//! Object file builder.
//!
//! Creates ELF image based on `Compilation` information. The ELF contains
//! functions and trampolines in the ".text" section. It also contains all
//! relocation records for the linking stage. If DWARF sections exist, their
//! content will be written as well.
//!
//! The object file has symbols for each function and trampoline, as well as
//! symbols that refer to libcalls.
//!
//! The function symbol names have format "_wasm_function_N", where N is
//! `FuncIndex`. The defined wasm function symbols refer to a JIT compiled
//! function body, the imported wasm function do not. The trampolines symbol
//! names have format "_trampoline_N", where N is `SignatureIndex`.

use crate::debug::{DwarfSection, DwarfSectionRelocTarget};
use crate::{CompiledFunction, Relocation, RelocationTarget};
use anyhow::Result;
use cranelift_codegen::binemit::Reloc;
use cranelift_codegen::ir::LibCall;
use cranelift_codegen::isa::{
    unwind::{systemv, UnwindInfo},
    TargetIsa,
};
use cranelift_codegen::TextSectionBuilder;
use gimli::write::{Address, EhFrame, EndianVec, FrameTable, Writer};
use gimli::RunTimeEndian;
use object::write::{
    Object, Relocation as ObjectRelocation, SectionId, StandardSegment, Symbol, SymbolId,
    SymbolSection,
};
use object::{
    Architecture, RelocationEncoding, RelocationKind, SectionKind, SymbolFlags, SymbolKind,
    SymbolScope,
};
use std::collections::HashMap;
use std::convert::TryFrom;
use std::mem;
use std::ops::Range;
use wasmtime_environ::obj;
use wasmtime_environ::{
    DefinedFuncIndex, EntityRef, FuncIndex, Module, PrimaryMap, SignatureIndex, Trampoline,
};

const TEXT_SECTION_NAME: &[u8] = b".text";

/// Iterates through all `LibCall` members and all runtime exported functions.
#[macro_export]
macro_rules! for_each_libcall {
    ($op:ident) => {
        $op![
            (UdivI64, wasmtime_i64_udiv),
            (UdivI64, wasmtime_i64_udiv),
            (SdivI64, wasmtime_i64_sdiv),
            (UremI64, wasmtime_i64_urem),
            (SremI64, wasmtime_i64_srem),
            (IshlI64, wasmtime_i64_ishl),
            (UshrI64, wasmtime_i64_ushr),
            (SshrI64, wasmtime_i64_sshr),
            (CeilF32, wasmtime_f32_ceil),
            (FloorF32, wasmtime_f32_floor),
            (TruncF32, wasmtime_f32_trunc),
            (NearestF32, wasmtime_f32_nearest),
            (CeilF64, wasmtime_f64_ceil),
            (FloorF64, wasmtime_f64_floor),
            (TruncF64, wasmtime_f64_trunc),
            (NearestF64, wasmtime_f64_nearest)
        ];
    };
}

fn write_libcall_symbols(obj: &mut Object) -> HashMap<LibCall, SymbolId> {
    let mut libcalls = HashMap::new();
    macro_rules! add_libcall_symbol {
        [$(($libcall:ident, $export:ident)),*] => {{
            $(
                let symbol_id = obj.add_symbol(Symbol {
                    name: stringify!($export).as_bytes().to_vec(),
                    value: 0,
                    size: 0,
                    kind: SymbolKind::Text,
                    scope: SymbolScope::Linkage,
                    weak: true,
                    section: SymbolSection::Undefined,
                    flags: SymbolFlags::None,
                });
                libcalls.insert(LibCall::$libcall, symbol_id);
            )+
        }};
    }
    for_each_libcall!(add_libcall_symbol);

    libcalls
}

/// A helper structure used to assemble the final text section of an exectuable,
/// plus unwinding information and other related details.
///
/// This builder relies on Cranelift-specific internals but assembles into a
/// generic `Object` which will get further appended to in a compiler-agnostic
/// fashion later.
pub struct ObjectBuilder<'a> {
    /// The target that we're compiling for, used to query target-specific
    /// information as necessary.
    isa: &'a dyn TargetIsa,

