diff options
Diffstat (limited to 'yjit/src/asm/arm64/arg/bitmask_imm.rs')
| -rw-r--r-- | yjit/src/asm/arm64/arg/bitmask_imm.rs | 255 |
1 files changed, 255 insertions, 0 deletions
diff --git a/yjit/src/asm/arm64/arg/bitmask_imm.rs b/yjit/src/asm/arm64/arg/bitmask_imm.rs new file mode 100644 index 0000000000..6b71a73d2c --- /dev/null +++ b/yjit/src/asm/arm64/arg/bitmask_imm.rs @@ -0,0 +1,255 @@ +/// Immediates used by the logical immediate instructions are not actually the +/// immediate value, but instead are encoded into a 13-bit wide mask of 3 +/// elements. This allows many more values to be represented than 13 bits would +/// normally allow, at the expense of not being able to represent every possible +/// value. +/// +/// In order for a number to be encodeable in this form, the binary +/// representation must consist of a single set of contiguous 1s. That pattern +/// must then be replicatable across all of the bits either 1, 2, 4, 8, 16, or +/// 32 times (rotated or not). +/// +/// For example, 1 (0b1), 2 (0b10), 3 (0b11), and 4 (0b100) are all valid. +/// However, 5 (0b101) is invalid, because it contains 2 sets of 1s and cannot +/// be replicated across 64 bits. +/// +/// Some more examples to illustrate the idea of replication: +/// * 0x5555555555555555 is a valid value (0b0101...) because it consists of a +/// single set of 1s which can be replicated across all of the bits 32 times. +/// * 0xf0f0f0f0f0f0f0f0 is a valid value (0b1111000011110000...) because it +/// consists of a single set of 1s which can be replicated across all of the +/// bits 8 times (rotated by 4 bits). +/// * 0x0ff00ff00ff00ff0 is a valid value (0000111111110000...) because it +/// consists of a single set of 1s which can be replicated across all of the +/// bits 4 times (rotated by 12 bits). +/// +/// To encode the values, there are 3 elements: +/// * n = 1 if the pattern is 64-bits wide, 0 otherwise +/// * imms = the size of the pattern, a 0, and then one less than the number of +/// sequential 1s +/// * immr = the number of right rotations to apply to the pattern to get the +/// target value +/// +pub struct BitmaskImmediate { + n: u8, + imms: u8, + immr: u8 +} + +impl TryFrom<u64> for BitmaskImmediate { + type Error = (); + + /// Attempt to convert a u64 into a BitmaskImmediate. + /// + /// The implementation here is largely based on this blog post: + /// https://dougallj.wordpress.com/2021/10/30/bit-twiddling-optimising-aarch64-logical-immediate-encoding-and-decoding/ + fn try_from(value: u64) -> Result<Self, Self::Error> { + if value == 0 || value == u64::MAX { + return Err(()); + } + + fn rotate_right(value: u64, rotations: u32) -> u64 { + (value >> (rotations & 0x3F)) | + (value << (rotations.wrapping_neg() & 0x3F)) + } + + let rotations = (value & (value + 1)).trailing_zeros(); + let normalized = rotate_right(value, rotations & 0x3F); + + let zeroes = normalized.leading_zeros(); + let ones = (!normalized).trailing_zeros(); + let size = zeroes + ones; + + if rotate_right(value, size & 0x3F) != value { + return Err(()); + } + + Ok(BitmaskImmediate { + n: ((size >> 6) & 1) as u8, + imms: (((size << 1).wrapping_neg() | (ones - 1)) & 0x3F) as u8, + immr: ((rotations.wrapping_neg() & (size - 1)) & 0x3F) as u8 + }) + } +} + +impl BitmaskImmediate { + /// Attempt to make a BitmaskImmediate for a 32 bit register. + /// The result has N==0, which is required for some 32-bit instructions. + /// Note that the exact same BitmaskImmediate produces different values + /// depending on the size of the target register. + pub fn new_32b_reg(value: u32) -> Result<Self, ()> { + // The same bit pattern replicated to u64 + let value = value as u64; + let replicated: u64 = (value << 32) | value; + let converted = Self::try_from(replicated); + if let Ok(ref imm) = converted { + assert_eq!(0, imm.n); + } + + converted + } +} + +impl BitmaskImmediate { + /// Encode a bitmask immediate into a 32-bit value. + pub fn encode(self) -> u32 { + 0 + | ((self.n as u32) << 12) + | ((self.immr as u32) << 6) + | (self.imms as u32) + } +} + +#[cfg(test)] +mod tests { + use super::*; + + #[test] + fn test_failures() { + [5, 9, 10, 11, 13, 17, 18, 19].iter().for_each(|&imm| { + assert!(BitmaskImmediate::try_from(imm).is_err()); + }); + } + + #[test] + fn test_negative() { + let bitmask: BitmaskImmediate = (-9_i64 as u64).try_into().unwrap(); + let encoded: u32 = bitmask.encode(); + assert_eq!