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Only save callee-saved registers that are used (#293)

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* Only save callee-saved registers that are actually being used.

* Rename AllocatableSet to RegisterSet

* Style cleanup and small renames for readability.

* Adjust x86 prologue-epilogue test to account for callee-saved register optimization.

* Add more tests for prologue-epilogue optimizations.
This commit is contained in:
Tyler McMullen
2018-04-03 14:44:12 -07:00
committed by Dan Gohman
parent 0948ca9963
commit 775c674b38
15 changed files with 310 additions and 123 deletions
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321
lib/cretonne/src/regalloc/register_set.rs Normal file
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//! Set of allocatable registers as a bit vector of register units.
//!
//! While allocating registers, we need to keep track of which registers are available and which
//! registers are in use. Since registers can alias in different ways, we track this via the
//! "register unit" abstraction. Every register contains one or more register units. Registers that
//! share a register unit can't be in use at the same time.
use isa::registers::{RegClass, RegInfo, RegUnit, RegUnitMask};
use std::char;
use std::fmt;
use std::iter::ExactSizeIterator;
use std::mem::size_of_val;
/// Set of registers available for allocation.
#[derive(Clone)]
pub struct RegisterSet {
avail: RegUnitMask,
}
// Given a register class and a register unit in the class, compute a word index and a bit mask of
// register units representing that register.
//
// Note that a register is not allowed to straddle words.
fn bitmask(rc: RegClass, reg: RegUnit) -> (usize, u32) {
// Bit mask representing the register. It is `rc.width` consecutive units.
let width_bits = (1 << rc.width) - 1;
// Index into avail[] of the word containing `reg`.
let word_index = (reg / 32) as usize;
// The actual bits in the word that cover `reg`.
let reg_bits = width_bits << (reg % 32);
(word_index, reg_bits)
}
impl RegisterSet {
/// Create a new register set with all registers available.
///
/// Note that this includes *all* registers. Query the `TargetIsa` object to get a set of
/// allocatable registers where reserved registers have been filtered out.
pub fn new() -> Self {
Self { avail: [!0; 3] }
}
/// Create a new register set with no registers available.
pub fn empty() -> Self {
Self { avail: [0; 3] }
}
/// Returns `true` if the specified register is available.
pub fn is_avail(&self, rc: RegClass, reg: RegUnit) -> bool {
let (idx, bits) = bitmask(rc, reg);
(self.avail[idx] & bits) == bits
}
/// Allocate `reg` from `rc` so it is no longer available.
///
/// It is an error to take a register that doesn't have all of its register units available.
pub fn take(&mut self, rc: RegClass, reg: RegUnit) {
let (idx, bits) = bitmask(rc, reg);
debug_assert!(
(self.avail[idx] & bits) == bits,
"{}:{} not available in {}",
rc,
rc.info.display_regunit(reg),
self.display(rc.info)
);
self.avail[idx] &= !bits;
}
/// Return `reg` and all of its register units to the set of available registers.
pub fn free(&mut self, rc: RegClass, reg: RegUnit) {
let (idx, bits) = bitmask(rc, reg);
debug_assert!(
(self.avail[idx] & bits) == 0,
"{}:{} not allocated in {}",
rc,
rc.info.display_regunit(reg),
self.display(rc.info)
);
self.avail[idx] |= bits;
}
/// Return an iterator over all available registers belonging to the register class `rc`.
///
/// This doesn't allocate anything from the set; use `take()` for that.
pub fn iter(&self, rc: RegClass) -> RegSetIter {
// Start by copying the RC mask. It is a single set bit for each register in the class.
let mut rsi = RegSetIter { regs: rc.mask };
// Mask out the unavailable units.
for idx in 0..self.avail.len() {
// If a single unit in a register is unavailable, the whole register can't be used.
// If a register straddles a word boundary, it will be marked as unavailable.
// There's an assertion in `cdsl/registers.py` to check for that.
for i in 0..rc.width {
rsi.regs[idx] &= self.avail[idx] >> i;
}
}
rsi
}
/// Check if any register units allocated out of this set interferes with units allocated out
/// of `other`.
///
/// This assumes that unused bits are 1.
pub fn interferes_with(&self, other: &RegisterSet) -> bool {
self.avail.iter().zip(&other.avail).any(
|(&x, &y)| (x | y) != !0,
)
}
/// Intersect this set of registers with `other`. This has the effect of removing any register
/// units from this set that are not in `other`.
pub fn intersect(&mut self, other: &RegisterSet) {
