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authorbors <bors@rust-lang.org>2025-07-21 19:48:22 +0000
committerbors <bors@rust-lang.org>2025-07-21 19:48:22 +0000
commit9748d87dc70a9a6725c5dbd76ce29d04752b4f90 (patch)
tree4cffbc92abe5995b0348e464ffe6a256f973e653 /compiler/rustc_codegen_llvm/src/builder/gpu_offload.rs
parent3f9f20f71dd945fe7d044e274094a53c90788269 (diff)
parent6628a4afb5d08555f09878a48f0c4179e7582aa2 (diff)
downloadrust-9748d87dc70a9a6725c5dbd76ce29d04752b4f90.tar.gz
rust-9748d87dc70a9a6725c5dbd76ce29d04752b4f90.zip
Auto merge of #144269 - jieyouxu:rollup-137ysl2, r=jieyouxu
Rollup of 14 pull requests

Successful merges:

 - rust-lang/rust#142097 (gpu offload host code generation)
 - rust-lang/rust#143430 (Lower extra lifetimes before normal generic params.)
 - rust-lang/rust#143768 (Constify Try, From, TryFrom and relevant traits)
 - rust-lang/rust#143816 (Implement `check` for compiletest and RA using tool macro)
 - rust-lang/rust#143985 (rustc_public: de-StableMIR-ize)
 - rust-lang/rust#144027 (clippy: make tests work in stage 1)
 - rust-lang/rust#144080 (Mitigate `#[align]` name resolution ambiguity regression with a rename)
 - rust-lang/rust#144176 (Add approval blocking labels for new bors)
 - rust-lang/rust#144187 (fix handling of base address for TypeId allocations)
 - rust-lang/rust#144212 (Remove the ptr_unique lang item)
 - rust-lang/rust#144243 (Subtree update of `rust-analyzer`)
 - rust-lang/rust#144246 (Don't use another main test file as auxiliary)
 - rust-lang/rust#144251 (rustc-dev-guide subtree update)
 - rust-lang/rust#144254 (opt-dist: make `artifact-dir` an absolute path for `opt-dist local`)

r? `@ghost`
`@rustbot` modify labels: rollup
Diffstat (limited to 'compiler/rustc_codegen_llvm/src/builder/gpu_offload.rs')
-rw-r--r--compiler/rustc_codegen_llvm/src/builder/gpu_offload.rs439
1 files changed, 439 insertions, 0 deletions
diff --git a/compiler/rustc_codegen_llvm/src/builder/gpu_offload.rs b/compiler/rustc_codegen_llvm/src/builder/gpu_offload.rs
new file mode 100644
index 00000000000..1280ab1442a
--- /dev/null
+++ b/compiler/rustc_codegen_llvm/src/builder/gpu_offload.rs
@@ -0,0 +1,439 @@
+use std::ffi::CString;
+
+use llvm::Linkage::*;
+use rustc_abi::Align;
+use rustc_codegen_ssa::back::write::CodegenContext;
+use rustc_codegen_ssa::traits::BaseTypeCodegenMethods;
+
+use crate::builder::SBuilder;
+use crate::common::AsCCharPtr;
+use crate::llvm::AttributePlace::Function;
+use crate::llvm::{self, Linkage, Type, Value};
+use crate::{LlvmCodegenBackend, SimpleCx, attributes};
+
+pub(crate) fn handle_gpu_code<'ll>(
+    _cgcx: &CodegenContext<LlvmCodegenBackend>,
+    cx: &'ll SimpleCx<'_>,
+) {
+    // The offload memory transfer type for each kernel
+    let mut o_types = vec![];
+    let mut kernels = vec![];
+    let offload_entry_ty = add_tgt_offload_entry(&cx);
+    for num in 0..9 {
+        let kernel = cx.get_function(&format!("kernel_{num}"));
+        if let Some(kernel) = kernel {
+            o_types.push(gen_define_handling(&cx, kernel, offload_entry_ty, num));
+            kernels.push(kernel);