    /// The object file that we're generating code into.
    obj: &'a mut Object<'static>,

    /// The WebAssembly module we're generating code for.
    module: &'a Module,

    /// Map of injected symbols for all possible libcalls, used whenever there's
    /// a relocation against a libcall.
    libcalls: HashMap<LibCall, SymbolId>,

    windows_unwind_info_id: Option<SectionId>,

    /// Packed form of windows unwind tables which, if present, will get emitted
    /// to a windows-specific unwind info section.
    windows_unwind_info: Vec<RUNTIME_FUNCTION>,

    systemv_unwind_info_id: Option<SectionId>,

    /// Pending unwinding information for DWARF-based platforms. This is used to
    /// build a `.eh_frame` lookalike at the very end of object building.
    systemv_unwind_info: Vec<(u64, &'a systemv::UnwindInfo)>,

    /// The corresponding symbol for each function, inserted as they're defined.
    ///
    /// If an index isn't here yet then it hasn't been defined yet.
    func_symbols: PrimaryMap<FuncIndex, SymbolId>,

    /// `object`-crate identifier for the text section.
    text_section: SectionId,

    /// Relocations to be added once we've got all function symbols available to
    /// us. The first entry is the relocation that we're applying, relative
    /// within a function, and the second entry here is the offset of the
    /// function that contains this relocation.
    relocations: Vec<(&'a Relocation, u64)>,

    /// In-progress text section that we're using cranelift's `MachBuffer` to
    /// build to resolve relocations (calls) between functions.
    pub text: Box<dyn TextSectionBuilder>,

    /// The unwind info _must_ come directly after the text section. Our FDE's
    /// instructions are encoded to rely on this placement. We use this `bool`
    /// for debug assertions to ensure that we get the ordering correct.
    added_unwind_info: bool,
}

// This is a mirror of `RUNTIME_FUNCTION` in the Windows API, but defined here
// to ensure everything is always `u32` and to have it available on all
// platforms. Note that all of these specifiers here are relative to a "base
// address" which we define as the base of where the text section is eventually
// loaded.
#[allow(non_camel_case_types)]
struct RUNTIME_FUNCTION {
    begin: u32,
    end: u32,
    unwind_address: u32,
}

impl<'a> ObjectBuilder<'a> {
    pub fn new(obj: &'a mut Object<'static>, module: &'a Module, isa: &'a dyn TargetIsa) -> Self {
        // Entire code (functions and trampolines) will be placed
        // in the ".text" section.
        let text_section = obj.add_section(
            obj.segment_name(StandardSegment::Text).to_vec(),
            TEXT_SECTION_NAME.to_vec(),
            SectionKind::Text,
        );

        // Create symbols for imports -- needed during linking.
        let mut func_symbols = PrimaryMap::with_capacity(module.functions.len());
        for index in 0..module.num_imported_funcs {
            let symbol_id = obj.add_symbol(Symbol {
                name: obj::func_symbol_name(FuncIndex::new(index))
                    .as_bytes()
                    .to_vec(),
                value: 0,
                size: 0,
                kind: SymbolKind::Text,
                scope: SymbolScope::Linkage,
                weak: false,
                section: SymbolSection::Undefined,
                flags: SymbolFlags::None,
            });
            func_symbols.push(symbol_id);
        }

        let libcalls = write_libcall_symbols(obj);

        Self {
            isa,
            obj,
            module,
            text_section,
            func_symbols,
            libcalls,
            windows_unwind_info_id: None,
            windows_unwind_info: Vec::new(),
            systemv_unwind_info_id: None,
            systemv_unwind_info: Vec::new(),
            relocations: Vec::new(),
            text: isa
                .text_section_builder((module.functions.len() - module.num_imported_funcs) as u32),
            added_unwind_info: false,
        }
    }

