Merge pull request #149 from rust-embedded/remove-fcsr

Remove FCSR

CHANGELOG.md

@@ -39,6 +39,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Removed
 
 - User mode registers removed, as they are no longer supported in RISC-V
+- FCSR register operations removed to avoid UB (#148)
 
 ## [v0.10.1] - 2023-01-18
 

src/register.rs

@@ -10,14 +10,35 @@
 //! - minstreth
 //! - mhpmcounter<3-31>h
 //! - mstatush
+//!
+//! # On Floating-Point CSRs
+//!
+//! We are deliberately *not* providing instructions that could change the floating-point rounding
+//! mode or exception behavior or read the accrued exceptions flags: `frcsr`, `fscsr`, `fsrm`,
+//! `frflags`, `fsflags`.
+//!
+//! Rust makes no guarantees whatsoever about the contents of the accrued exceptions register: Rust
+//! floating-point operations may or may not result in this register getting updated with exception
+//! state, and the register can change between two invocations of this function even when no
+//! floating-point operations appear in the source code (since floating-point operations appearing
+//! earlier or later can be reordered).
+//!
+//! Modifying the rounding mode leads to **immediate Undefined Behavior**: Rust assumes that the
+//! default rounding mode is always set and will optimize accordingly. This even applies when the
+//! rounding mode is altered and later reset to its original value without any floating-point
+//! operations appearing in the source code between those operations (since floating-point
+//! operations appearing earlier or later can be reordered).
+//!
+//! If you need to perform some floating-point operations and check whether they raised an
+//! exception, use a single inline assembly block for the entire sequence of operations.
+//!
+//! If you need to perform some floating-point operations under a different rounding mode, use a
+//! single inline assembly block and make sure to restore the original rounding mode before the end
+//! of the block.
 
 #[macro_use]
 mod macros;
 
-// User Floating-Point CSRs
-// TODO: frm, fflags
-pub mod fcsr;
-
 // User Counter/Timers
 pub mod cycle;
 pub mod cycleh;
@@ -29,7 +50,6 @@ pub mod time;
 pub mod timeh;
 
 // Supervisor Trap Setup
-// TODO: sedeleg, sideleg
 pub mod scounteren;
 pub mod sie;
 pub mod sstatus;

src/register/fcsr.rs

@@ -1,134 +0,0 @@
-//! Floating-point control and status register
-
-/// Floating-point control and status register
-#[derive(Clone, Copy, Debug)]
-pub struct FCSR {
-    bits: u32,
-}
-
-/// Accrued Exception Flags
-#[derive(Clone, Copy, Debug)]
-pub struct Flags(u32);
-
-/// Accrued Exception Flag
-#[derive(Clone, Copy, Debug)]
-pub enum Flag {
-    /// Inexact
-    NX = 0b00001,
-
-    /// Underflow
-    UF = 0b00010,
-
-    /// Overflow
-    OF = 0b00100,
-
-    /// Divide by Zero
-    DZ = 0b01000,
-
-    /// Invalid Operation
-    NV = 0b10000,
-}
-
-impl Flags {
-    /// Inexact
-    #[inline]
-    pub fn nx(&self) -> bool {
-        self.0 & (1 << 0) != 0
-    }
-
-    /// Underflow
-    #[inline]
-    pub fn uf(&self) -> bool {
-        self.0 & (1 << 1) != 0
-    }
-
-    /// Overflow
-    #[inline]
-    pub fn of(&self) -> bool {
-        self.0 & (1 << 2) != 0
-    }
-
-    /// Divide by Zero
-    #[inline]
-    pub fn dz(&self) -> bool {
-        self.0 & (1 << 3) != 0
-    }
-
-    /// Invalid Operation
-    #[inline]
-    pub fn nv(&self) -> bool {
-        self.0 & (1 << 4) != 0
-    }
-}
-
-/// Rounding Mode
-#[derive(Clone, Copy, Debug, Eq, PartialEq)]
-pub enum RoundingMode {
-    RoundToNearestEven = 0b000,
-    RoundTowardsZero = 0b001,
-    RoundDown = 0b010,
-    RoundUp = 0b011,
-    RoundToNearestMaxMagnitude = 0b100,
-    Invalid = 0b111,
-}
-
-impl FCSR {
-    /// Returns the contents of the register as raw bits
-    #[inline]
-    pub fn bits(&self) -> u32 {
-        self.bits
-    }
-
-    /// Accrued Exception Flags
-    #[inline]
-    pub fn fflags(&self) -> Flags {
-        Flags(self.bits & 0x1F) // bits 0-4
-    }
-
-    /// Rounding Mode
-    #[inline]
-    pub fn frm(&self) -> RoundingMode {
-        let frm = (self.bits >> 5) & 0x7; // bits 5-7
-        match frm {
-            0b000 => RoundingMode::RoundToNearestEven,
-            0b001 => RoundingMode::RoundTowardsZero,
-            0b010 => RoundingMode::RoundDown,
-            0b011 => RoundingMode::RoundUp,
-            0b100 => RoundingMode::RoundToNearestMaxMagnitude,
-            _ => RoundingMode::Invalid,
-        }
-    }
-}
-
-read_csr!(0x003);
-write_csr!(0x003);
-clear!(0x003);
-
-/// Reads the CSR
-#[inline]
-pub fn read() -> FCSR {
-    FCSR {
-        bits: unsafe { _read() as u32 },
-    }
-}
-
-/// Writes the CSR
-#[inline]
-pub unsafe fn set_rounding_mode(frm: RoundingMode) {
-    let old = read();
-    let bits = ((frm as u32) << 5) | old.fflags().0;
-    _write(bits as usize);
-}
-
-/// Resets `fflags` field bits
-#[inline]
-pub unsafe fn clear_flags() {
-    let mask = 0b11111;
-    _clear(mask);
-}
-
-/// Resets `fflags` field bit
-#[inline]
-pub unsafe fn clear_flag(flag: Flag) {
-    _clear(flag as usize);
-}