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Rollup merge of #70595 - wesleywiser:remove_unused_discriminant_reads, r=oli-obk

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Remove unused discriminant reads from MIR bodies

Allow the `SimplifyLocals` pass to remove reads of discriminants if the
read is never used.

Fixes #70531

r? @oli-obk
This commit is contained in:
Mazdak Farrokhzad 2020-04-03 00:32:01 +02:00 committed by GitHub
commit 4cba69e585
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GPG Key ID: 4AEE18F83AFDEB23
4 changed files with 71 additions and 20 deletions
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26
src/librustc_mir/transform/simplify.rs
View File

@ -368,18 +368,22 @@ fn visit_local(&mut self, local: &Local, ctx: PlaceContext, location: Location)
if location.statement_index != block.statements.len() {
let stmt = &block.statements[location.statement_index];
if let StatementKind::Assign(box (p, Rvalue::Use(Operand::Constant(c)))) =
&stmt.kind
{
match c.literal.val {
// Keep assignments from unevaluated constants around, since the evaluation
// may report errors, even if the use of the constant is dead code.
ty::ConstKind::Unevaluated(..) => {}
_ => {
if !p.is_indirect() {
trace!("skipping store of const value {:?} to {:?}", c, p);
return;
if let StatementKind::Assign(box (dest, rvalue)) = &stmt.kind {
if !dest.is_indirect() && dest.local == *local {
if let Rvalue::Use(Operand::Constant(c)) = rvalue {
match c.literal.val {
// Keep assignments from unevaluated constants around, since the
// evaluation may report errors, even if the use of the constant
// is dead code.
ty::ConstKind::Unevaluated(..) => {}
_ => {
trace!("skipping store of const value {:?} to {:?}", c, dest);
return;
}
}
} else if let Rvalue::Discriminant(d) = rvalue {
trace!("skipping store of discriminant value {:?} to {:?}", d, dest);
return;
}
}
}

12
src/test/mir-opt/simplify-locals-removes-unused-discriminant-reads.rs Normal file
View File

@ -0,0 +1,12 @@
fn map(x: Option<Box<()>>) -> Option<Box<()>> {
match x {
None => None,
Some(x) => Some(x),
}
}
fn main() {
map(None);
}
// EMIT_MIR rustc.map.SimplifyLocals.diff

37
src/test/mir-opt/simplify-locals-removes-unused-discriminant-reads/rustc.map.SimplifyLocals.diff Normal file
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@ -0,0 +1,37 @@
- // MIR for `map` before SimplifyLocals
+ // MIR for `map` after SimplifyLocals
fn map(_1: std::option::Option<std::boxed::Box<()>>) -> std::option::Option<std::boxed::Box<()>> {
debug x => _1; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:1:8: 1:9
let mut _0: std::option::Option<std::boxed::Box<()>>; // return place in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:1:31: 1:46
let mut _2: isize; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:3:9: 3:13
let _3: std::boxed::Box<()>; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:4:14: 4:15
- let mut _4: std::boxed::Box<()>; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:4:25: 4:26
- let mut _5: isize; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:6:1: 6:2
- let mut _6: isize; // in scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:6:1: 6:2
scope 1 {
debug x => _3; // in scope 1 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:4:14: 4:15
}
bb0: {
_2 = discriminant(_1); // bb0[0]: scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:3:9: 3:13
switchInt(move _2) -> [0isize: bb2, otherwise: bb1]; // bb0[1]: scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:3:9: 3:13
}
bb1: {
_0 = move _1; // bb1[0]: scope 1 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:4:20: 4:27
goto -> bb3; // bb1[1]: scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:2:5: 5:6
}
bb2: {
discriminant(_0) = 0; // bb2[0]: scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:3:17: 3:21
goto -> bb3; // bb2[1]: scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:2:5: 5:6
}
bb3: {
- _5 = discriminant(_1); // bb3[0]: scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:6:1: 6:2
- return; // bb3[1]: scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:6:2: 6:2
+ return; // bb3[0]: scope 0 at $DIR/simplify-locals-removes-unused-discriminant-reads.rs:6:2: 6:2
}
}

16
src/test/mir-opt/simplify_try.rs
View File

@ -183,25 +183,24 @@ fn main() {
// fn try_identity(_1: std::result::Result<u32, i32>) -> std::result::Result<u32, i32> {
// debug x => _1;
// let mut _0: std::result::Result<u32, i32>;
// let mut _2: isize;
// let _3: i32;
// let _4: u32;
// let _2: i32;
// let _3: u32;
// scope 1 {
// debug y => _4;
// debug y => _3;
// }
// scope 2 {
// debug err => _3;
// debug err => _2;
// scope 3 {
// scope 7 {
// debug t => _3;
// debug t => _2;
// }
// scope 8 {
// debug v => _3;
// debug v => _2;
// }
// }
// }
// scope 4 {
// debug val => _4;
// debug val => _3;
// scope 5 {
// }
// }
@ -209,7 +208,6 @@ fn main() {
// debug self => _1;
// }
// bb0: {
// _2 = discriminant(_1);
// _0 = move _1;
// return;
// }