    /// Appends the `func` specified named `name` to this object.
    ///
    /// Returns the symbol associated with the function as well as the range
    /// that the function resides within the text section.
    fn append_func(
        &mut self,
        wat: bool,
        name: Vec<u8>,
        func: &'a CompiledFunction,
    ) -> (SymbolId, Range<u64>) {
        let body_len = func.body.len() as u64;
        let off = self.text.append(wat, &func.body, 1);

        let symbol_id = self.obj.add_symbol(Symbol {
            name,
            value: off,
            size: body_len,
            kind: SymbolKind::Text,
            scope: SymbolScope::Compilation,
            weak: false,
            section: SymbolSection::Section(self.text_section),
            flags: SymbolFlags::None,
        });

        match &func.unwind_info {
            // Windows unwind information is preferred to come after the code
            // itself. The information is appended here just after the function,
            // aligned to 4-bytes as required by Windows.
            //
            // The location of the unwind info, and the function it describes,
            // is then recorded in an unwind info table to get embedded into the
            // object at the end of compilation.
            Some(UnwindInfo::WindowsX64(info)) => {
                // Windows prefers Unwind info after the code -- writing it here.
                let unwind_size = info.emit_size();
                let mut unwind_info = vec![0; unwind_size];
                info.emit(&mut unwind_info);
                let unwind_off = self.text.append(false, &unwind_info, 4);
                self.windows_unwind_info.push(RUNTIME_FUNCTION {
                    begin: u32::try_from(off).unwrap(),
                    end: u32::try_from(off + body_len).unwrap(),
                    unwind_address: u32::try_from(unwind_off).unwrap(),
                });
            }

            // System-V is different enough that we just record the unwinding
            // information to get processed at a later time.
            Some(UnwindInfo::SystemV(info)) => {
                self.systemv_unwind_info.push((off, info));
            }

            Some(_) => panic!("some unwind info isn't handled here"),
            None => {}
        }

        for r in func.relocations.iter() {
            let (symbol, symbol_offset) = match r.reloc_target {
                // Relocations against user-defined functions means that this is
                // a relocation against a module-local function, typically a
                // call between functions. The `text` field is given priority to
                // resolve this relocation before we actually emit an object
                // file, but if it can't handle it then we pass through the
                // relocation.
                RelocationTarget::UserFunc(index) => {
                    let defined_index = self.module.defined_func_index(index).unwrap();
                    if self.text.resolve_reloc(
                        off + u64::from(r.offset),
                        r.reloc,
                        r.addend,
                        defined_index.as_u32(),
                    ) {
                        continue;
                    }

                    // FIXME(#3009) once the old backend is removed all
                    // inter-function relocations should be handled by
                    // `self.text`. This can become `unreachable!()` in that
                    // case.
                    self.relocations.push((r, off));
                    continue;
                }

                // These relocations, unlike against user funcs above, typically
                // involve absolute addresses and need to get resolved at load
                // time. These are persisted immediately into the object file.
                //
                // FIXME: these, like user-defined-functions, should probably
                // use relative jumps and avoid absolute relocations. They don't
                // seem too common though so aren't necessarily that important
                // to optimize.
                RelocationTarget::LibCall(call) => (self.libcalls[&call], 0),
            };
            let (kind, encoding, size) = match r.reloc {
                Reloc::Abs4 => (RelocationKind::Absolute, RelocationEncoding::Generic, 32),
                Reloc::Abs8 => (RelocationKind::Absolute, RelocationEncoding::Generic, 64),

                other => unimplemented!("Unimplemented relocation {:?}", other),
            };
            self.obj
                .add_relocation(
                    self.text_section,
                    ObjectRelocation {
                        offset: off + r.offset as u64,
                        size,
                        kind,
                        encoding,
                        symbol,
                        addend: r.addend.wrapping_add(symbol_offset as i64),
                    },
                )
                .unwrap();
        }
        (symbol_id, off..off + body_len)
    }

