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Auto merge of rust-lang#105119 - JakobDegen:inline-experiments, r=cjg…
…illot Disable top down MIR inlining The current MIR inliner has exponential behavior in some cases: <https://godbolt.org/z/7jnWah4fE>. The cause of this is top-down inlining, where we repeatedly do inlining like `call_a() => { call_b(); call_b(); }`. Each decision on its own seems to make sense, but the result is exponential. Disabling top-down inlining fundamentally prevents this. Each call site in the original, unoptimized source code is now considered for inlining exactly one time, which means that the total growth in MIR size is limited to number of call sites * inlining threshold. Top down inlining may be worth re-introducing at some point, but it needs to be accompanied with a principled way to prevent this kind of behavior.
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src/test/mir-opt/inline/exponential_runtime.main.Inline.diff
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,50 @@ | ||
| - // MIR for `main` before Inline | ||
| + // MIR for `main` after Inline | ||
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|
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| fn main() -> () { | ||
| let mut _0: (); // return place in scope 0 at $DIR/exponential_runtime.rs:+0:11: +0:11 | ||
| let _1: (); // in scope 0 at $DIR/exponential_runtime.rs:+1:5: +1:22 | ||
| + scope 1 (inlined <() as G>::call) { // at $DIR/exponential_runtime.rs:86:5: 86:22 | ||
| + let _2: (); // in scope 1 at $DIR/exponential_runtime.rs:73:9: 73:25 | ||
| + let _3: (); // in scope 1 at $DIR/exponential_runtime.rs:74:9: 74:25 | ||
| + let _4: (); // in scope 1 at $DIR/exponential_runtime.rs:75:9: 75:25 | ||
| + } | ||
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|
||
| bb0: { | ||
| StorageLive(_1); // scope 0 at $DIR/exponential_runtime.rs:+1:5: +1:22 | ||
| - _1 = <() as G>::call() -> bb1; // scope 0 at $DIR/exponential_runtime.rs:+1:5: +1:22 | ||
| + StorageLive(_2); // scope 1 at $DIR/exponential_runtime.rs:73:9: 73:25 | ||
| + _2 = <() as F>::call() -> bb1; // scope 1 at $DIR/exponential_runtime.rs:73:9: 73:25 | ||
| // mir::Constant | ||
| - // + span: $DIR/exponential_runtime.rs:86:5: 86:20 | ||
| - // + literal: Const { ty: fn() {<() as G>::call}, val: Value(<ZST>) } | ||
| + // + span: $DIR/exponential_runtime.rs:73:9: 73:23 | ||
| + // + literal: Const { ty: fn() {<() as F>::call}, val: Value(<ZST>) } | ||
| } | ||
|
|
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| bb1: { | ||
| + StorageDead(_2); // scope 1 at $DIR/exponential_runtime.rs:73:25: 73:26 | ||
| + StorageLive(_3); // scope 1 at $DIR/exponential_runtime.rs:74:9: 74:25 | ||
| + _3 = <() as F>::call() -> bb2; // scope 1 at $DIR/exponential_runtime.rs:74:9: 74:25 | ||
| + // mir::Constant | ||
| + // + span: $DIR/exponential_runtime.rs:74:9: 74:23 | ||
| + // + literal: Const { ty: fn() {<() as F>::call}, val: Value(<ZST>) } | ||
| + } | ||
| + | ||
| + bb2: { | ||
| + StorageDead(_3); // scope 1 at $DIR/exponential_runtime.rs:74:25: 74:26 | ||
| + StorageLive(_4); // scope 1 at $DIR/exponential_runtime.rs:75:9: 75:25 | ||
| + _4 = <() as F>::call() -> bb3; // scope 1 at $DIR/exponential_runtime.rs:75:9: 75:25 | ||
| + // mir::Constant | ||
| + // + span: $DIR/exponential_runtime.rs:75:9: 75:23 | ||
| + // + literal: Const { ty: fn() {<() as F>::call}, val: Value(<ZST>) } | ||
| + } | ||
| + | ||
| + bb3: { | ||
| + StorageDead(_4); // scope 1 at $DIR/exponential_runtime.rs:75:25: 75:26 | ||
| StorageDead(_1); // scope 0 at $DIR/exponential_runtime.rs:+1:22: +1:23 | ||
| _0 = const (); // scope 0 at $DIR/exponential_runtime.rs:+0:11: +2:2 | ||
| return; // scope 0 at $DIR/exponential_runtime.rs:+2:2: +2:2 | ||
| } | ||
| } | ||
|
|
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,87 @@ | ||
| // Checks that code with exponential runtime does not have exponential behavior in inlining. | ||
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| trait A { | ||
| fn call(); | ||
| } | ||
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| trait B { | ||
| fn call(); | ||
| } | ||
| impl<T: A> B for T { | ||
| #[inline] | ||
| fn call() { | ||
| <T as A>::call(); | ||
| <T as A>::call(); | ||
| <T as A>::call(); | ||
| } | ||
| } | ||
|
|
||
| trait C { | ||
| fn call(); | ||
| } | ||
| impl<T: B> C for T { | ||
| #[inline] | ||
| fn call() { | ||
| <T as B>::call(); | ||
| <T as B>::call(); | ||
| <T as B>::call(); | ||
| } | ||
| } | ||
|
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| trait D { | ||
| fn call(); | ||
| } | ||
| impl<T: C> D for T { | ||
| #[inline] | ||
| fn call() { | ||
| <T as C>::call(); | ||
| <T as C>::call(); | ||
| <T as C>::call(); | ||
| } | ||
| } | ||
|
|
||
| trait E { | ||
| fn call(); | ||
| } | ||
| impl<T: D> E for T { | ||
| #[inline] | ||
| fn call() { | ||
| <T as D>::call(); | ||
| <T as D>::call(); | ||
| <T as D>::call(); | ||
| } | ||
| } | ||
|
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| trait F { | ||
| fn call(); | ||
| } | ||
| impl<T: E> F for T { | ||
| #[inline] | ||
| fn call() { | ||
| <T as E>::call(); | ||
| <T as E>::call(); | ||
| <T as E>::call(); | ||
| } | ||
| } | ||
|
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| trait G { | ||
| fn call(); | ||
| } | ||
| impl<T: F> G for T { | ||
| #[inline] | ||
| fn call() { | ||
| <T as F>::call(); | ||
| <T as F>::call(); | ||
| <T as F>::call(); | ||
| } | ||
| } | ||
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| impl A for () { | ||
| #[inline(never)] | ||
| fn call() {} | ||
| } | ||
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| // EMIT_MIR exponential_runtime.main.Inline.diff | ||
| fn main() { | ||
| <() as G>::call(); | ||
| } |
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