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Optimize switch codegen and improve docs (#712)

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This commit is contained in:
bjorn3
2019-03-26 11:04:02 +01:00
committed by Benjamin Bouvier
parent 5e12f4f2b8
commit 45efbf7973
1 changed files with 114 additions and 27 deletions
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141
cranelift/frontend/src/switch.rs
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@@ -9,6 +9,36 @@ type EntryIndex = u64;
/// Unlike with `br_table`, `Switch` cases may be sparse or non-0-based.
/// They emit efficient code using branches, jump tables, or a combination of both.
///
/// # Example
///
/// ```rust
/// # use cranelift_codegen::ir::types::*;
/// # use cranelift_codegen::ir::{ExternalName, Function, Signature, InstBuilder};
/// # use cranelift_codegen::isa::CallConv;
/// # use cranelift_frontend::{FunctionBuilder, FunctionBuilderContext, Switch};
/// #
/// # let mut sig = Signature::new(CallConv::SystemV);
/// # let mut fn_builder_ctx = FunctionBuilderContext::new();
/// # let mut func = Function::with_name_signature(ExternalName::user(0, 0), sig);
/// # let mut builder = FunctionBuilder::new(&mut func, &mut fn_builder_ctx);
/// #
/// # let entry = builder.create_ebb();
/// # builder.switch_to_block(entry);
/// #
/// let block0 = builder.create_ebb();
/// let block1 = builder.create_ebb();
/// let block2 = builder.create_ebb();
/// let fallback = builder.create_ebb();
///
/// let val = builder.ins().iconst(I32, 1);
///
/// let mut switch = Switch::new();
/// switch.set_entry(0, block0);
/// switch.set_entry(1, block1);
/// switch.set_entry(7, block2);
/// switch.emit(&mut builder, val, fallback);
/// ```
#[derive(Debug, Default)]
pub struct Switch {
cases: HashMap<EntryIndex, Ebb>,
@@ -32,6 +62,14 @@ impl Switch {
);
}
/// Turn the `cases` `HashMap` into a list of `ContiguousCaseRange`s.
///
/// # Postconditions
///
/// * Every entry will be represented.
/// * The `ContiguousCaseRange`s will not overlap.
/// * Between two `ContiguousCaseRange`s there will be at least one entry index.
/// * No `ContiguousCaseRange`s will be empty.
fn collect_contiguous_case_ranges(self) -> Vec<ContiguousCaseRange> {
debug!("build_contiguous_case_ranges before: {:#?}", self.cases);
let mut cases = self.cases.into_iter().collect::<Vec<(_, _)>>();
@@ -60,6 +98,7 @@ impl Switch {
contiguous_case_ranges
}
/// Binary search for the right `ContiguousCaseRange`.
fn build_search_tree(
bx: &mut FunctionBuilder,
val: Value,
@@ -120,6 +159,7 @@ impl Switch {
cases_and_jt_ebbs
}
/// Linear search for the right `ContiguousCaseRange`.
fn build_search_branches(
bx: &mut FunctionBuilder,
val: Value,
@@ -128,22 +168,41 @@ impl Switch {
cases_and_jt_ebbs: &mut Vec<(EntryIndex, Ebb, Vec<Ebb>)>,
) {
for ContiguousCaseRange { first_index, ebbs } in contiguous_case_ranges.into_iter().rev() {
if ebbs.len() == 1 {
let is_good_val = bx.ins().icmp_imm(IntCC::Equal, val, first_index as i64);
bx.ins().brnz(is_good_val, ebbs[0], &[]);
} else {
let jt_ebb = bx.create_ebb();
let is_good_val =
bx.ins()
.icmp_imm(IntCC::UnsignedGreaterThanOrEqual, val, first_index as i64);
bx.ins().brnz(is_good_val, jt_ebb, &[]);
cases_and_jt_ebbs.push((first_index, jt_ebb, ebbs));
match (ebbs.len(), first_index) {
(1, 0) => {
bx.ins().brz(val, ebbs[0], &[]);
}
(1, _) => {
let is_good_val = bx.ins().icmp_imm(IntCC::Equal, val, first_index as i64);
bx.ins().brnz(is_good_val, ebbs[0], &[]);
}
(_, 0) => {
// if `first_index` is 0, then `icmp_imm uge val, first_index` is trivially true
let jt_ebb = bx.create_ebb();
bx.ins().jump(jt_ebb, &[]);
cases_and_jt_ebbs.push((first_index, jt_ebb, ebbs));
// `jump otherwise` below must not be hit, because the current block has been
// filled above. This is the last iteration anyway, as 0 is the smallest
// unsigned int, so just return here.