    /// Appends a function to this object file.
    ///
    /// This is expected to be called in-order for ascending `index` values.
    pub fn func(&mut self, index: DefinedFuncIndex, func: &'a CompiledFunction) -> Range<u64> {
        assert!(!self.added_unwind_info);
        let index = self.module.func_index(index);
        let name = obj::func_symbol_name(index);
        let (symbol_id, range) = self.append_func(true, name.into_bytes(), func);
        assert_eq!(self.func_symbols.push(symbol_id), index);
        range
    }

    pub fn trampoline(&mut self, sig: SignatureIndex, func: &'a CompiledFunction) -> Trampoline {
        assert!(!self.added_unwind_info);
        let name = obj::trampoline_symbol_name(sig);
        let (_, range) = self.append_func(false, name.into_bytes(), func);
        Trampoline {
            signature: sig,
            start: range.start,
            length: u32::try_from(range.end - range.start).unwrap(),
        }
    }

    pub fn dwarf_sections(&mut self, sections: &[DwarfSection]) -> Result<()> {
        assert!(
            self.added_unwind_info,
            "can't add dwarf yet; unwind info must directly follow the text section"
        );

        // If we have DWARF data, write it in the object file.
        let (debug_bodies, debug_relocs): (Vec<_>, Vec<_>) = sections
            .iter()
            .map(|s| ((s.name, &s.body), (s.name, &s.relocs)))
            .unzip();
        let mut dwarf_sections_ids = HashMap::new();
        for (name, body) in debug_bodies {
            let segment = self.obj.segment_name(StandardSegment::Debug).to_vec();
            let section_id =
                self.obj
                    .add_section(segment, name.as_bytes().to_vec(), SectionKind::Debug);
            dwarf_sections_ids.insert(name, section_id);
            self.obj.append_section_data(section_id, &body, 1);
        }

        // Write all debug data relocations.
        for (name, relocs) in debug_relocs {
            let section_id = *dwarf_sections_ids.get(name).unwrap();
            for reloc in relocs {
                let target_symbol = match reloc.target {
                    DwarfSectionRelocTarget::Func(index) => {
                        self.func_symbols[self.module.func_index(DefinedFuncIndex::new(index))]
                    }
                    DwarfSectionRelocTarget::Section(name) => {
                        self.obj.section_symbol(dwarf_sections_ids[name])
                    }
                };
                self.obj.add_relocation(
                    section_id,
                    ObjectRelocation {
                        offset: u64::from(reloc.offset),
                        size: reloc.size << 3,
                        kind: RelocationKind::Absolute,
                        encoding: RelocationEncoding::Generic,
                        symbol: target_symbol,
                        addend: i64::from(reloc.addend),
                    },
                )?;
            }
        }

        Ok(())
    }

    pub fn unwind_info(&mut self) {
        assert!(!self.added_unwind_info);

        if self.windows_unwind_info.len() > 0 {
            let segment = self.obj.segment_name(StandardSegment::Data).to_vec();
            self.windows_unwind_info_id = Some(self.obj.add_section(
                segment,
                b"_wasmtime_winx64_unwind".to_vec(),
                SectionKind::ReadOnlyData,
            ));
        }
        if self.systemv_unwind_info.len() > 0 {
            let segment = self.obj.segment_name(StandardSegment::Data).to_vec();
            self.systemv_unwind_info_id = Some(self.obj.add_section(
                segment,
                b".eh_frame".to_vec(),
                SectionKind::ReadOnlyData,
            ));
        }