+        }
+    }
+
+    gen_call_handling(&cx, &kernels, &o_types);
+}
+
+// What is our @1 here? A magic global, used in our data_{begin/update/end}_mapper:
+// @0 = private unnamed_addr constant [23 x i8] c";unknown;unknown;0;0;;\00", align 1
+// @1 = private unnamed_addr constant %struct.ident_t { i32 0, i32 2, i32 0, i32 22, ptr @0 }, align 8
+fn generate_at_one<'ll>(cx: &'ll SimpleCx<'_>) -> &'ll llvm::Value {
+    // @0 = private unnamed_addr constant [23 x i8] c";unknown;unknown;0;0;;\00", align 1
+    let unknown_txt = ";unknown;unknown;0;0;;";
+    let c_entry_name = CString::new(unknown_txt).unwrap();
+    let c_val = c_entry_name.as_bytes_with_nul();
+    let initializer = crate::common::bytes_in_context(cx.llcx, c_val);
+    let at_zero = add_unnamed_global(&cx, &"", initializer, PrivateLinkage);
+    llvm::set_alignment(at_zero, Align::ONE);
+
+    // @1 = private unnamed_addr constant %struct.ident_t { i32 0, i32 2, i32 0, i32 22, ptr @0 }, align 8
+    let struct_ident_ty = cx.type_named_struct("struct.ident_t");
+    let struct_elems = vec![
+        cx.get_const_i32(0),
+        cx.get_const_i32(2),
+        cx.get_const_i32(0),
+        cx.get_const_i32(22),
+        at_zero,
+    ];
+    let struct_elems_ty: Vec<_> = struct_elems.iter().map(|&x| cx.val_ty(x)).collect();
+    let initializer = crate::common::named_struct(struct_ident_ty, &struct_elems);
+    cx.set_struct_body(struct_ident_ty, &struct_elems_ty, false);
+    let at_one = add_unnamed_global(&cx, &"", initializer, PrivateLinkage);
+    llvm::set_alignment(at_one, Align::EIGHT);
+    at_one
+}
+
+pub(crate) fn add_tgt_offload_entry<'ll>(cx: &'ll SimpleCx<'_>) -> &'ll llvm::Type {
+    let offload_entry_ty = cx.type_named_struct("struct.__tgt_offload_entry");
+    let tptr = cx.type_ptr();
+    let ti64 = cx.type_i64();
+    let ti32 = cx.type_i32();
+    let ti16 = cx.type_i16();
+    // For each kernel to run on the gpu, we will later generate one entry of this type.
+    // copied from LLVM
+    // typedef struct {
+    //   uint64_t Reserved;
+    //   uint16_t Version;
+    //   uint16_t Kind;
+    //   uint32_t Flags; Flags associated with the entry (see Target Region Entry Flags)
+    //   void *Address; Address of global symbol within device image (function or global)
+    //   char *SymbolName;
+    //   uint64_t Size; Size of the entry info (0 if it is a function)
+    //   uint64_t Data;
+    //   void *AuxAddr;
+    // } __tgt_offload_entry;
+    let entry_elements = vec![ti64, ti16, ti16, ti32, tptr, tptr, ti64, ti64, tptr];
+    cx.set_struct_body(offload_entry_ty, &entry_elements, false);
+    offload_entry_ty
+}
+
+fn gen_tgt_kernel_global<'ll>(cx: &'ll SimpleCx<'_>) {
+    let kernel_arguments_ty = cx.type_named_struct("struct.__tgt_kernel_arguments");
+    let tptr = cx.type_ptr();
+    let ti64 = cx.type_i64();
+    let ti32 = cx.type_i32();
+    let tarr = cx.type_array(ti32, 3);
+
+    // Taken from the LLVM APITypes.h declaration:
+    //struct KernelArgsTy {
+    //  uint32_t Version = 0; // Version of this struct for ABI compatibility.
