Files
wasmtime/lib/codegen/src/preopt.rs

566 lines
21 KiB
Rust

//! A pre-legalization rewriting pass.
#![allow(non_snake_case)]
use cursor::{Cursor, FuncCursor};
use divconst_magic_numbers::{magicS32, magicS64, magicU32, magicU64};
use divconst_magic_numbers::{MS32, MS64, MU32, MU64};
use ir::dfg::ValueDef;
use ir::instructions::Opcode;
use ir::types::{I32, I64};
use ir::Inst;
use ir::{DataFlowGraph, Function, InstBuilder, InstructionData, Type, Value};
use timing;
//----------------------------------------------------------------------
//
// Pattern-match helpers and transformation for div and rem by constants.
// Simple math helpers
/// if `x` is a power of two, or the negation thereof, return the power along
/// with a boolean that indicates whether `x` is negative. Else return None.
#[inline]
fn isPowerOf2_S32(x: i32) -> Option<(bool, u32)> {
// We have to special-case this because abs(x) isn't representable.
if x == -0x8000_0000 {
return Some((true, 31));
}
let abs_x = i32::wrapping_abs(x) as u32;
if abs_x.is_power_of_two() {
return Some((x < 0, abs_x.trailing_zeros()));
}
None
}
/// Same comments as for isPowerOf2_S64 apply.
#[inline]
fn isPowerOf2_S64(x: i64) -> Option<(bool, u32)> {
// We have to special-case this because abs(x) isn't representable.
if x == -0x8000_0000_0000_0000 {
return Some((true, 63));
}
let abs_x = i64::wrapping_abs(x) as u64;
if abs_x.is_power_of_two() {
return Some((x < 0, abs_x.trailing_zeros()));
}
None
}
#[derive(Debug)]
enum DivRemByConstInfo {
DivU32(Value, u32), // In all cases, the arguments are:
DivU64(Value, u64), // left operand, right operand
DivS32(Value, i32),
DivS64(Value, i64),
RemU32(Value, u32),
RemU64(Value, u64),
RemS32(Value, i32),
RemS64(Value, i64),
}
/// Possibly create a DivRemByConstInfo from the given components, by
/// figuring out which, if any, of the 8 cases apply, and also taking care to
/// sanity-check the immediate.
fn package_up_divrem_info(
argL: Value,
argL_ty: Type,
argRs: i64,
isSigned: bool,
isRem: bool,
) -> Option<DivRemByConstInfo> {
let argRu: u64 = argRs as u64;
if !isSigned && argL_ty == I32 && argRu < 0x1_0000_0000 {
let con = if isRem {
DivRemByConstInfo::RemU32
} else {
DivRemByConstInfo::DivU32
};
return Some(con(argL, argRu as u32));
}
if !isSigned && argL_ty == I64 {
// unsigned 64, no range constraint
let con = if isRem {
DivRemByConstInfo::RemU64
} else {
DivRemByConstInfo::DivU64
};
return Some(con(argL, argRu));
}
if isSigned && argL_ty == I32 && (argRu <= 0x7fff_ffff || argRu >= 0xffff_ffff_8000_0000) {
let con = if isRem {
DivRemByConstInfo::RemS32
} else {
DivRemByConstInfo::DivS32
};
return Some(con(argL, argRu as i32));
}
if isSigned && argL_ty == I64 {
// signed 64, no range constraint
let con = if isRem {
DivRemByConstInfo::RemS64
} else {
DivRemByConstInfo::DivS64
};
return Some(con(argL, argRu as i64));
}
None
}
/// Examine `idata` to see if it is a div or rem by a constant, and if so
/// return the operands, signedness, operation size and div-vs-rem-ness in a
/// handy bundle.
fn get_div_info(inst: Inst, dfg: &DataFlowGraph) -> Option<DivRemByConstInfo> {
let idata: &InstructionData = &dfg[inst];
if let InstructionData::BinaryImm { opcode, arg, imm } = *idata {
let (isSigned, isRem) = match opcode {
Opcode::UdivImm => (false, false),
Opcode::UremImm => (false, true),
Opcode::SdivImm => (true, false),
Opcode::SremImm => (true, true),
_other => return None,
};
// Pull the operation size (type) from the left arg
let argL_ty = dfg.value_type(arg);
return package_up_divrem_info(arg, argL_ty, imm.into(), isSigned, isRem);
}
None
}
/// Actually do the transformation given a bundle containing the relevant
/// information. `divrem_info` describes a div or rem by a constant, that
/// `pos` currently points at, and `inst` is the associated instruction.