(7998, encoded); + } + + #[test] + fn test_size_2_minimum() { + let bitmask = BitmaskImmediate::try_from(0x5555555555555555); + assert!(matches!(bitmask, Ok(BitmaskImmediate { n: 0, immr: 0b000000, imms: 0b111100 }))); + } + + #[test] + fn test_size_2_maximum() { + let bitmask = BitmaskImmediate::try_from(0xaaaaaaaaaaaaaaaa); + assert!(matches!(bitmask, Ok(BitmaskImmediate { n: 0, immr: 0b000001, imms: 0b111100 }))); + } + + #[test] + fn test_size_4_minimum() { + let bitmask = BitmaskImmediate::try_from(0x1111111111111111); + assert!(matches!(bitmask, Ok(BitmaskImmediate { n: 0, immr: 0b000000, imms: 0b111000 }))); + } + + #[test] + fn test_size_4_rotated() { + let bitmask = BitmaskImmediate::try_from(0x6666666666666666); + assert!(matches!(bitmask, Ok(BitmaskImmediate { n: 0, immr: 0b000011, imms: 0b111001 }))); + } + + #[test] + fn test_size_4_maximum() { + let bitmask = BitmaskImmediate::try_from(0xeeeeeeeeeeeeeeee); + assert!(matches!(bitmask, Ok(BitmaskImmediate { n: 0, immr: 0b000011, imms: 0b111010 }))); + } + + #[test] + fn test_size_8_minimum() { + let bitmask = BitmaskImmediate::try_from(0x0101010101010101); + assert!(matches!(bitmask, Ok(BitmaskImmediate { n: 0, immr: 0b000000, imms: 0b110000 }))); + } + + #[test] + fn test_size_8_rotated() { + let bitmask = BitmaskImmediate::try_from(0x1818181818181818); + assert!(matches!(bitmask, Ok(BitmaskImmediate { n: 0, immr: 0b000101, imms: 0b110001 }))); + } + + #[test] + fn test_size_8_maximum() { + let bitmask = BitmaskImmediate::try_from(0xfefefefefefefefe); + assert!(matches!(bitmask, Ok(BitmaskImmediate { n: 0, immr: 0b000111, imms: 0b110110 }))); + } + + #[test] + fn test_size_16_minimum() { + let bitmask = BitmaskImmediate::try_from(0x0001000100010001); + assert!(matches!(bitmask, Ok(BitmaskImmediate { n: 0, immr: 0b000000, imms: 0b100000 }))); + } + + #[test] + fn test_size_16_rotated() { + let bitmask = BitmaskImmediate::try_from(0xff8fff8fff8fff8f); + assert!(matches!(bitmask, Ok(BitmaskImmediate { n: 0, immr: 0b001001, imms: 0b101100 }))); + } + + #[test] + fn test_size_16_maximum() { + let bitmask = BitmaskImmediate::try_from(0xfffefffefffefffe); + assert!(matches!(bitmask, Ok(BitmaskImmediate { n: 0, immr: 0b001111, imms: 0b101110 }))); + } + + #[test] + fn test_size_32_minimum() { + let bitmask = BitmaskImmediate::try_from(0x0000000100000001); + assert!(matches!(bitmask, Ok(BitmaskImmediate { n: 0, immr: 0b000000, imms: 0b000000 }))); + } + + #[test] + fn test_size_32_rotated() { + let bitmask = BitmaskImmediate::try_from(0x3fffff003fffff00); + assert!(matches!(bitmask, Ok(BitmaskImmediate { n: 0, immr: 0b011000, imms: 0b010101 }))); + } + + #[test] + fn test_size_32_maximum() { + let bitmask = BitmaskImmediate::try_from(0xfffffffefffffffe); + assert!(matches!(bitmask, Ok(BitmaskImmediate { n: 0, immr: 0b011111, imms: 0b011110 }))); + } + + #[test] + fn test_size_64_minimum() { + let bitmask = BitmaskImmediate::try_from(0x0000000000000001); + assert!(matches!(bitmask, Ok(BitmaskImmediate { n: 1, immr: 0b000000, imms: 0b000000 }))); + } + + #[test] + fn test_size_64_rotated() { + let bitmask = BitmaskImmediate::try_from(0x0000001fffff0000); + assert!(matches!(bitmask, Ok(BitmaskImmediate { n: 1, immr: 0b110000, imms: 0b010100 }))); + } + + #[test] + fn test_size_64_maximum() { + let bitmask = BitmaskImmediate::try_from(0xfffffffffffffffe); + assert!(matches!(bitmask, Ok(BitmaskImmediate { n: 1, immr: 0b111111, imms: 0b111110 }))); + } + + #[test] + fn test_size_64_invalid() { + let bitmask = BitmaskImmediate::try_from(u64::MAX); + assert!(matches!(bitmask, Err(()))); + } + + #[test] + fn test_all_valid_32b_pattern() { + let mut patterns = vec![]; + for pattern_size in [2, 4, 8, 16, 32_u64] { + for ones_count in 1..pattern_size { + for rotation in 0..pattern_size { + let ones = (1_u64 << ones_count) - 1; + let rotated = (ones >> rotation) | + ((ones & ((1 << rotation) - 1)) << (pattern_size - rotation)); + let mut replicated = rotated; + let mut shift = pattern_size; + while shift < 32 { + replicated |= replicated << shift; + shift *= 2; + } + let replicated: u32 = replicated.try_into().unwrap(); + assert!(BitmaskImmediate::new_32b_reg(replicated).is_ok()); + patterns.push(replicated); + } + } + } + patterns.sort(); + patterns.dedup(); + // Up to {size}-1 ones, and a total of {size} possible rotations. + assert_eq!(1*2 + 3*4 + 7*8 + 15*16 + 31*32, patterns.len()); + } +} |