+    //  uint32_t NumArgs = 0; // Number of arguments in each input pointer.
+    //  void **ArgBasePtrs =
+    //      nullptr;                 // Base pointer of each argument (e.g. a struct).
+    //  void **ArgPtrs = nullptr;    // Pointer to the argument data.
+    //  int64_t *ArgSizes = nullptr; // Size of the argument data in bytes.
+    //  int64_t *ArgTypes = nullptr; // Type of the data (e.g. to / from).
+    //  void **ArgNames = nullptr;   // Name of the data for debugging, possibly null.
+    //  void **ArgMappers = nullptr; // User-defined mappers, possibly null.
+    //  uint64_t Tripcount =
+    //      0; // Tripcount for the teams / distribute loop, 0 otherwise.
+    //  struct {
+    //    uint64_t NoWait : 1; // Was this kernel spawned with a `nowait` clause.
+    //    uint64_t IsCUDA : 1; // Was this kernel spawned via CUDA.
+    //    uint64_t Unused : 62;
+    //  } Flags = {0, 0, 0};
+    //  // The number of teams (for x,y,z dimension).
+    //  uint32_t NumTeams[3] = {0, 0, 0};
+    //  // The number of threads (for x,y,z dimension).
+    //  uint32_t ThreadLimit[3] = {0, 0, 0};
+    //  uint32_t DynCGroupMem = 0; // Amount of dynamic cgroup memory requested.
+    //};
+    let kernel_elements =
+        vec![ti32, ti32, tptr, tptr, tptr, tptr, tptr, tptr, ti64, ti64, tarr, tarr, ti32];
+
+    cx.set_struct_body(kernel_arguments_ty, &kernel_elements, false);
+    // For now we don't handle kernels, so for now we just add a global dummy
+    // to make sure that the __tgt_offload_entry is defined and handled correctly.
+    cx.declare_global("my_struct_global2", kernel_arguments_ty);
+}
+
+fn gen_tgt_data_mappers<'ll>(
+    cx: &'ll SimpleCx<'_>,
+) -> (&'ll llvm::Value, &'ll llvm::Value, &'ll llvm::Value, &'ll llvm::Type) {
+    let tptr = cx.type_ptr();
+    let ti64 = cx.type_i64();
+    let ti32 = cx.type_i32();
+
+    let args = vec![tptr, ti64, ti32, tptr, tptr, tptr, tptr, tptr, tptr];
+    let mapper_fn_ty = cx.type_func(&args, cx.type_void());
+    let mapper_begin = "__tgt_target_data_begin_mapper";
+    let mapper_update = "__tgt_target_data_update_mapper";
+    let mapper_end = "__tgt_target_data_end_mapper";
+    let begin_mapper_decl = declare_offload_fn(&cx, mapper_begin, mapper_fn_ty);
+    let update_mapper_decl = declare_offload_fn(&cx, mapper_update, mapper_fn_ty);
+    let end_mapper_decl = declare_offload_fn(&cx, mapper_end, mapper_fn_ty);
+
+    let nounwind = llvm::AttributeKind::NoUnwind.create_attr(cx.llcx);
+    attributes::apply_to_llfn(begin_mapper_decl, Function, &[nounwind]);
+    attributes::apply_to_llfn(update_mapper_decl, Function, &[nounwind]);
+    attributes::apply_to_llfn(end_mapper_decl, Function, &[nounwind]);
+
+    (begin_mapper_decl, update_mapper_decl, end_mapper_decl, mapper_fn_ty)
+}
+
+fn add_priv_unnamed_arr<'ll>(cx: &SimpleCx<'ll>, name: &str, vals: &[u64]) -> &'ll llvm::Value {
+    let ti64 = cx.type_i64();
+    let mut size_val = Vec::with_capacity(vals.len());
+    for &val in vals {
+        size_val.push(cx.get_const_i64(val));
+    }
+    let initializer = cx.const_array(ti64, &size_val);
+    add_unnamed_global(cx, name, initializer, PrivateLinkage)
+}
