diff options
Diffstat (limited to 'yjit/src/asm/arm64/arg/truncate.rs')
-rw-r--r-- | yjit/src/asm/arm64/arg/truncate.rs | 66 |
1 files changed, 66 insertions, 0 deletions
diff --git a/yjit/src/asm/arm64/arg/truncate.rs b/yjit/src/asm/arm64/arg/truncate.rs new file mode 100644 index 0000000000..52f2c012cb --- /dev/null +++ b/yjit/src/asm/arm64/arg/truncate.rs @@ -0,0 +1,66 @@ +// There are many instances in AArch64 instruction encoding where you represent +// an integer value with a particular bit width that isn't a power of 2. These +// functions represent truncating those integer values down to the appropriate +// number of bits. + +/// Truncate a signed immediate to fit into a compile-time known width. It is +/// assumed before calling this function that the value fits into the correct +/// size. If it doesn't, then this function will panic. +/// +/// When the value is positive, this should effectively be a no-op since we're +/// just dropping leading zeroes. When the value is negative we should only be +/// dropping leading ones. +pub fn truncate_imm<T: Into<i32>, const WIDTH: usize>(imm: T) -> u32 { + let value: i32 = imm.into(); + let masked = (value as u32) & ((1 << WIDTH) - 1); + + // Assert that we didn't drop any bits by truncating. + if value >= 0 { + assert_eq!(value as u32, masked); + } else { + assert_eq!(value as u32, masked | (u32::MAX << WIDTH)); + } + + masked +} + +/// Truncate an unsigned immediate to fit into a compile-time known width. It is +/// assumed before calling this function that the value fits into the correct +/// size. If it doesn't, then this function will panic. +/// +/// This should effectively be a no-op since we're just dropping leading zeroes. +pub fn truncate_uimm<T: Into<u32>, const WIDTH: usize>(uimm: T) -> u32 { + let value: u32 = uimm.into(); + let masked = (value & ((1 << WIDTH) - 1)); + + // Assert that we didn't drop any bits by truncating. + assert_eq!(value, masked); + + masked +} + +#[cfg(test)] +mod tests { + use super::*; + + #[test] + fn test_truncate_imm_positive() { + let inst = truncate_imm::<i32, 4>(5); + let result: u32 = inst.into(); + assert_eq!(0b0101, result); + } + + #[test] + fn test_truncate_imm_negative() { + let inst = truncate_imm::<i32, 4>(-5); + let result: u32 = inst.into(); + assert_eq!(0b1011, result); + } + + #[test] + fn test_truncate_uimm() { + let inst = truncate_uimm::<u32, 4>(5); + let result: u32 = inst.into(); + assert_eq!(0b0101, result); + } +} |