for (x, &y) in self.avail.iter_mut().zip(&other.avail) {
*x &= y;
}
}
/// Return an object that can display this register set, using the register info from the
/// target ISA.
pub fn display<'a, R: Into<Option<&'a RegInfo>>>(&self, regs: R) -> DisplayRegisterSet<'a> {
DisplayRegisterSet(self.clone(), regs.into())
}
}
/// Iterator over available registers in a register class.
pub struct RegSetIter {
regs: RegUnitMask,
}
impl Iterator for RegSetIter {
type Item = RegUnit;
fn next(&mut self) -> Option<RegUnit> {
let mut unit_offset = 0;
// Find the first set bit in `self.regs`.
for word in &mut self.regs {
if *word != 0 {
// Compute the register unit number from the lowest set bit in the word.
let unit = unit_offset + word.trailing_zeros() as RegUnit;
// Clear that lowest bit so we won't find it again.
*word &= *word - 1;
return Some(unit);
}
// How many register units was there in the word? This is a constant 32 for `u32` etc.
unit_offset += 8 * size_of_val(word) as RegUnit;
}
// All of `self.regs` is 0.
None
}
fn size_hint(&self) -> (usize, Option<usize>) {
let bits = self.regs.iter().map(|&w| w.count_ones() as usize).sum();
(bits, Some(bits))
}
}
impl ExactSizeIterator for RegSetIter {}
/// Displaying an `RegisterSet` correctly requires the associated `RegInfo` from the target ISA.
pub struct DisplayRegisterSet<'a>(RegisterSet, Option<&'a RegInfo>);
impl<'a> fmt::Display for DisplayRegisterSet<'a> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "[")?;
match self.1 {
None => {
for w in &self.0.avail {
write!(f, " #{:08x}", w)?;
}
}
Some(reginfo) => {
let toprcs = reginfo
.banks
.iter()
.map(|b| b.first_toprc + b.num_toprcs)
.max()
.expect("No register banks");
for rc in &reginfo.classes[0..toprcs] {
if rc.width == 1 {
let bank = &reginfo.banks[rc.bank as usize];
write!(f, " {}: ", rc)?;
for offset in 0..bank.units {
let reg = bank.first_unit + offset;
if !rc.contains(reg) {
continue;
}
if !self.0.is_avail(rc, reg) {
write!(f, "-")?;
continue;
}
// Display individual registers as either the second letter of their
// name or the last digit of their number.
// This works for Intel (rax, rbx, ...) and for numbered regs.
write!(
f,
"{}",
bank.names
.get(offset as usize)
.and_then(|name| name.chars().nth(1))
.unwrap_or_else(
|| char::from_digit(u32::from(offset % 10), 10).unwrap(),
)
)?;
}
}
}
}
}
write!(f, " ]")
}
}
impl fmt::Display for RegisterSet {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
self.display(None).fmt(f)
}
}
#[cfg(test)]
mod tests {
use super::*;
use isa::registers::{RegClass, RegClassData};
use std::vec::Vec;
// Register classes for testing.
const GPR: RegClass = &RegClassData {
name: "GPR",
index: 0,
width: 1,
bank: 0,
toprc: 0,
first: 28,
subclasses: 0,
mask: [0xf0000000, 0x0000000f, 0],
info: &INFO,
};
const DPR: RegClass = &RegClassData {
name: "DPR",
index: 0,
width: 2,
bank: 0,
toprc: 0,
first: 28,
subclasses: 0,
mask: [0x50000000, 0x0000000a, 0],
info: &INFO,
};
const INFO: RegInfo = RegInfo {
banks: &[],
classes: &[],
};
#[test]
fn put_and_take() {
let mut regs = RegisterSet::new();
// `GPR` has units 28-36.
assert_eq!(regs.iter(GPR).len(), 8);
assert_eq!(regs.iter(GPR).count(), 8);
assert_eq!(regs.iter(DPR).collect::<Vec<_>>(), [28, 30, 33, 35]);
assert!(regs.is_avail(GPR, 29));
regs.take(&GPR, 29);
assert!(!regs.is_avail(GPR, 29));
assert_eq!(regs.iter(GPR).count(), 7);
assert_eq!(regs.iter(DPR).collect::<Vec<_>>(), [30, 33, 35]);
assert!(regs.is_avail(GPR, 30));
regs.take(&GPR, 30);
assert!(!regs.is_avail(GPR, 30));
assert_eq!(regs.iter(GPR).count(), 6);
assert_eq!(regs.iter(DPR).collect::<Vec<_>>(), [33, 35]);
assert!(regs.is_avail(GPR, 32));
regs.take(&GPR, 32);
assert!(!regs.is_avail(GPR, 32));
assert_eq!(regs.iter(GPR).count(), 5);
assert_eq!(regs.iter(DPR).collect::<Vec<_>>(), [33, 35]);
regs.free(&GPR, 30);
assert!(regs.is_avail(GPR, 30));
assert!(!regs.is_avail(GPR, 29));
assert!(!regs.is_avail(GPR, 32));
assert_eq!(regs.iter(GPR).count(), 6);
assert_eq!(regs.iter(DPR).collect::<Vec<_>>(), [30, 33, 35]);
regs.free(&GPR, 32);
assert!(regs.is_avail(GPR, 31));
assert!(!regs.is_avail(GPR, 29));
assert!(regs.is_avail(GPR, 32));
assert_eq!(regs.iter(GPR).count(), 7);
assert_eq!(regs.iter(DPR).collect::<Vec<_>>(), [30, 33, 35]);
}
#[test]
fn interference() {
let mut regs1 = RegisterSet::new();
let mut regs2 = RegisterSet::new();
assert!(!regs1.interferes_with(&regs2));
regs1.take(&GPR, 32);
assert!(!regs1.interferes_with(&regs2));
regs2.take(&GPR, 31);
assert!(!regs1.interferes_with(&regs2));
regs1.intersect(&regs2);
assert!(regs1.interferes_with(&regs2));
}
}