+
+pub(crate) fn add_unnamed_global<'ll>(
+    cx: &SimpleCx<'ll>,
+    name: &str,
+    initializer: &'ll llvm::Value,
+    l: Linkage,
+) -> &'ll llvm::Value {
+    let llglobal = add_global(cx, name, initializer, l);
+    llvm::LLVMSetUnnamedAddress(llglobal, llvm::UnnamedAddr::Global);
+    llglobal
+}
+
+pub(crate) fn add_global<'ll>(
+    cx: &SimpleCx<'ll>,
+    name: &str,
+    initializer: &'ll llvm::Value,
+    l: Linkage,
+) -> &'ll llvm::Value {
+    let c_name = CString::new(name).unwrap();
+    let llglobal: &'ll llvm::Value = llvm::add_global(cx.llmod, cx.val_ty(initializer), &c_name);
+    llvm::set_global_constant(llglobal, true);
+    llvm::set_linkage(llglobal, l);
+    llvm::set_initializer(llglobal, initializer);
+    llglobal
+}
+
+fn gen_define_handling<'ll>(
+    cx: &'ll SimpleCx<'_>,
+    kernel: &'ll llvm::Value,
+    offload_entry_ty: &'ll llvm::Type,
+    num: i64,
+) -> &'ll llvm::Value {
+    let types = cx.func_params_types(cx.get_type_of_global(kernel));
+    // It seems like non-pointer values are automatically mapped. So here, we focus on pointer (or
+    // reference) types.
+    let num_ptr_types = types
+        .iter()
+        .map(|&x| matches!(cx.type_kind(x), rustc_codegen_ssa::common::TypeKind::Pointer))
+        .count();
+
+    // We do not know their size anymore at this level, so hardcode a placeholder.
+    // A follow-up pr will track these from the frontend, where we still have Rust types.
+    // Then, we will be able to figure out that e.g. `&[f32;256]` will result in 4*256 bytes.
+    // I decided that 1024 bytes is a great placeholder value for now.
+    add_priv_unnamed_arr(&cx, &format!(".offload_sizes.{num}"), &vec![1024; num_ptr_types]);
+    // Here we figure out whether something needs to be copied to the gpu (=1), from the gpu (=2),
+    // or both to and from the gpu (=3). Other values shouldn't affect us for now.
+    // A non-mutable reference or pointer will be 1, an array that's not read, but fully overwritten
+    // will be 2. For now, everything is 3, until we have our frontend set up.
+    let o_types =
+        add_priv_unnamed_arr(&cx, &format!(".offload_maptypes.{num}"), &vec![3; num_ptr_types]);
+    // Next: For each function, generate these three entries. A weak constant,
+    // the llvm.rodata entry name, and  the omp_offloading_entries value
+
+    let name = format!(".kernel_{num}.region_id");
+    let initializer = cx.get_const_i8(0);
+    let region_id = add_unnamed_global(&cx, &name, initializer, WeakAnyLinkage);
+
+    let c_entry_name = CString::new(format!("kernel_{num}")).unwrap();
+    let c_val = c_entry_name.as_bytes_with_nul();
+    let offload_entry_name = format!(".offloading.entry_name.{num}");
+
+    let initializer = crate::common::bytes_in_context(cx.llcx, c_val);
+    let llglobal = add_unnamed_global(&cx, &offload_entry_name, initializer, InternalLinkage);
+    llvm::set_alignment(llglobal, Align::ONE);
+    llvm::set_section(llglobal, c".llvm.rodata.offloading");
+
+    // Not actively used yet, for calling real kernels
+    let name = format!(".offloading.entry.kernel_{num}");
+
+    // See the __tgt_offload_entry documentation above.
