118 lines
3.4 KiB
Rust
118 lines
3.4 KiB
Rust
//! Memory operation flags.
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use core::fmt;
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enum FlagBit {
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Notrap,
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Aligned,
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Readonly,
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}
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const NAMES: [&str; 3] = ["notrap", "aligned", "readonly"];
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/// Flags for memory operations like load/store.
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///
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/// Each of these flags introduce a limited form of undefined behavior. The flags each enable
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/// certain optimizations that need to make additional assumptions. Generally, the semantics of a
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/// program does not change when a flag is removed, but adding a flag will.
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#[derive(Clone, Copy, Debug, Hash, PartialEq, Eq)]
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pub struct MemFlags {
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bits: u8,
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}
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impl MemFlags {
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/// Create a new empty set of flags.
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pub fn new() -> Self {
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Self { bits: 0 }
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}
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/// Create a set of flags representing an access from a "trusted" address, meaning it's
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/// known to be aligned and non-trapping.
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pub fn trusted() -> Self {
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let mut result = Self::new();
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result.set_notrap();
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result.set_aligned();
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result
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}
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/// Read a flag bit.
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fn read(self, bit: FlagBit) -> bool {
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self.bits & (1 << bit as usize) != 0
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}
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/// Set a flag bit.
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fn set(&mut self, bit: FlagBit) {
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self.bits |= 1 << bit as usize
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}
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/// Set a flag bit by name.
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///
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/// Returns true if the flag was found and set, false for an unknown flag name.
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pub fn set_by_name(&mut self, name: &str) -> bool {
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match NAMES.iter().position(|&s| s == name) {
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Some(bit) => {
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self.bits |= 1 << bit;
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true
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}
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None => false,
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}
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}
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/// Test if the `notrap` flag is set.
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///
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/// Normally, trapping is part of the semantics of a load/store operation. If the platform
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/// would cause a trap when accessing the effective address, the Cranelift memory operation is
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/// also required to trap.
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///
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/// The `notrap` flag tells Cranelift that the memory is *accessible*, which means that
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/// accesses will not trap. This makes it possible to delete an unused load or a dead store
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/// instruction.
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pub fn notrap(self) -> bool {
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self.read(FlagBit::Notrap)
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}
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/// Set the `notrap` flag.
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pub fn set_notrap(&mut self) {
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self.set(FlagBit::Notrap)
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}
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/// Test if the `aligned` flag is set.
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///
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/// By default, Cranelift memory instructions work with any unaligned effective address. If the
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/// `aligned` flag is set, the instruction is permitted to trap or return a wrong result if the
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/// effective address is misaligned.
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pub fn aligned(self) -> bool {
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self.read(FlagBit::Aligned)
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}
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/// Set the `aligned` flag.
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pub fn set_aligned(&mut self) {
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self.set(FlagBit::Aligned)
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}
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/// Test if the `readonly` flag is set.
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///
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/// Loads with this flag have no memory dependencies.
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/// This results in undefined behavior if the dereferenced memory is mutated at any time
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/// between when the function is called and when it is exited.
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pub fn readonly(self) -> bool {
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self.read(FlagBit::Readonly)
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}
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/// Set the `readonly` flag.
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pub fn set_readonly(&mut self) {
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self.set(FlagBit::Readonly)
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}
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}
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impl fmt::Display for MemFlags {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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for (i, n) in NAMES.iter().enumerate() {
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if self.bits & (1 << i) != 0 {
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write!(f, " {}", n)?;
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}
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}
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Ok(())
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}
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}
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