src/librustc_trans/back/bytecode.rs

   1 // Copyright 2017 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Management of the encoding of LLVM bytecode into rlibs
  12 //!
  13 //! This module contains the management of encoding LLVM bytecode into rlibs,
  14 //! primarily for the usage in LTO situations. Currently the compiler will
  15 //! unconditionally encode LLVM-IR into rlibs regardless of what's happening
  16 //! elsewhere, so we currently compress the bytecode via deflate to avoid taking
  17 //! up too much space on disk.
  18 //!
  19 //! After compressing the bytecode we then have the rest of the format to
  20 //! basically deal with various bugs in various archive implementations. The
  21 //! format currently is:
  22 //!
  23 //!     RLIB LLVM-BYTECODE OBJECT LAYOUT
  24 //!     Version 2
  25 //!     Bytes    Data
  26 //!     0..10    "RUST_OBJECT" encoded in ASCII
  27 //!     11..14   format version as little-endian u32
  28 //!     15..19   the length of the module identifier string
  29 //!     20..n    the module identifier string
  30 //!     n..n+8   size in bytes of deflate compressed LLVM bitcode as
  31 //!              little-endian u64
  32 //!     n+9..    compressed LLVM bitcode
  33 //!     ?        maybe a byte to make this whole thing even length
  34
  35 use std::io::{Read, Write};
  36 use std::ptr;
  37 use std::str;
  38
  39 use flate2::Compression;
  40 use flate2::read::DeflateDecoder;
  41 use flate2::write::DeflateEncoder;
  42
  43 // This is the "magic number" expected at the beginning of a LLVM bytecode
  44 // object in an rlib.
  45 pub const RLIB_BYTECODE_OBJECT_MAGIC: &'static [u8] = b"RUST_OBJECT";
  46
  47 // The version number this compiler will write to bytecode objects in rlibs
  48 pub const RLIB_BYTECODE_OBJECT_VERSION: u8 = 2;
  49
  50 pub const RLIB_BYTECODE_EXTENSION: &str = "bc.z";
  51
  52 pub fn encode(identifier: &str, bytecode: &[u8]) -> Vec<u8> {
  53     let mut encoded = Vec::new();
  54
  55     // Start off with the magic string
  56     encoded.extend_from_slice(RLIB_BYTECODE_OBJECT_MAGIC);
  57
  58     // Next up is the version
  59     encoded.extend_from_slice(&[RLIB_BYTECODE_OBJECT_VERSION, 0, 0, 0]);
  60
  61     // Next is the LLVM module identifier length + contents
  62     let identifier_len = identifier.len();
  63     encoded.extend_from_slice(&[
  64         (identifier_len >>  0) as u8,
  65         (identifier_len >>  8) as u8,
  66         (identifier_len >> 16) as u8,
  67         (identifier_len >> 24) as u8,
  68     ]);
  69     encoded.extend_from_slice(identifier.as_bytes());
  70
  71     // Next is the LLVM module deflate compressed, prefixed with its length. We
  72     // don't know its length yet, so fill in 0s
  73     let deflated_size_pos = encoded.len();
  74     encoded.extend_from_slice(&[0, 0, 0, 0, 0, 0, 0, 0]);
  75
  76     let before = encoded.len();
  77     DeflateEncoder::new(&mut encoded, Compression::fast())
  78         .write_all(bytecode)
  79         .unwrap();
  80     let after = encoded.len();
  81
  82     // Fill in the length we reserved space for before
  83     let bytecode_len = (after - before) as u64;
  84     encoded[deflated_size_pos + 0] = (bytecode_len >>  0) as u8;
  85     encoded[deflated_size_pos + 1] = (bytecode_len >>  8) as u8;
  86     encoded[deflated_size_pos + 2] = (bytecode_len >> 16) as u8;
  87     encoded[deflated_size_pos + 3] = (bytecode_len >> 24) as u8;
  88     encoded[deflated_size_pos + 4] = (bytecode_len >> 32) as u8;
  89     encoded[deflated_size_pos + 5] = (bytecode_len >> 40) as u8;
  90     encoded[deflated_size_pos + 6] = (bytecode_len >> 48) as u8;
  91     encoded[deflated_size_pos + 7] = (bytecode_len >> 56) as u8;
  92
  93     // If the number of bytes written to the object so far is odd, add a
  94     // padding byte to make it even. This works around a crash bug in LLDB
  95     // (see issue #15950)
  96     if encoded.len() % 2 == 1 {
  97         encoded.push(0);
  98     }
  99
 100     return encoded
 101 }
 102
 103 pub struct DecodedBytecode<'a> {
 104     identifier: &'a str,
 105     encoded_bytecode: &'a [u8],
 106 }
 107
 108 impl<'a> DecodedBytecode<'a> {
 109     pub fn new(data: &'a [u8]) -> Result<DecodedBytecode<'a>, String> {
 110         if !data.starts_with(RLIB_BYTECODE_OBJECT_MAGIC) {
 111             return Err(format!("magic bytecode prefix not found"))
 112         }
 113         let data = &data[RLIB_BYTECODE_OBJECT_MAGIC.len()..];
 114         if !data.starts_with(&[RLIB_BYTECODE_OBJECT_VERSION, 0, 0, 0]) {
 115             return Err(format!("wrong version prefix found in bytecode"))
 116         }
 117         let data = &data[4..];
 118         if data.len() < 4 {
 119             return Err(format!("bytecode corrupted"))
 120         }
 121         let identifier_len = unsafe {
 122             u32::from_le(ptr::read_unaligned(data.as_ptr() as *const u32)) as usize
 123         };
 124         let data = &data[4..];
 125         if data.len() < identifier_len {
 126             return Err(format!("bytecode corrupted"))
 127         }
 128         let identifier = match str::from_utf8(&data[..identifier_len]) {
 129             Ok(s) => s,
 130             Err(_) => return Err(format!("bytecode corrupted"))
 131         };
 132         let data = &data[identifier_len..];
 133         if data.len() < 8 {
 134             return Err(format!("bytecode corrupted"))
 135         }
 136         let bytecode_len = unsafe {
 137             u64::from_le(ptr::read_unaligned(data.as_ptr() as *const u64)) as usize
 138         };
 139         let data = &data[8..];
 140         if data.len() < bytecode_len {
 141             return Err(format!("bytecode corrupted"))
 142         }
 143         let encoded_bytecode = &data[..bytecode_len];
 144
 145         Ok(DecodedBytecode {
 146             identifier,
 147             encoded_bytecode,
 148         })
 149     }
 150
 151     pub fn bytecode(&self) -> Vec<u8> {
 152         let mut data = Vec::new();
 153         DeflateDecoder::new(self.encoded_bytecode).read_to_end(&mut data).unwrap();
 154         return data
 155     }
 156
 157     pub fn identifier(&self) -> &'a str {
 158         self.identifier
 159     }
 160 }