+    let reserved = cx.get_const_i64(0);
+    let version = cx.get_const_i16(1);
+    let kind = cx.get_const_i16(1);
+    let flags = cx.get_const_i32(0);
+    let size = cx.get_const_i64(0);
+    let data = cx.get_const_i64(0);
+    let aux_addr = cx.const_null(cx.type_ptr());
+    let elems = vec![reserved, version, kind, flags, region_id, llglobal, size, data, aux_addr];
+
+    let initializer = crate::common::named_struct(offload_entry_ty, &elems);
+    let c_name = CString::new(name).unwrap();
+    let llglobal = llvm::add_global(cx.llmod, offload_entry_ty, &c_name);
+    llvm::set_global_constant(llglobal, true);
+    llvm::set_linkage(llglobal, WeakAnyLinkage);
+    llvm::set_initializer(llglobal, initializer);
+    llvm::set_alignment(llglobal, Align::ONE);
+    let c_section_name = CString::new(".omp_offloading_entries").unwrap();
+    llvm::set_section(llglobal, &c_section_name);
+    o_types
+}
+
+fn declare_offload_fn<'ll>(
+    cx: &'ll SimpleCx<'_>,
+    name: &str,
+    ty: &'ll llvm::Type,
+) -> &'ll llvm::Value {
+    crate::declare::declare_simple_fn(
+        cx,
+        name,
+        llvm::CallConv::CCallConv,
+        llvm::UnnamedAddr::No,
+        llvm::Visibility::Default,
+        ty,
+    )
+}
+
+// For each kernel *call*, we now use some of our previous declared globals to move data to and from
+// the gpu. We don't have a proper frontend yet, so we assume that every call to a kernel function
+// from main is intended to run on the GPU. For now, we only handle the data transfer part of it.
+// If two consecutive kernels use the same memory, we still move it to the host and back to the gpu.
+// Since in our frontend users (by default) don't have to specify data transfer, this is something
+// we should optimize in the future! We also assume that everything should be copied back and forth,
+// but sometimes we can directly zero-allocate on the device and only move back, or if something is
+// immutable, we might only copy it to the device, but not back.
+//
+// Current steps:
+// 0. Alloca some variables for the following steps
+// 1. set insert point before kernel call.
+// 2. generate all the GEPS and stores, to be used in 3)
+// 3. generate __tgt_target_data_begin calls to move data to the GPU
+//
+// unchanged: keep kernel call. Later move the kernel to the GPU
+//
+// 4. set insert point after kernel call.
+// 5. generate all the GEPS and stores, to be used in 6)
+// 6. generate __tgt_target_data_end calls to move data from the GPU
+fn gen_call_handling<'ll>(
+    cx: &'ll SimpleCx<'_>,
+    _kernels: &[&'ll llvm::Value],
+    o_types: &[&'ll llvm::Value],
+) {
+    // %struct.__tgt_bin_desc = type { i32, ptr, ptr, ptr }
+    let tptr = cx.type_ptr();
+    let ti32 = cx.type_i32();
+    let tgt_bin_desc_ty = vec![ti32, tptr, tptr, tptr];
+    let tgt_bin_desc = cx.type_named_struct("struct.__tgt_bin_desc");
+    cx.set_struct_body(tgt_bin_desc, &tgt_bin_desc_ty, false);
+
+    gen_tgt_kernel_global(&cx);
+    let (begin_mapper_decl, _, end_mapper_decl, fn_ty) = gen_tgt_data_mappers(&cx);
+
+    let main_fn = cx.get_function("main");
+    let Some(main_fn) = main_fn else { return };
+    let kernel_name = "kernel_1";
+    let call = unsafe {
+        llvm::LLVMRustGetFunctionCall(main_fn, kernel_name.as_c_char_ptr(), kernel_name.len())
+    };
+    let Some(kernel_call) = call else {
+        return;
+    };
+    let kernel_call_bb = unsafe { llvm::LLVMGetInstructionParent(kernel_call) };
+    let called = unsafe { llvm::LLVMGetCalledValue(kernel_call).unwrap() };
+    let mut builder = SBuilder::build(cx, kernel_call_bb);
+
+    let types = cx.func_params_types(cx.get_type_of_global(called));
+    let num_args = types.len() as u64;
+
+    // Step 0)
+    // %struct.__tgt_bin_desc = type { i32, ptr, ptr, ptr }
+    // %6 = alloca %struct.__tgt_bin_desc, align 8
+    unsafe { llvm::LLVMRustPositionBuilderPastAllocas(builder.llbuilder, main_fn) };
+
+    let tgt_bin_desc_alloca = builder.direct_alloca(tgt_bin_desc, Align::EIGHT, "EmptyDesc");
+
+    let ty = cx.type_array(cx.type_ptr(), num_args);
+    // Baseptr are just the input pointer to the kernel, stored in a local alloca
+    let a1 = builder.direct_alloca(ty, Align::EIGHT, ".offload_baseptrs");
+    // Ptrs are the result of a gep into the baseptr, at least for our trivial types.