/// `inst` is replaced by a sequence of other operations that calculate the
/// same result. Note that there are various `divrem_info` cases where we
/// cannot do any transformation, in which case `inst` is left unchanged.
fn do_divrem_transformation(divrem_info: &DivRemByConstInfo, pos: &mut FuncCursor, inst: Inst) {
let isRem = match *divrem_info {
DivRemByConstInfo::DivU32(_, _)
| DivRemByConstInfo::DivU64(_, _)
| DivRemByConstInfo::DivS32(_, _)
| DivRemByConstInfo::DivS64(_, _) => false,
DivRemByConstInfo::RemU32(_, _)
| DivRemByConstInfo::RemU64(_, _)
| DivRemByConstInfo::RemS32(_, _)
| DivRemByConstInfo::RemS64(_, _) => true,
};
match *divrem_info {
// -------------------- U32 --------------------
// U32 div, rem by zero: ignore
DivRemByConstInfo::DivU32(_n1, 0) | DivRemByConstInfo::RemU32(_n1, 0) => {}
// U32 div by 1: identity
// U32 rem by 1: zero
DivRemByConstInfo::DivU32(n1, 1) | DivRemByConstInfo::RemU32(n1, 1) => {
if isRem {
pos.func.dfg.replace(inst).iconst(I32, 0);
} else {
pos.func.dfg.replace(inst).copy(n1);
}
}
// U32 div, rem by a power-of-2
DivRemByConstInfo::DivU32(n1, d) | DivRemByConstInfo::RemU32(n1, d)
if d.is_power_of_two() =>
{
debug_assert!(d >= 2);
// compute k where d == 2^k
let k = d.trailing_zeros();
debug_assert!(k >= 1 && k <= 31);
if isRem {
let mask = (1u64 << k) - 1;
pos.func.dfg.replace(inst).band_imm(n1, mask as i64);
} else {
pos.func.dfg.replace(inst).ushr_imm(n1, k as i64);
}
}
// U32 div, rem by non-power-of-2
DivRemByConstInfo::DivU32(n1, d) | DivRemByConstInfo::RemU32(n1, d) => {
debug_assert!(d >= 3);
let MU32 {
mulBy,
doAdd,
shiftBy,
} = magicU32(d);
let qf; // final quotient
let q0 = pos.ins().iconst(I32, mulBy as i64);
let q1 = pos.ins().umulhi(n1, q0);
if doAdd {
debug_assert!(shiftBy >= 1 && shiftBy <= 32);
let t1 = pos.ins().isub(n1, q1);
let t2 = pos.ins().ushr_imm(t1, 1);
let t3 = pos.ins().iadd(t2, q1);
// I never found any case where shiftBy == 1 here.
// So there's no attempt to fold out a zero shift.
debug_assert_ne!(shiftBy, 1);
qf = pos.ins().ushr_imm(t3, (shiftBy - 1) as i64);
} else {
debug_assert!(shiftBy >= 0 && shiftBy <= 31);
// Whereas there are known cases here for shiftBy == 0.
if shiftBy > 0 {
qf = pos.ins().ushr_imm(q1, shiftBy as i64);
} else {
qf = q1;
}
}
// Now qf holds the final quotient. If necessary calculate the
// remainder instead.