        self.added_unwind_info = true;
    }

    pub fn finish(&mut self) -> Result<()> {
        // Now that all function symbols are available register all final
        // relocations between functions.
        //
        // FIXME(#3009) once the old backend is removed this loop should be
        // deleted since there won't be any relocations here.
        for (r, off) in mem::take(&mut self.relocations) {
            let symbol = match r.reloc_target {
                RelocationTarget::UserFunc(index) => self.func_symbols[index],
                _ => unreachable!("should be handled in `append_func`"),
            };
            let (kind, encoding, size) = match r.reloc {
                Reloc::X86CallPCRel4 => {
                    (RelocationKind::Relative, RelocationEncoding::X86Branch, 32)
                }
                other => unimplemented!("Unimplemented relocation {:?}", other),
            };
            self.obj.add_relocation(
                self.text_section,
                ObjectRelocation {
                    offset: off + u64::from(r.offset),
                    size,
                    kind,
                    encoding,
                    symbol,
                    addend: r.addend,
                },
            )?;
        }

        // Finish up the text section now that we're done adding functions.
        let text = self.text.finish();
        self.obj
            .section_mut(self.text_section)
            .set_data(text, self.isa.code_section_alignment());

        // With all functions added we can also emit the fully-formed unwinding
        // information sections.
        if self.windows_unwind_info.len() > 0 {
            self.append_windows_unwind_info();
        }
        if self.systemv_unwind_info.len() > 0 {
            self.append_systemv_unwind_info();
        }

        Ok(())
    }

    /// This function appends a nonstandard section to the object which is only
    /// used during `CodeMemory::allocate_for_object`.
    ///
    /// This custom section effectively stores a `[RUNTIME_FUNCTION; N]` into
    /// the object file itself. This way registration of unwind info can simply
    /// pass this slice to the OS itself and there's no need to recalculate
    /// anything on the other end of loading a module from a precompiled object.
    fn append_windows_unwind_info(&mut self) {
        // Currently the binary format supported here only supports
        // little-endian for x86_64, or at least that's all where it's tested.
        // This may need updates for other platforms.
        assert_eq!(self.obj.architecture(), Architecture::X86_64);

        let section_id = self.windows_unwind_info_id.unwrap();

        // Page-align the text section so the unwind info can reside on a
        // separate page that doesn't need executable permissions.
        self.obj
            .append_section_data(self.text_section, &[], self.isa.code_section_alignment());

        let mut unwind_info = Vec::with_capacity(self.windows_unwind_info.len() * 3 * 4);
        for info in self.windows_unwind_info.iter() {
            unwind_info.extend_from_slice(&info.begin.to_le_bytes());
            unwind_info.extend_from_slice(&info.end.to_le_bytes());
            unwind_info.extend_from_slice(&info.unwind_address.to_le_bytes());
        }
        self.obj.append_section_data(section_id, &unwind_info, 4);
    }

    /// This function appends a nonstandard section to the object which is only
    /// used during `CodeMemory::allocate_for_object`.
    ///
    /// This will generate a `.eh_frame` section, but not one that can be
    /// naively loaded. The goal of this section is that we can create the
    /// section once here and never again does it need to change. To describe
    /// dynamically loaded functions though each individual FDE needs to talk
    /// about the function's absolute address that it's referencing. Naturally
    /// we don't actually know the function's absolute address when we're
    /// creating an object here.
    ///
    /// To solve this problem the FDE address encoding mode is set to
    /// `DW_EH_PE_pcrel`. This means that the actual effective address that the
    /// FDE describes is a relative to the address of the FDE itself. By
    /// leveraging this relative-ness we can assume that the relative distance
    /// between the FDE and the function it describes is constant, which should
    /// allow us to generate an FDE ahead-of-time here.
    ///
    /// For now this assumes that all the code of functions will start at a
    /// page-aligned address when loaded into memory. The eh_frame encoded here
    /// then assumes that the text section is itself page aligned to its size
    /// and the eh_frame will follow just after the text section. This means
    /// that the relative offsets we're using here is the FDE going backwards
    /// into the text section itself.
    ///
    /// Note that the library we're using to create the FDEs, `gimli`, doesn't
    /// actually encode addresses relative to the FDE itself. Instead the
    /// addresses are encoded relative to the start of the `.eh_frame` section.
    /// This makes it much easier for us where we provide the relative offset
    /// from the start of `.eh_frame` to the function in the text section, which
    /// given our layout basically means the offset of the function in the text
    /// section from the end of the text section.
    ///
    /// A final note is that the reason we page-align the text section's size is
    /// so the .eh_frame lives on a separate page from the text section itself.
    /// This allows `.eh_frame` to have different virtual memory permissions,
    /// such as being purely read-only instead of read/execute like the code
    /// bits.
    fn append_systemv_unwind_info(&mut self) {
        let section_id = self.systemv_unwind_info_id.unwrap();
        let mut cie = self
            .isa
            .create_systemv_cie()
            .expect("must be able to create a CIE for system-v unwind info");
        let mut table = FrameTable::default();
        cie.fde_address_encoding = gimli::constants::DW_EH_PE_pcrel;
        let cie_id = table.add_cie(cie);