+    let a2 = builder.direct_alloca(ty, Align::EIGHT, ".offload_ptrs");
+    // These represent the sizes in bytes, e.g. the entry for `&[f64; 16]` will be 8*16.
+    let ty2 = cx.type_array(cx.type_i64(), num_args);
+    let a4 = builder.direct_alloca(ty2, Align::EIGHT, ".offload_sizes");
+    // Now we allocate once per function param, a copy to be passed to one of our maps.
+    let mut vals = vec![];
+    let mut geps = vec![];
+    let i32_0 = cx.get_const_i32(0);
+    for (index, in_ty) in types.iter().enumerate() {
+        // get function arg, store it into the alloca, and read it.
+        let p = llvm::get_param(called, index as u32);
+        let name = llvm::get_value_name(p);
+        let name = str::from_utf8(&name).unwrap();
+        let arg_name = format!("{name}.addr");
+        let alloca = builder.direct_alloca(in_ty, Align::EIGHT, &arg_name);
+
+        builder.store(p, alloca, Align::EIGHT);
+        let val = builder.load(in_ty, alloca, Align::EIGHT);
+        let gep = builder.inbounds_gep(cx.type_f32(), val, &[i32_0]);
+        vals.push(val);
+        geps.push(gep);
+    }
+
+    // Step 1)
+    unsafe { llvm::LLVMRustPositionBefore(builder.llbuilder, kernel_call) };
+    builder.memset(tgt_bin_desc_alloca, cx.get_const_i8(0), cx.get_const_i64(32), Align::EIGHT);
+
+    let mapper_fn_ty = cx.type_func(&[cx.type_ptr()], cx.type_void());
+    let register_lib_decl = declare_offload_fn(&cx, "__tgt_register_lib", mapper_fn_ty);
+    let unregister_lib_decl = declare_offload_fn(&cx, "__tgt_unregister_lib", mapper_fn_ty);
+    let init_ty = cx.type_func(&[], cx.type_void());
+    let init_rtls_decl = declare_offload_fn(cx, "__tgt_init_all_rtls", init_ty);
+
+    // call void @__tgt_register_lib(ptr noundef %6)
+    builder.call(mapper_fn_ty, register_lib_decl, &[tgt_bin_desc_alloca], None);
+    // call void @__tgt_init_all_rtls()
+    builder.call(init_ty, init_rtls_decl, &[], None);
+
+    for i in 0..num_args {
+        let idx = cx.get_const_i32(i);
+        let gep1 = builder.inbounds_gep(ty, a1, &[i32_0, idx]);
+        builder.store(vals[i as usize], gep1, Align::EIGHT);
+        let gep2 = builder.inbounds_gep(ty, a2, &[i32_0, idx]);
+        builder.store(geps[i as usize], gep2, Align::EIGHT);
+        let gep3 = builder.inbounds_gep(ty2, a4, &[i32_0, idx]);
+        // As mentioned above, we don't use Rust type information yet. So for now we will just
+        // assume that we have 1024 bytes, 256 f32 values.