if isRem {
let tt = pos.ins().imul_imm(qf, d as i64);
pos.func.dfg.replace(inst).isub(n1, tt);
} else {
pos.func.dfg.replace(inst).copy(qf);
}
}
// -------------------- U64 --------------------
// U64 div, rem by zero: ignore
DivRemByConstInfo::DivU64(_n1, 0) | DivRemByConstInfo::RemU64(_n1, 0) => {}
// U64 div by 1: identity
// U64 rem by 1: zero
DivRemByConstInfo::DivU64(n1, 1) | DivRemByConstInfo::RemU64(n1, 1) => {
if isRem {
pos.func.dfg.replace(inst).iconst(I64, 0);
} else {
pos.func.dfg.replace(inst).copy(n1);
}
}
// U64 div, rem by a power-of-2
DivRemByConstInfo::DivU64(n1, d) | DivRemByConstInfo::RemU64(n1, d)
if d.is_power_of_two() =>
{
debug_assert!(d >= 2);
// compute k where d == 2^k
let k = d.trailing_zeros();
debug_assert!(k >= 1 && k <= 63);
if isRem {
let mask = (1u64 << k) - 1;
pos.func.dfg.replace(inst).band_imm(n1, mask as i64);
} else {
pos.func.dfg.replace(inst).ushr_imm(n1, k as i64);
}
}
// U64 div, rem by non-power-of-2
DivRemByConstInfo::DivU64(n1, d) | DivRemByConstInfo::RemU64(n1, d) => {
debug_assert!(d >= 3);
let MU64 {
mulBy,
doAdd,
shiftBy,
} = magicU64(d);
let qf; // final quotient
let q0 = pos.ins().iconst(I64, mulBy as i64);
let q1 = pos.ins().umulhi(n1, q0);
if doAdd {
debug_assert!(shiftBy >= 1 && shiftBy <= 64);
let t1 = pos.ins().isub(n1, q1);
let t2 = pos.ins().ushr_imm(t1, 1);
let t3 = pos.ins().iadd(t2, q1);
// I never found any case where shiftBy == 1 here.
// So there's no attempt to fold out a zero shift.
debug_assert_ne!(shiftBy, 1);
qf = pos.ins().ushr_imm(t3, (shiftBy - 1) as i64);
} else {
debug_assert!(shiftBy >= 0 && shiftBy <= 63);
// Whereas there are known cases here for shiftBy == 0.
if shiftBy > 0 {
qf = pos.ins().ushr_imm(q1, shiftBy as i64);
} else {
qf = q1;
}
}
// Now qf holds the final quotient. If necessary calculate the
// remainder instead.
if isRem {
let tt = pos.ins().imul_imm(qf, d as i64);
pos.func.dfg.replace(inst).isub(n1, tt);
} else {
pos.func.dfg.replace(inst).copy(qf);
}
}
// -------------------- S32 --------------------
// S32 div, rem by zero or -1: ignore
DivRemByConstInfo::DivS32(_n1, -1)
| DivRemByConstInfo::RemS32(_n1, -1)
| DivRemByConstInfo::DivS32(_n1, 0)
| DivRemByConstInfo::RemS32(_n1, 0) => {}
// S32 div by 1: identity
// S32 rem by 1: zero
DivRemByConstInfo::DivS32(n1, 1) | DivRemByConstInfo::RemS32(n1, 1) => {
if isRem {
pos.func.dfg.replace(inst).iconst(I32, 0);
} else {
pos.func.dfg.replace(inst).copy(n1);
}
}
DivRemByConstInfo::DivS32(n1, d) | DivRemByConstInfo::RemS32(n1, d) => {
if let Some((isNeg, k)) = isPowerOf2_S32(d) {
// k can be 31 only in the case that d is -2^31.
debug_assert!(k >= 1 && k <= 31);
let t1 = if k - 1 == 0 {
n1
} else {
pos.ins().sshr_imm(n1, (k - 1) as i64)
};
let t2 = pos.ins().ushr_imm(t1, (32 - k) as i64);
let t3 = pos.ins().iadd(n1, t2);
if isRem {
// S32 rem by a power-of-2
let t4 = pos.ins().band_imm(t3, i32::wrapping_neg(1 << k) as i64);
// Curiously, we don't care here what the sign of d is.
