rustc_codegen_llvm/builder/

gpu_offload.rs

use std::ffi::CString;

use llvm::Linkage::*;
use rustc_abi::Align;
use rustc_codegen_ssa::traits::{BaseTypeCodegenMethods, BuilderMethods};
use rustc_middle::ty::offload_meta::OffloadMetadata;

use crate::builder::Builder;
use crate::common::CodegenCx;
use crate::llvm::AttributePlace::Function;
use crate::llvm::{self, Linkage, Type, Value};
use crate::{SimpleCx, attributes};

// LLVM kernel-independent globals required for offloading
pub(crate) struct OffloadGlobals<'ll> {
    pub launcher_fn: &'ll llvm::Value,
    pub launcher_ty: &'ll llvm::Type,

    pub bin_desc: &'ll llvm::Type,

    pub kernel_args_ty: &'ll llvm::Type,

    pub offload_entry_ty: &'ll llvm::Type,

    pub begin_mapper: &'ll llvm::Value,
    pub end_mapper: &'ll llvm::Value,
    pub mapper_fn_ty: &'ll llvm::Type,

    pub ident_t_global: &'ll llvm::Value,

    pub register_lib: &'ll llvm::Value,
    pub unregister_lib: &'ll llvm::Value,
    pub init_rtls: &'ll llvm::Value,
}

impl<'ll> OffloadGlobals<'ll> {
    pub(crate) fn declare(cx: &CodegenCx<'ll, '_>) -> Self {
        let (launcher_fn, launcher_ty) = generate_launcher(cx);
        let kernel_args_ty = KernelArgsTy::new_decl(cx);
        let offload_entry_ty = TgtOffloadEntry::new_decl(cx);
        let (begin_mapper, _, end_mapper, mapper_fn_ty) = gen_tgt_data_mappers(cx);
        let ident_t_global = generate_at_one(cx);

        let tptr = cx.type_ptr();
        let ti32 = cx.type_i32();
        let tgt_bin_desc_ty = vec![ti32, tptr, tptr, tptr];
        let bin_desc = cx.type_named_struct("struct.__tgt_bin_desc");
        cx.set_struct_body(bin_desc, &tgt_bin_desc_ty, false);

        let register_lib = declare_offload_fn(&cx, "__tgt_register_lib", mapper_fn_ty);
        let unregister_lib = declare_offload_fn(&cx, "__tgt_unregister_lib", mapper_fn_ty);
        let init_ty = cx.type_func(&[], cx.type_void());
        let init_rtls = declare_offload_fn(cx, "__tgt_init_all_rtls", init_ty);

        OffloadGlobals {
            launcher_fn,
            launcher_ty,
            bin_desc,
            kernel_args_ty,
            offload_entry_ty,
            begin_mapper,
            end_mapper,
            mapper_fn_ty,
            ident_t_global,
            register_lib,
            unregister_lib,
            init_rtls,
        }
    }
}

// ; Function Attrs: nounwind
// declare i32 @__tgt_target_kernel(ptr, i64, i32, i32, ptr, ptr) #2
fn generate_launcher<'ll>(cx: &CodegenCx<'ll, '_>) -> (&'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, ti32, tptr, tptr];
    let tgt_fn_ty = cx.type_func(&args, ti32);
    let name = "__tgt_target_kernel";
    let tgt_decl = declare_offload_fn(&cx, name, tgt_fn_ty);
    let nounwind = llvm::AttributeKind::NoUnwind.create_attr(cx.llcx);
    attributes::apply_to_llfn(tgt_decl, Function, &[nounwind]);
    (tgt_decl, tgt_fn_ty)
}

// 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
// FIXME(offload): @0 should include the file name (e.g. lib.rs) in which the function to be
// offloaded was defined.
pub(crate) fn generate_at_one<'ll>(cx: &CodegenCx<'ll, '_>) -> &'ll llvm::Value {
    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) struct TgtOffloadEntry {
    //   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;
}

impl TgtOffloadEntry {
    pub(crate) fn new_decl<'ll>(cx: &CodegenCx<'ll, '_>) -> &'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
        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 new<'ll>(
        cx: &CodegenCx<'ll, '_>,
        region_id: &'ll Value,
        llglobal: &'ll Value,
    ) -> [&'ll Value; 9] {
        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());
        [reserved, version, kind, flags, region_id, llglobal, size, data, aux_addr]
    }
}