+        // FIXME(offload): write an offload frontend and handle arbitrary types.
+        builder.store(cx.get_const_i64(1024), gep3, Align::EIGHT);
+    }
+
+    // For now we have a very simplistic indexing scheme into our
+    // offload_{baseptrs,ptrs,sizes}. We will probably improve this along with our gpu frontend pr.
+    fn get_geps<'a, 'll>(
+        builder: &mut SBuilder<'a, 'll>,
+        cx: &'ll SimpleCx<'ll>,
+        ty: &'ll Type,
+        ty2: &'ll Type,
+        a1: &'ll Value,
+        a2: &'ll Value,
+        a4: &'ll Value,
+    ) -> (&'ll Value, &'ll Value, &'ll Value) {
+        let i32_0 = cx.get_const_i32(0);
+
+        let gep1 = builder.inbounds_gep(ty, a1, &[i32_0, i32_0]);
+        let gep2 = builder.inbounds_gep(ty, a2, &[i32_0, i32_0]);
+        let gep3 = builder.inbounds_gep(ty2, a4, &[i32_0, i32_0]);
+        (gep1, gep2, gep3)
+    }
+
+    fn generate_mapper_call<'a, 'll>(
+        builder: &mut SBuilder<'a, 'll>,
+        cx: &'ll SimpleCx<'ll>,
+        geps: (&'ll Value, &'ll Value, &'ll Value),
+        o_type: &'ll Value,
+        fn_to_call: &'ll Value,
+        fn_ty: &'ll Type,
+        num_args: u64,
+        s_ident_t: &'ll Value,
+    ) {
+        let nullptr = cx.const_null(cx.type_ptr());
+        let i64_max = cx.get_const_i64(u64::MAX);
+        let num_args = cx.get_const_i32(num_args);
+        let args =
+            vec![s_ident_t, i64_max, num_args, geps.0, geps.1, geps.2, o_type, nullptr, nullptr];
+        builder.call(fn_ty, fn_to_call, &args, None);
+    }
+
+    // Step 2)
+    let s_ident_t = generate_at_one(&cx);
+    let o = o_types[0];
+    let geps = get_geps(&mut builder, &cx, ty, ty2, a1, a2, a4);
+    generate_mapper_call(&mut builder, &cx, geps, o, begin_mapper_decl, fn_ty, num_args, s_ident_t);
+
+    // Step 3)
+    // Here we will add code for the actual kernel launches in a follow-up PR.
+    // FIXME(offload): launch kernels
+
+    // Step 4)
+    unsafe { llvm::LLVMRustPositionAfter(builder.llbuilder, kernel_call) };
+
+    let geps = get_geps(&mut builder, &cx, ty, ty2, a1, a2, a4);
+    generate_mapper_call(&mut builder, &cx, geps, o, end_mapper_decl, fn_ty, num_args, s_ident_t);
+
+    builder.call(mapper_fn_ty, unregister_lib_decl, &[tgt_bin_desc_alloca], None);
+
+    // With this we generated the following begin and end mappers. We could easily generate the
+    // update mapper in an update.
+    // call void @__tgt_target_data_begin_mapper(ptr @1, i64 -1, i32 3, ptr %27, ptr %28, ptr %29, ptr @.offload_maptypes, ptr null, ptr null)
+    // call void @__tgt_target_data_update_mapper(ptr @1, i64 -1, i32 2, ptr %46, ptr %47, ptr %48, ptr @.offload_maptypes.1, ptr null, ptr null)
+    // call void @__tgt_target_data_end_mapper(ptr @1, i64 -1, i32 3, ptr %49, ptr %50, ptr %51, ptr @.offload_maptypes, ptr null, ptr null)
+}