pos.func.dfg.replace(inst).isub(n1, t4);
} else {
// S32 div by a power-of-2
let t4 = pos.ins().sshr_imm(t3, k as i64);
if isNeg {
pos.func.dfg.replace(inst).irsub_imm(t4, 0);
} else {
pos.func.dfg.replace(inst).copy(t4);
}
}
} else {
// S32 div, rem by a non-power-of-2
debug_assert!(d < -2 || d > 2);
let MS32 { mulBy, shiftBy } = magicS32(d);
let q0 = pos.ins().iconst(I32, mulBy as i64);
let q1 = pos.ins().smulhi(n1, q0);
let q2 = if d > 0 && mulBy < 0 {
pos.ins().iadd(q1, n1)
} else if d < 0 && mulBy > 0 {
pos.ins().isub(q1, n1)
} else {
q1
};
debug_assert!(shiftBy >= 0 && shiftBy <= 31);
let q3 = if shiftBy == 0 {
q2
} else {
pos.ins().sshr_imm(q2, shiftBy as i64)
};
let t1 = pos.ins().ushr_imm(q3, 31);
let qf = pos.ins().iadd(q3, t1);
// Now qf holds the final quotient. If necessary calculate
// the remainder instead.
if isRem {
let tt = pos.ins().imul_imm(qf, d as i64);
pos.func.dfg.replace(inst).isub(n1, tt);
} else {
pos.func.dfg.replace(inst).copy(qf);
}
}
}
// -------------------- S64 --------------------
// S64 div, rem by zero or -1: ignore
DivRemByConstInfo::DivS64(_n1, -1)
| DivRemByConstInfo::RemS64(_n1, -1)
| DivRemByConstInfo::DivS64(_n1, 0)
| DivRemByConstInfo::RemS64(_n1, 0) => {}
// S64 div by 1: identity
// S64 rem by 1: zero
DivRemByConstInfo::DivS64(n1, 1) | DivRemByConstInfo::RemS64(n1, 1) => {
if isRem {
pos.func.dfg.replace(inst).iconst(I64, 0);
} else {
pos.func.dfg.replace(inst).copy(n1);
}
}
DivRemByConstInfo::DivS64(n1, d) | DivRemByConstInfo::RemS64(n1, d) => {
if let Some((isNeg, k)) = isPowerOf2_S64(d) {
// k can be 63 only in the case that d is -2^63.
debug_assert!(k >= 1 && k <= 63);
let t1 = if k - 1 == 0 {
n1
} else {
pos.ins().sshr_imm(n1, (k - 1) as i64)
};
let t2 = pos.ins().ushr_imm(t1, (64 - k) as i64);
let t3 = pos.ins().iadd(n1, t2);
if isRem {
// S64 rem by a power-of-2
let t4 = pos.ins().band_imm(t3, i64::wrapping_neg(1 << k));
// Curiously, we don't care here what the sign of d is.
pos.func.dfg.replace(inst).isub(n1, t4);
} else {
// S64 div by a power-of-2
let t4 = pos.ins().sshr_imm(t3, k as i64);
if isNeg {
pos.func.dfg.replace(inst).irsub_imm(t4, 0);
} else {
pos.func.dfg.replace(inst).copy(t4);
}
}
} else {
// S64 div, rem by a non-power-of-2
debug_assert!(d < -2 || d > 2);
let MS64 { mulBy, shiftBy } = magicS64(d);
let q0 = pos.ins().iconst(I64, mulBy);
let q1 = pos.ins().smulhi(n1, q0);
let q2 = if d > 0 && mulBy < 0 {
pos.ins().iadd(q1, n1)
} else if d < 0 && mulBy > 0 {
pos.ins().isub(q1, n1)
} else {
q1
};
debug_assert!(shiftBy >= 0 && shiftBy <= 63);
let q3 = if shiftBy == 0 {
q2
} else {
pos.ins().sshr_imm(q2, shiftBy as i64)
};
let t1 = pos.ins().ushr_imm(q3, 63);
let qf = pos.ins().iadd(q3, t1);
// Now qf holds the final quotient. If necessary calculate
// the remainder instead.
if isRem {
let tt = pos.ins().imul_imm(qf, d);
pos.func.dfg.replace(inst).isub(n1, tt);
} else {
pos.func.dfg.replace(inst).copy(qf);
}
}
}
}
}
/// Apply basic simplifications.