// 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}; // totals to 64 Bit, 8 Byte
    //  // 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.
}

impl KernelArgsTy {
    const OFFLOAD_VERSION: u64 = 3;
    const FLAGS: u64 = 0;
    const TRIPCOUNT: u64 = 0;
    fn new_decl<'ll>(cx: &CodegenCx<'ll, '_>) -> &'ll Type {
        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);

        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);
        kernel_arguments_ty
    }

    fn new<'ll, 'tcx>(
        cx: &CodegenCx<'ll, 'tcx>,
        num_args: u64,
        memtransfer_types: &'ll Value,
        geps: [&'ll Value; 3],
    ) -> [(Align, &'ll Value); 13] {
        let four = Align::from_bytes(4).expect("4 Byte alignment should work");
        let eight = Align::EIGHT;

        let ti32 = cx.type_i32();
        let ci32_0 = cx.get_const_i32(0);
        [
            (four, cx.get_const_i32(KernelArgsTy::OFFLOAD_VERSION)),
            (four, cx.get_const_i32(num_args)),
            (eight, geps[0]),
            (eight, geps[1]),
            (eight, geps[2]),
            (eight, memtransfer_types),
            // The next two are debug infos. FIXME(offload): set them
            (eight, cx.const_null(cx.type_ptr())), // dbg
            (eight, cx.const_null(cx.type_ptr())), // dbg
            (eight, cx.get_const_i64(KernelArgsTy::TRIPCOUNT)),
            (eight, cx.get_const_i64(KernelArgsTy::FLAGS)),
            (four, cx.const_array(ti32, &[cx.get_const_i32(2097152), ci32_0, ci32_0])),
            (four, cx.const_array(ti32, &[cx.get_const_i32(256), ci32_0, ci32_0])),
            (four, cx.get_const_i32(0)),
        ]
    }
}

// Contains LLVM values needed to manage offloading for a single kernel.
#[derive(Copy, Clone)]
pub(crate) struct OffloadKernelGlobals<'ll> {
    pub offload_sizes: &'ll llvm::Value,
    pub memtransfer_types: &'ll llvm::Value,
    pub region_id: &'ll llvm::Value,
    pub offload_entry: &'ll llvm::Value,
}

fn gen_tgt_data_mappers<'ll>(
    cx: &CodegenCx<'ll, '_>,
) -> (&'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
}

// This function returns a memtransfer value which encodes how arguments to this kernel shall be
// mapped to/from the gpu. It also returns a region_id with the name of this kernel, to be
// concatenated into the list of region_ids.
pub(crate) fn gen_define_handling<'ll>(
    cx: &CodegenCx<'ll, '_>,
    metadata: &[OffloadMetadata],
    types: &[&'ll Type],
    symbol: String,
    offload_globals: &OffloadGlobals<'ll>,
) -> OffloadKernelGlobals<'ll> {
    if let Some(entry) = cx.offload_kernel_cache.borrow().get(&symbol) {
        return *entry;
    }

    let offload_entry_ty = offload_globals.offload_entry_ty;

    // It seems like non-pointer values are automatically mapped. So here, we focus on pointer (or
    // reference) types.
    let ptr_meta = types.iter().zip(metadata).filter_map(|(&x, meta)| match cx.type_kind(x) {
        rustc_codegen_ssa::common::TypeKind::Pointer => Some(meta),
        _ => None,
    });

