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Implement algebraic simplifications.

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This commit is contained in:
Camille GILLOT 2023-04-15 17:00:43 +00:00
parent 74a967bcec
commit 22986b72e5
5 changed files with 103 additions and 4 deletions
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36
compiler/rustc_mir_transform/src/dataflow_const_prop.rs
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@ -309,7 +309,10 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
) -> (FlatSet<ScalarInt>, FlatSet<bool>) {
let left = self.eval_operand(left, state);
let right = self.eval_operand(right, state);
match (left, right) {
(FlatSet::Bottom, _) | (_, FlatSet::Bottom) => (FlatSet::Bottom, FlatSet::Bottom),
// Both sides are known, do the actual computation.
(FlatSet::Elem(left), FlatSet::Elem(right)) => {
match self.ecx.overflowing_binary_op(op, &left, &right) {
Ok((Scalar::Int(val), overflow, _)) => {
@ -318,11 +321,36 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
_ => (FlatSet::Top, FlatSet::Top),
}
}
(FlatSet::Bottom, _) | (_, FlatSet::Bottom) => (FlatSet::Bottom, FlatSet::Bottom),
(_, _) => {
// Could attempt some algebraic simplifications here.
(FlatSet::Top, FlatSet::Top)
// Exactly one side is known, attempt some algebraic simplifications.
(FlatSet::Elem(const_arg), _) | (_, FlatSet::Elem(const_arg)) => {
let layout = const_arg.layout;
if !matches!(layout.abi, rustc_target::abi::Abi::Scalar(..)) {
return (FlatSet::Top, FlatSet::Top);
}
let arg_scalar = const_arg.to_scalar();
let Ok(arg_scalar) = arg_scalar.try_to_int() else {
return (FlatSet::Top, FlatSet::Top);
};
let Ok(arg_value) = arg_scalar.to_bits(layout.size) else {
return (FlatSet::Top, FlatSet::Top);
};
match op {
BinOp::BitAnd if arg_value == 0 => (FlatSet::Elem(arg_scalar), FlatSet::Bottom),
BinOp::BitOr
if arg_value == layout.size.truncate(u128::MAX)
|| (layout.ty.is_bool() && arg_value == 1) =>
{
(FlatSet::Elem(arg_scalar), FlatSet::Bottom)
}
BinOp::Mul if layout.ty.is_integral() && arg_value == 0 => {
(FlatSet::Elem(arg_scalar), FlatSet::Elem(false))
}
_ => (FlatSet::Top, FlatSet::Top),
}
}
(FlatSet::Top, FlatSet::Top) => (FlatSet::Top, FlatSet::Top),
}
}

10
tests/mir-opt/dataflow-const-prop/boolean_identities.rs Normal file
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@ -0,0 +1,10 @@
// unit-test: DataflowConstProp
// EMIT_MIR boolean_identities.test.DataflowConstProp.diff
pub fn test(x: bool, y: bool) -> bool {
(y | true) & (x & false)
}
fn main() {
test(true, false);
}

33
tests/mir-opt/dataflow-const-prop/boolean_identities.test.DataflowConstProp.diff Normal file
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@ -0,0 +1,33 @@
- // MIR for `test` before DataflowConstProp
+ // MIR for `test` after DataflowConstProp
fn test(_1: bool, _2: bool) -> bool {
debug x => _1;
debug y => _2;
let mut _0: bool;
let mut _3: bool;
let mut _4: bool;
let mut _5: bool;
let mut _6: bool;
bb0: {
StorageLive(_3);
StorageLive(_4);
_4 = _2;
- _3 = BitOr(move _4, const true);
+ _3 = const true;
StorageDead(_4);
StorageLive(_5);
StorageLive(_6);
_6 = _1;
- _5 = BitAnd(move _6, const false);
+ _5 = const false;
StorageDead(_6);
- _0 = BitAnd(move _3, move _5);
+ _0 = const false;
StorageDead(_5);
StorageDead(_3);
return;
}
}

10
tests/mir-opt/dataflow-const-prop/mult_by_zero.rs Normal file
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@ -0,0 +1,10 @@
// unit-test: DataflowConstProp
// EMIT_MIR mult_by_zero.test.DataflowConstProp.diff
fn test(x : i32) -> i32 {
x * 0
}
fn main() {
test(10);
}

18
tests/mir-opt/dataflow-const-prop/mult_by_zero.test.DataflowConstProp.diff Normal file
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@ -0,0 +1,18 @@
- // MIR for `test` before DataflowConstProp
+ // MIR for `test` after DataflowConstProp
fn test(_1: i32) -> i32 {
debug x => _1;
let mut _0: i32;
let mut _2: i32;
bb0: {
StorageLive(_2);
_2 = _1;
- _0 = Mul(move _2, const 0_i32);
+ _0 = const 0_i32;
StorageDead(_2);
return;
}
}