///
/// This folds constants with arithmetic to form `_imm` instructions, and other
/// minor simplifications.
fn simplify(pos: &mut FuncCursor, inst: Inst) {
match pos.func.dfg[inst] {
InstructionData::Binary { opcode, args } => {
if let ValueDef::Result(iconst_inst, _) = pos.func.dfg.value_def(args[1]) {
if let InstructionData::UnaryImm {
opcode: Opcode::Iconst,
mut imm,
} = pos.func.dfg[iconst_inst]
{
let new_opcode = match opcode {
Opcode::Iadd => Opcode::IaddImm,
Opcode::Imul => Opcode::ImulImm,
Opcode::Sdiv => Opcode::SdivImm,
Opcode::Udiv => Opcode::UdivImm,
Opcode::Srem => Opcode::SremImm,
Opcode::Urem => Opcode::UremImm,
Opcode::Band => Opcode::BandImm,
Opcode::Bor => Opcode::BorImm,
Opcode::Bxor => Opcode::BxorImm,
Opcode::Rotl => Opcode::RotlImm,
Opcode::Rotr => Opcode::RotrImm,
Opcode::Ishl => Opcode::IshlImm,
Opcode::Ushr => Opcode::UshrImm,
Opcode::Sshr => Opcode::SshrImm,
Opcode::Isub => {
imm = imm.wrapping_neg();
Opcode::IaddImm
}
_ => return,
};
let ty = pos.func.dfg.ctrl_typevar(inst);
pos.func
.dfg
.replace(inst)
.BinaryImm(new_opcode, ty, imm, args[0]);
}
} else if let ValueDef::Result(iconst_inst, _) = pos.func.dfg.value_def(args[0]) {
if let InstructionData::UnaryImm {
opcode: Opcode::Iconst,
imm,
} = pos.func.dfg[iconst_inst]
{
let new_opcode = match opcode {
Opcode::Isub => Opcode::IrsubImm,
_ => return,
};
let ty = pos.func.dfg.ctrl_typevar(inst);
pos.func
.dfg
.replace(inst)
.BinaryImm(new_opcode, ty, imm, args[1]);
}
}
}
InstructionData::IntCompare { opcode, cond, args } => {
debug_assert_eq!(opcode, Opcode::Icmp);
if let ValueDef::Result(iconst_inst, _) = pos.func.dfg.value_def(args[1]) {
if let InstructionData::UnaryImm {
opcode: Opcode::Iconst,
imm,
} = pos.func.dfg[iconst_inst]
{
pos.func.dfg.replace(inst).icmp_imm(cond, args[0], imm);
}
}
}
InstructionData::CondTrap { .. }
| InstructionData::Branch { .. }
| InstructionData::Ternary {
opcode: Opcode::Select,
..
} => {
// Fold away a redundant `bint`.
let maybe = {
let args = pos.func.dfg.inst_args(inst);
if let ValueDef::Result(def_inst, _) = pos.func.dfg.value_def(args[0]) {
if let InstructionData::Unary {
opcode: Opcode::Bint,
arg: bool_val,
} = pos.func.dfg[def_inst]
{
Some(bool_val)
} else {
None
}
} else {
None
}
};
if let Some(bool_val) = maybe {
let args = pos.func.dfg.inst_args_mut(inst);
args[0] = bool_val;
}
}
_ => {}
}
}
/// The main pre-opt pass.
pub fn do_preopt(func: &mut Function) {
let _tt = timing::preopt();
let mut pos = FuncCursor::new(func);
while let Some(_ebb) = pos.next_ebb() {
while let Some(inst) = pos.next_inst() {
// Apply basic simplifications.
simplify(&mut pos, inst);
//-- BEGIN -- division by constants ----------------
let mb_dri = get_div_info(inst, &pos.func.dfg);
if let Some(divrem_info) = mb_dri {
do_divrem_transformation(&divrem_info, &mut pos, inst);
continue;
}
//-- END -- division by constants ------------------
}
}
}