    // FIXME(Sa4dUs): add `OMP_MAP_TARGET_PARAM = 0x20` only if necessary
    let (ptr_sizes, ptr_transfer): (Vec<_>, Vec<_>) =
        ptr_meta.map(|m| (m.payload_size, m.mode.bits() | 0x20)).unzip();

    let offload_sizes = add_priv_unnamed_arr(&cx, &format!(".offload_sizes.{symbol}"), &ptr_sizes);
    // 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.
    // 1+2+32: 1 (MapTo), 2 (MapFrom), 32 (Add one extra input ptr per function, to be used later).
    let memtransfer_types =
        add_priv_unnamed_arr(&cx, &format!(".offload_maptypes.{symbol}"), &ptr_transfer);

    // Next: For each function, generate these three entries. A weak constant,
    // the llvm.rodata entry name, and  the llvm_offload_entries value

    let name = format!(".{symbol}.region_id");
    let initializer = cx.get_const_i8(0);
    let region_id = add_global(&cx, &name, initializer, WeakAnyLinkage);

    let c_entry_name = CString::new(symbol.clone()).unwrap();
    let c_val = c_entry_name.as_bytes_with_nul();
    let offload_entry_name = format!(".offloading.entry_name.{symbol}");

    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");

    let name = format!(".offloading.entry.{symbol}");

    // See the __tgt_offload_entry documentation above.
    let elems = TgtOffloadEntry::new(&cx, region_id, llglobal);

    let initializer = crate::common::named_struct(offload_entry_ty, &elems);
    let c_name = CString::new(name).unwrap();
    let offload_entry = llvm::add_global(cx.llmod, offload_entry_ty, &c_name);
    llvm::set_global_constant(offload_entry, true);
    llvm::set_linkage(offload_entry, WeakAnyLinkage);
    llvm::set_initializer(offload_entry, initializer);
    llvm::set_alignment(offload_entry, Align::EIGHT);
    let c_section_name = CString::new("llvm_offload_entries").unwrap();
    llvm::set_section(offload_entry, &c_section_name);

    let result =
        OffloadKernelGlobals { offload_sizes, memtransfer_types, region_id, offload_entry };

    cx.offload_kernel_cache.borrow_mut().insert(symbol, result);

    result
}

fn declare_offload_fn<'ll>(
    cx: &CodegenCx<'ll, '_>,
    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. 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
pub(crate) fn gen_call_handling<'ll, 'tcx>(
    builder: &mut Builder<'_, 'll, 'tcx>,
    offload_data: &OffloadKernelGlobals<'ll>,
    args: &[&'ll Value],
    types: &[&Type],
    metadata: &[OffloadMetadata],
    offload_globals: &OffloadGlobals<'ll>,
) {
    let cx = builder.cx;
    let OffloadKernelGlobals { offload_sizes, offload_entry, memtransfer_types, region_id } =
        offload_data;

    let tgt_decl = offload_globals.launcher_fn;
    let tgt_target_kernel_ty = offload_globals.launcher_ty;

    // %struct.__tgt_bin_desc = type { i32, ptr, ptr, ptr }
    let tgt_bin_desc = offload_globals.bin_desc;

    let tgt_kernel_decl = offload_globals.kernel_args_ty;
    let begin_mapper_decl = offload_globals.begin_mapper;
    let end_mapper_decl = offload_globals.end_mapper;
    let fn_ty = offload_globals.mapper_fn_ty;

    let num_args = types.len() as u64;
    let ip = unsafe { llvm::LLVMRustGetInsertPoint(&builder.llbuilder) };

    // FIXME(Sa4dUs): dummy loads are a temp workaround, we should find a proper way to prevent these
    // variables from being optimized away
    for val in [offload_sizes, offload_entry] {
        unsafe {
            let dummy = llvm::LLVMBuildLoad2(
                &builder.llbuilder,
                llvm::LLVMTypeOf(val),
                val,
                b"dummy\0".as_ptr() as *const _,
            );
            llvm::LLVMSetVolatile(dummy, llvm::TRUE);
        }
    }

    // Step 0)
    // %struct.__tgt_bin_desc = type { i32, ptr, ptr, ptr }
    // %6 = alloca %struct.__tgt_bin_desc, align 8
    unsafe {
        llvm::LLVMRustPositionBuilderPastAllocas(&builder.llbuilder, builder.llfn());
    }
    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");