return;
}
(_, _) => {
let jt_ebb = bx.create_ebb();
let is_good_val = bx.ins().icmp_imm(
IntCC::UnsignedGreaterThanOrEqual,
val,
first_index as i64,
);
bx.ins().brnz(is_good_val, jt_ebb, &[]);
cases_and_jt_ebbs.push((first_index, jt_ebb, ebbs));
}
}
}
bx.ins().jump(otherwise, &[]);
}
/// For every item in `cases_and_jt_ebbs` this will create a jump table in the specified ebb.
fn build_jump_tables(
bx: &mut FunctionBuilder,
val: Value,
@@ -158,7 +217,11 @@ impl Switch {
let jump_table = bx.create_jump_table(jt_data);
bx.switch_to_block(jt_ebb);
let discr = bx.ins().iadd_imm(val, (first_index as i64).wrapping_neg());
let discr = if first_index == 0 {
val
} else {
bx.ins().iadd_imm(val, (first_index as i64).wrapping_neg())
};
bx.ins().br_table(discr, otherwise, jump_table);
}
}
@@ -183,9 +246,22 @@ impl Switch {
}
}
/// This represents a contiguous range of cases to switch on.
///
/// For example 10 => ebb1, 11 => ebb2, 12 => ebb7 will be represented as:
///
/// ```plain
/// ContiguousCaseRange {
/// first_index: 10,
/// ebbs: vec![Ebb::from_u32(1), Ebb::from_u32(2), Ebb::from_u32(7)]
/// }
/// ```
#[derive(Debug)]
struct ContiguousCaseRange {
/// The entry index of the first case. Eg. 10 when the entry indexes are 10, 11, 12 and 13.
first_index: EntryIndex,
/// The ebbs to jump to sorted in ascending order of entry index.
ebbs: Vec<Ebb>,
}
@@ -237,8 +313,7 @@ mod tests {
"ebb0:
v0 = iconst.i8 0
v1 = uextend.i32 v0
v2 = icmp_imm eq v1, 0
brnz v2, ebb1
brz v1, ebb1
jump ebb0"
);
}
@@ -267,13 +342,10 @@ mod tests {
ebb0:
v0 = iconst.i8 0
v1 = uextend.i32 v0
v2 = icmp_imm uge v1, 0
brnz v2, ebb3
jump ebb0
jump ebb3
ebb3:
v3 = iadd_imm.i32 v1, 0
br_table v3, ebb0, jt0"
br_table.i32 v1, ebb0, jt0"
);
}
@@ -287,8 +359,7 @@ ebb3:
v1 = uextend.i32 v0
v2 = icmp_imm eq v1, 2
brnz v2, ebb2
v3 = icmp_imm eq v1, 0
brnz v3, ebb1
brz v1, ebb1
jump ebb0"
);
}
@@ -318,17 +389,14 @@ ebb9:
ebb8:
v5 = icmp_imm.i32 eq v1, 5
brnz v5, ebb3
v6 = icmp_imm.i32 uge v1, 0
brnz v6, ebb11
jump ebb0
jump ebb11
ebb11:
v7 = iadd_imm.i32 v1, 0
br_table v7, ebb0, jt0
br_table.i32 v1, ebb0, jt0
ebb10:
v8 = iadd_imm.i32 v1, -10
br_table v8, ebb0, jt1"
v6 = iadd_imm.i32 v1, -10
br_table v6, ebb0, jt1"
);
}
@@ -363,4 +431,23 @@ ebb10:
jump ebb0"
)
}
#[test]
fn switch_optimal_codegen() {
let func = setup!(0, [-1i64 as u64, 0, 1,]);
assert_eq!(
func,
" jt0 = jump_table [ebb2, ebb3]
ebb0:
v0 = iconst.i8 0
v1 = uextend.i32 v0
v2 = icmp_imm eq v1, -1
brnz v2, ebb1
jump ebb4
ebb4:
br_table.i32 v1, ebb0, jt0"
);
}
}