wasmtime_environ/compile/
address_map.rs

1//! Data structures to provide transformation of the source
2
3use crate::InstructionAddressMap;
4use crate::obj::ELF_WASMTIME_ADDRMAP;
5use crate::prelude::*;
6use object::write::{Object, StandardSegment};
7use object::{LittleEndian, SectionKind, U32Bytes};
8use std::ops::Range;
9
10/// Builder for the address map section of a wasmtime compilation image.
11///
12/// This builder is used to conveniently built the `ELF_WASMTIME_ADDRMAP`
13/// section by compilers, and provides utilities to directly insert the results
14/// into an `Object`.
15#[derive(Default)]
16pub struct AddressMapSection {
17    offsets: Vec<U32Bytes<LittleEndian>>,
18    positions: Vec<U32Bytes<LittleEndian>>,
19    last_offset: u32,
20}
21
22impl AddressMapSection {
23    /// Pushes a new set of instruction mapping information for a function added
24    /// in the executable.
25    ///
26    /// The `func` argument here is the range of the function, relative to the
27    /// start of the text section in the executable. The `instrs` provided are
28    /// the descriptors for instructions in the function and their various
29    /// mappings back to original source positions.
30    ///
31    /// This is required to be called for `func` values that are strictly
32    /// increasing in addresses (e.g. as the object is built). Additionally the
33    /// `instrs` map must be sorted based on code offset in the native text
34    /// section.
35    pub fn push(&mut self, func: Range<u64>, instrs: &[InstructionAddressMap]) {
36        // NB: for now this only supports <=4GB text sections in object files.
37        // Alternative schemes will need to be created for >32-bit offsets to
38        // avoid making this section overly large.
39        let func_start = u32::try_from(func.start).unwrap();
40        let func_end = u32::try_from(func.end).unwrap();
41
42        self.offsets.reserve(instrs.len());
43        self.positions.reserve(instrs.len());
44        let mut last_srcloc = None;
45        for map in instrs {
46            // Sanity-check to ensure that functions are pushed in-order, otherwise
47            // the `offsets` array won't be sorted which is our goal.
48            let pos = func_start + map.code_offset;
49            assert!(pos >= self.last_offset);
50            self.last_offset = pos;
51
52            // Drop duplicate instruction mappings that match what was
53            // previously pushed into the array since the representation used
54            // here will naturally cover `pos` with the previous entry.
55            let srcloc = map.srcloc.file_offset().unwrap_or(u32::MAX);
56            if Some(srcloc) == last_srcloc {
57                continue;
58            }
59            last_srcloc = Some(srcloc);
60
61            self.offsets.push(U32Bytes::new(LittleEndian, pos));
62            self.positions.push(U32Bytes::new(LittleEndian, srcloc));
63        }
64        self.last_offset = func_end;
65    }
66
67    /// Finishes encoding this section into the `Object` provided.
68    pub fn append_to(self, obj: &mut Object) {
69        let section = obj.add_section(
70            obj.segment_name(StandardSegment::Data).to_vec(),
71            ELF_WASMTIME_ADDRMAP.as_bytes().to_vec(),
72            SectionKind::ReadOnlyData,
73        );
74
75        // NB: this matches the encoding expected by `lookup` below.
76        let amt = u32::try_from(self.offsets.len()).unwrap();
77        obj.append_section_data(section, &amt.to_le_bytes(), 1);
78        obj.append_section_data(section, object::bytes_of_slice(&self.offsets), 1);
79        obj.append_section_data(section, object::bytes_of_slice(&self.positions), 1);
80    }
81}
wasmtime_environ/compile/address_map.rs

wasmtime_environ/compile/
address_map.rs
