wasmtime_environ/compile/trap_encoding.rs
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use crate::obj::ELF_WASMTIME_TRAPS;
use crate::prelude::*;
use crate::TrapInformation;
use object::write::{Object, StandardSegment};
use object::{LittleEndian, SectionKind, U32Bytes};
use std::ops::Range;
/// A helper structure to build the custom-encoded section of a wasmtime
/// compilation image which encodes trap information.
///
/// This structure is incrementally fed the results of compiling individual
/// functions and handles all the encoding internally, allowing usage of
/// `lookup_trap_code` below with the resulting section.
#[derive(Default)]
pub struct TrapEncodingBuilder {
offsets: Vec<U32Bytes<LittleEndian>>,
traps: Vec<u8>,
last_offset: u32,
}
impl TrapEncodingBuilder {
/// Appends trap information about a function into this section.
///
/// This function is called to describe traps for the `func` range
/// specified. The `func` offsets are specified relative to the text section
/// itself, and the `traps` offsets are specified relative to the start of
/// `func`.
///
/// This is required to be called in-order for increasing ranges of `func`
/// to ensure the final array is properly sorted. Additionally `traps` must
/// be sorted.
pub fn push(&mut self, func: Range<u64>, traps: &[TrapInformation]) {
// NB: for now this only supports <=4GB text sections in object files.
// Alternative schemes will need to be created for >32-bit offsets to
// avoid making this section overly large.
let func_start = u32::try_from(func.start).unwrap();
let func_end = u32::try_from(func.end).unwrap();
// Sanity-check to ensure that functions are pushed in-order, otherwise
// the `offsets` array won't be sorted which is our goal.
assert!(func_start >= self.last_offset);
self.offsets.reserve(traps.len());
self.traps.reserve(traps.len());
for info in traps {
let pos = func_start + info.code_offset;
assert!(pos >= self.last_offset);
self.offsets.push(U32Bytes::new(LittleEndian, pos));
self.traps.push(info.trap_code as u8);
self.last_offset = pos;
}
self.last_offset = func_end;
}
/// Encodes this section into the object provided.
pub fn append_to(self, obj: &mut Object) {
let section = obj.add_section(
obj.segment_name(StandardSegment::Data).to_vec(),
ELF_WASMTIME_TRAPS.as_bytes().to_vec(),
SectionKind::ReadOnlyData,
);
// NB: this matches the encoding expected by `lookup` below.
let amt = u32::try_from(self.traps.len()).unwrap();
obj.append_section_data(section, &amt.to_le_bytes(), 1);
obj.append_section_data(section, object::bytes_of_slice(&self.offsets), 1);
obj.append_section_data(section, &self.traps, 1);
}
}