    //%kernel_args = alloca %struct.__tgt_kernel_arguments, align 8
    let a5 = builder.direct_alloca(tgt_kernel_decl, Align::EIGHT, "kernel_args");

    // Step 1)
    unsafe {
        llvm::LLVMRustRestoreInsertPoint(&builder.llbuilder, ip);
    }
    builder.memset(tgt_bin_desc_alloca, cx.get_const_i8(0), cx.get_const_i64(32), Align::EIGHT);

    // 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 &v in args {
        let gep = builder.inbounds_gep(cx.type_f32(), v, &[i32_0]);
        vals.push(v);
        geps.push(gep);
    }

    let mapper_fn_ty = cx.type_func(&[cx.type_ptr()], cx.type_void());
    let register_lib_decl = offload_globals.register_lib;
    let unregister_lib_decl = offload_globals.unregister_lib;
    let init_ty = cx.type_func(&[], cx.type_void());
    let init_rtls_decl = offload_globals.init_rtls;

    // FIXME(offload): Later we want to add them to the wrapper code, rather than our main function.
    // call void @__tgt_register_lib(ptr noundef %6)
    builder.call(mapper_fn_ty, None, None, register_lib_decl, &[tgt_bin_desc_alloca], None, None);
    // call void @__tgt_init_all_rtls()
    builder.call(init_ty, None, None, init_rtls_decl, &[], None, 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]);
        // FIXME(offload): write an offload frontend and handle arbitrary types.
        builder.store(cx.get_const_i64(metadata[i as usize].payload_size), 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<'ll, 'tcx>(
        builder: &mut Builder<'_, 'll, 'tcx>,
        ty: &'ll Type,
        ty2: &'ll Type,
        a1: &'ll Value,
        a2: &'ll Value,
        a4: &'ll Value,
    ) -> [&'ll Value; 3] {
        let cx = builder.cx;
        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<'ll, 'tcx>(
        builder: &mut Builder<'_, 'll, 'tcx>,
        geps: [&'ll Value; 3],
        o_type: &'ll Value,
        fn_to_call: &'ll Value,
        fn_ty: &'ll Type,
        num_args: u64,
        s_ident_t: &'ll Value,
    ) {
        let cx = builder.cx;
        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, None, None, fn_to_call, &args, None, None);
    }

    // Step 2)
    let s_ident_t = offload_globals.ident_t_global;
    let geps = get_geps(builder, ty, ty2, a1, a2, a4);
    generate_mapper_call(
        builder,
        geps,
        memtransfer_types,
        begin_mapper_decl,
        fn_ty,
        num_args,
        s_ident_t,
    );
    let values = KernelArgsTy::new(&cx, num_args, memtransfer_types, geps);

    // Step 3)
    // Here we fill the KernelArgsTy, see the documentation above
    for (i, value) in values.iter().enumerate() {
        let ptr = builder.inbounds_gep(tgt_kernel_decl, a5, &[i32_0, cx.get_const_i32(i as u64)]);
        builder.store(value.1, ptr, value.0);
    }

    let args = vec![
        s_ident_t,
        // FIXME(offload) give users a way to select which GPU to use.
        cx.get_const_i64(u64::MAX), // MAX == -1.
        // FIXME(offload): Don't hardcode the numbers of threads in the future.
        cx.get_const_i32(2097152),
        cx.get_const_i32(256),
        region_id,
        a5,
    ];
    builder.call(tgt_target_kernel_ty, None, None, tgt_decl, &args, None, None);
    // %41 = call i32 @__tgt_target_kernel(ptr @1, i64 -1, i32 2097152, i32 256, ptr @.kernel_1.region_id, ptr %kernel_args)

    // Step 4)
    let geps = get_geps(builder, ty, ty2, a1, a2, a4);
    generate_mapper_call(
        builder,
        geps,
        memtransfer_types,
        end_mapper_decl,
        fn_ty,
        num_args,
        s_ident_t,
    );

    builder.call(mapper_fn_ty, None, None, unregister_lib_decl, &[tgt_bin_desc_alloca], None, None);
}