        // This write will align the text section to a page boundary
        // and then return the offset at that point. This gives us the full size
        // of the text section at that point, after alignment.
        let text_section_size =
            self.obj
                .append_section_data(self.text_section, &[], self.isa.code_section_alignment());
        for (text_section_off, unwind_info) in self.systemv_unwind_info.iter() {
            let backwards_off = text_section_size - text_section_off;
            let actual_offset = -i64::try_from(backwards_off).unwrap();
            // Note that gimli wants an unsigned 64-bit integer here, but
            // unwinders just use this constant for a relative addition with the
            // address of the FDE, which means that the sign doesn't actually
            // matter.
            let fde = unwind_info.to_fde(Address::Constant(actual_offset as u64));
            table.add_fde(cie_id, fde);
        }
        let endian = match self.isa.triple().endianness().unwrap() {
            target_lexicon::Endianness::Little => RunTimeEndian::Little,
            target_lexicon::Endianness::Big => RunTimeEndian::Big,
        };
        let mut eh_frame = EhFrame(MyVec(EndianVec::new(endian)));
        table.write_eh_frame(&mut eh_frame).unwrap();

        // Some unwinding implementations expect a terminating "empty" length so
        // a 0 is written at the end of the table for those implementations.
        let mut endian_vec = (eh_frame.0).0;
        endian_vec.write_u32(0).unwrap();
        self.obj
            .append_section_data(section_id, endian_vec.slice(), 1);

        use gimli::constants;
        use gimli::write::Error;

        struct MyVec(EndianVec<RunTimeEndian>);

        impl Writer for MyVec {
            type Endian = RunTimeEndian;

            fn endian(&self) -> RunTimeEndian {
                self.0.endian()
            }

            fn len(&self) -> usize {
                self.0.len()
            }

            fn write(&mut self, buf: &[u8]) -> Result<(), Error> {
                self.0.write(buf)
            }

            fn write_at(&mut self, pos: usize, buf: &[u8]) -> Result<(), Error> {
                self.0.write_at(pos, buf)
            }

            // FIXME(gimli-rs/gimli#576) this is the definition we want for
            // `write_eh_pointer` but the default implementation, at the time
            // of this writing, uses `offset - val` instead of `val - offset`.
            // A PR has been merged to fix this but until that's published we
            // can't use it.
            fn write_eh_pointer(
                &mut self,
                address: Address,
                eh_pe: constants::DwEhPe,
                size: u8,
            ) -> Result<(), Error> {
                let val = match address {
                    Address::Constant(val) => val,
                    Address::Symbol { .. } => unreachable!(),
                };
                assert_eq!(eh_pe.application(), constants::DW_EH_PE_pcrel);
                let offset = self.len() as u64;
                let val = val.wrapping_sub(offset);
                self.write_eh_pointer_data(val, eh_pe.format(), size)
            }
        }
    }
}