wasmtime_environ/compile/
trap_encoding.rs

1use crate::obj::ELF_WASMTIME_TRAPS;
2use crate::prelude::*;
3use crate::TrapInformation;
4use object::write::{Object, StandardSegment};
5use object::{LittleEndian, SectionKind, U32Bytes};
6use std::ops::Range;
7
8/// A helper structure to build the custom-encoded section of a wasmtime
9/// compilation image which encodes trap information.
10///
11/// This structure is incrementally fed the results of compiling individual
12/// functions and handles all the encoding internally, allowing usage of
13/// `lookup_trap_code` below with the resulting section.
14#[derive(Default)]
15pub struct TrapEncodingBuilder {
16    offsets: Vec<U32Bytes<LittleEndian>>,
17    traps: Vec<u8>,
18    last_offset: u32,
19}
20
21impl TrapEncodingBuilder {
22    /// Appends trap information about a function into this section.
23    ///
24    /// This function is called to describe traps for the `func` range
25    /// specified. The `func` offsets are specified relative to the text section
26    /// itself, and the `traps` offsets are specified relative to the start of
27    /// `func`.
28    ///
29    /// This is required to be called in-order for increasing ranges of `func`
30    /// to ensure the final array is properly sorted. Additionally `traps` must
31    /// be sorted.
32    pub fn push(&mut self, func: Range<u64>, traps: &[TrapInformation]) {
33        // NB: for now this only supports <=4GB text sections in object files.
34        // Alternative schemes will need to be created for >32-bit offsets to
35        // avoid making this section overly large.
36        let func_start = u32::try_from(func.start).unwrap();
37        let func_end = u32::try_from(func.end).unwrap();
38
39        // Sanity-check to ensure that functions are pushed in-order, otherwise
40        // the `offsets` array won't be sorted which is our goal.
41        assert!(func_start >= self.last_offset);
42
43        self.offsets.reserve(traps.len());
44        self.traps.reserve(traps.len());
45        for info in traps {
46            let pos = func_start + info.code_offset;
47            assert!(pos >= self.last_offset);
48            self.offsets.push(U32Bytes::new(LittleEndian, pos));
49            self.traps.push(info.trap_code as u8);
50            self.last_offset = pos;
51        }
52
53        self.last_offset = func_end;
54    }
55
56    /// Encodes this section into the object provided.
57    pub fn append_to(self, obj: &mut Object) {
58        let section = obj.add_section(
59            obj.segment_name(StandardSegment::Data).to_vec(),
60            ELF_WASMTIME_TRAPS.as_bytes().to_vec(),
61            SectionKind::ReadOnlyData,
62        );
63
64        // NB: this matches the encoding expected by `lookup` below.
65        let amt = u32::try_from(self.traps.len()).unwrap();
66        obj.append_section_data(section, &amt.to_le_bytes(), 1);
67        obj.append_section_data(section, object::bytes_of_slice(&self.offsets), 1);
68        obj.append_section_data(section, &self.traps, 1);
69    }
70}
wasmtime_environ/compile/trap_encoding.rs

wasmtime_environ/compile/
trap_encoding.rs
