[MIR trans] Optimize trans for biased switches #33566
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r? @arielb1 (rust_highfive has picked a reviewer for you, use r? to override) |
| // If a single target basic blocks is predominant, promote that to be the | ||
| // default case for the switch instruction to reduce the size of the generated | ||
| // code. This is especially helpful in cases like an if-let on a huge enum. | ||
| Some((&&bb, &c)) if c as f32 / targets.len() as f32 >= 0.05 => { |
| // instruction so LLVM knows that | ||
| let unreachable_blk = self.unreachable_block(); | ||
| let switch = bcx.switch(discr, unreachable_blk.llbb, targets.len()); | ||
| let mut bb_hist = FnvHashMap(); |
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You should add a FIXME here to check for the validity of such transformation. While currently the otherwise branch is always unreachable (and contains a panic), this might not be the case after a bunch of optimisation passes are added to the MIR thus making this transformation invalid.
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TerminatorKind::Switch can't have an otherwise branch in any case.
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There's also the assert_eq!(adt_def.variants.len(), targets.len()); which should in theory make sure we've covered all the cases.
Currently, all switches in MIR are exhausitive, meaning that we can have a lot of arms that all go to the same basic block, the extreme case being an if-let expression which results in just 2 possible cases, be might end up with hundreds of arms for large enums. To improve this situation and give LLVM less code to chew on, we can detect whether there's a pre-dominant target basic block in a switch and then promote this to be the default target, not translating the corresponding arms at all. In combination with rust-lang#33544 this makes unoptimized MIR trans of nickel.rs as fast as using old trans and greatly improves the times for optimized builds, which are only 30-40% slower instead of ~300%. cc rust-lang#33111
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I guess this is fine to land. I’d still prefer it to become a proper MIR transformation down the line but this can be done later. @bors r+ |
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📌 Commit 49b2cdf has been approved by |
Manishearth
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[MIR trans] Optimize trans for biased switches Currently, all switches in MIR are exhausitive, meaning that we can have a lot of arms that all go to the same basic block, the extreme case being an if-let expression which results in just 2 possible cases, be might end up with hundreds of arms for large enums. To improve this situation and give LLVM less code to chew on, we can detect whether there's a pre-dominant target basic block in a switch and then promote this to be the default target, not translating the corresponding arms at all. In combination with rust-lang#33544 this makes unoptimized MIR trans of nickel.rs as fast as using old trans and greatly improves the times for optimized builds, which are only 30-40% slower instead of ~300%. cc rust-lang#33111
eddyb
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[MIR trans] Optimize trans for biased switches Currently, all switches in MIR are exhausitive, meaning that we can have a lot of arms that all go to the same basic block, the extreme case being an if-let expression which results in just 2 possible cases, be might end up with hundreds of arms for large enums. To improve this situation and give LLVM less code to chew on, we can detect whether there's a pre-dominant target basic block in a switch and then promote this to be the default target, not translating the corresponding arms at all. In combination with rust-lang#33544 this makes unoptimized MIR trans of nickel.rs as fast as using old trans and greatly improves the times for optimized builds, which are only 30-40% slower instead of ~300%. cc rust-lang#33111
eddyb
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May 13, 2016
[MIR trans] Optimize trans for biased switches Currently, all switches in MIR are exhausitive, meaning that we can have a lot of arms that all go to the same basic block, the extreme case being an if-let expression which results in just 2 possible cases, be might end up with hundreds of arms for large enums. To improve this situation and give LLVM less code to chew on, we can detect whether there's a pre-dominant target basic block in a switch and then promote this to be the default target, not translating the corresponding arms at all. In combination with rust-lang#33544 this makes unoptimized MIR trans of nickel.rs as fast as using old trans and greatly improves the times for optimized builds, which are only 30-40% slower instead of ~300%. cc rust-lang#33111
Manishearth
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May 14, 2016
[MIR trans] Optimize trans for biased switches Currently, all switches in MIR are exhausitive, meaning that we can have a lot of arms that all go to the same basic block, the extreme case being an if-let expression which results in just 2 possible cases, be might end up with hundreds of arms for large enums. To improve this situation and give LLVM less code to chew on, we can detect whether there's a pre-dominant target basic block in a switch and then promote this to be the default target, not translating the corresponding arms at all. In combination with rust-lang#33544 this makes unoptimized MIR trans of nickel.rs as fast as using old trans and greatly improves the times for optimized builds, which are only 30-40% slower instead of ~300%. cc rust-lang#33111
bors
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Currently, all switches in MIR are exhausitive, meaning that we can have
a lot of arms that all go to the same basic block, the extreme case
being an if-let expression which results in just 2 possible cases, be
might end up with hundreds of arms for large enums.
To improve this situation and give LLVM less code to chew on, we can
detect whether there's a pre-dominant target basic block in a switch
and then promote this to be the default target, not translating the
corresponding arms at all.
In combination with #33544 this makes unoptimized MIR trans of
nickel.rs as fast as using old trans and greatly improves the times for
optimized builds, which are only 30-40% slower instead of ~300%.
cc #33111