 'll, 'tcx> {
                 return Ok(());
             }
 
+            sym::gpu_launch_sized_workgroup_mem => {
+                // The name of the global variable is not relevant, the important properties are.
+                // 1. The global is in the address space for workgroup memory
+                // 2. It is an extern global
+                // All instances of extern addrspace(gpu_workgroup) globals are merged in the LLVM backend.
+                // Generate an unnamed global per intrinsic call, so that different kernels can have
+                // different minimum alignments.
+                // See https://docs.nvidia.com/cuda/cuda-c-programming-guide/#shared
+                // FIXME Workaround an nvptx backend issue that extern globals must have a name
+                let name = if tcx.sess.target.arch == Arch::Nvptx64 {
+                    "gpu_launch_sized_workgroup_mem"
+                } else {
+                    ""
+                };
+                let global = self.declare_global_in_addrspace(
+                    name,
+                    self.type_array(self.type_i8(), 0),
+                    AddressSpace::GPU_WORKGROUP,
+                );
+                let ty::RawPtr(inner_ty, _) = result.layout.ty.kind() else { unreachable!() };
+                // The alignment of the global is used to specify the *minimum* alignment that
+                // must be obeyed by the GPU runtime.
+                // When multiple of these global variables are used by a kernel, the maximum alignment is taken.
+                // See https://github.com/llvm/llvm-project/blob/a271d07488a85ce677674bbe8101b10efff58c95/llvm/lib/Target/AMDGPU/AMDGPULowerModuleLDSPass.cpp#L821
+                let alignment = self.align_of(*inner_ty).bytes() as u32;
+                unsafe {
+                    // FIXME Workaround the above issue by taking maximum alignment if the global existed
+                    if tcx.sess.target.arch == Arch::Nvptx64 {
+                        if alignment > llvm::LLVMGetAlignment(global) {
+                            llvm::LLVMSetAlignment(global, alignment);
+                        }
+                    } else {
+                        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

@@ -2017,6 +2017,13 @@ 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 LLVMRustGetNamedValue(
         M: &Module,
         Name: *const c_char,

compiler/rustc_codegen_ssa/src/mir/intrinsic.rs

@@ -111,6 +111,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 sym::abort
                 | sym::unreachable
                 | sym::cold_path
+                | sym::gpu_launch_sized_workgroup_mem
                 | sym::breakpoint
                 | sym::assert_zero_valid
                 | sym::assert_mem_uninitialized_valid

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -132,6 +132,7 @@ fn intrinsic_operation_unsafety(tcx: TyCtxt<'_>, intrinsic_id: LocalDefId) -> hi
         | sym::forget
         | sym::frem_algebraic
         | sym::fsub_algebraic
+        | sym::gpu_launch_sized_workgroup_mem
         | sym::is_val_statically_known
         | sym::log2f16
         | sym::log2f32
@@ -293,6 +294,7 @@ pub(crate) fn check_intrinsic_type(
         sym::offset_of => (1, 0, vec![tcx.types.u32, tcx.types.u32], tcx.types.usize),
         sym::rustc_peek => (1, 0, vec![param(0)], param(0)),
         sym::caller_location => (0, 0, vec![], tcx.caller_location_ty()),
+        sym::gpu_launch_sized_workgroup_mem => (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

@@ -261,10 +261,10 @@ 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);
 
@@ -275,10 +275,21 @@ 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);
+}
+
 // Must match the layout of `rustc_codegen_llvm::llvm::ffi::AttributeKind`.
 enum class LLVMRustAttributeKind {
   AlwaysInline = 0,

compiler/rustc_span/src/symbol.rs

@@ -1152,6 +1152,7 @@ symbols! {
         global_asm,
         global_registration,
         globs,
+        gpu_launch_sized_workgroup_mem,
         gt,
         guard_patterns,
         half_open_range_patterns,

library/core/src/intrinsics/mod.rs

@@ -3436,6 +3436,45 @@ pub(crate) const fn miri_promise_symbolic_alignment(ptr: *const (), align: usize
     )
 }
 
+/// Returns the pointer to workgroup memory allocated at launch-time on GPUs.
+///
+/// Workgroup memory is a memory region that is shared between all threads in
+/// the same workgroup. It is faster to access than other memory but pointers do not
+/// work outside the workgroup where they were obtained.
+/// Workgroup memory can be allocated statically or after compilation, when
+/// launching a gpu-kernel. `gpu_launch_sized_workgroup_mem` returns the pointer to
+/// the memory that is allocated at launch-time.
+/// The size of this memory can differ between launches of a gpu-kernel, depending on
+/// what is specified at launch-time.
+/// However, the alignment is fixed by the kernel itself, at compile-time.
+///
+/// The returned pointer is the start of the workgroup memory region that is
+/// allocated at launch-time.
+/// All calls to `gpu_launch_sized_workgroup_mem` in a workgroup, independent of the
+/// generic type, return the same address, so alias the same memory.
+/// The returned pointer is aligned by at least the alignment of `T`.
+///
+/// # Safety
+///
+/// The pointer is safe to dereference from the start (the returned pointer) up to the
+/// size of workgroup memory that was specified when launching the current gpu-kernel.
+///
+/// The user must take care of synchronizing access to workgroup memory between
+/// threads in a workgroup. The usual data race requirements apply.
+///
+/// # Other APIs
+///
+/// CUDA and HIP call this dynamic shared memory, shared between threads in a block.
+/// OpenCL and SYCL call this local memory, shared between threads in a work-group.
+/// GLSL calls this shared memory, shared between invocations in a work group.
+/// DirectX calls this groupshared memory, shared between threads in a thread-group.
+#[must_use = "returns a pointer that does nothing unless used"]
+#[rustc_intrinsic]
+#[rustc_nounwind]
+#[unstable(feature = "gpu_launch_sized_workgroup_mem", issue = "135513")]
+#[cfg(any(target_arch = "amdgpu", target_arch = "nvptx64"))]
+pub fn gpu_launch_sized_workgroup_mem<T>() -> *mut T;
+
 /// Copies the current location of arglist `src` to the arglist `dst`.
 ///
 /// # Safety

src/tools/tidy/src/style.rs

@@ -222,6 +222,10 @@ fn should_ignore(line: &str) -> bool {
         || static_regex!(
             "\\s*//@ \\!?(count|files|has|has-dir|hasraw|matches|matchesraw|snapshot)\\s.*"
         ).is_match(line)
+        // Matching for FileCheck checks
+        || static_regex!(
+            "\\s*// [a-zA-Z0-9-_]*:\\s.*"
+        ).is_match(line)
 }
 
 /// Returns `true` if `line` is allowed to be longer than the normal limit.