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[experiment] Try using a very simple heuristic for the inliner #68795

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[experiment] Try using a very simple heuristic for the inliner #68795

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[experiment] Try using a very simple heuristic for the inliner
wesleywiser Feb 3, 2020
cf0876f
tweak suggested by eddyb
wesleywiser Feb 4, 2020
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21 changes: 19 additions & 2 deletions src/librustc_mir/transform/inline.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -39,15 +39,19 @@ struct CallSite<'tcx> {

impl<'tcx> MirPass<'tcx> for Inline {
fn run_pass(&self, tcx: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>) {
if tcx.sess.opts.debugging_opts.mir_opt_level >= 2 {
Inliner { tcx, source }.run_pass(body);
let mir_opt_level = tcx.sess.opts.debugging_opts.mir_opt_level;
if mir_opt_level == 0 {
return;
}

Inliner { tcx, source, use_simple_heuristic: mir_opt_level == 1 }.run_pass(body);
}
}

struct Inliner<'tcx> {
tcx: TyCtxt<'tcx>,
source: MirSource<'tcx>,
use_simple_heuristic: bool,
}

impl Inliner<'tcx> {
Expand Down Expand Up @@ -251,6 +255,19 @@ impl Inliner<'tcx> {
}
}

if self.use_simple_heuristic {
use rustc::mir::StatementKind::*;

return callee_body.basic_blocks().len() == 1
&& callee_body.basic_blocks()[BasicBlock::from_u32(0)]
.statements
.iter()
.filter(|stmt| !matches!(stmt.kind, StorageLive(_) | StorageDead(_)))
.take(5)
.count()
<= 4;
}

let mut threshold = if hinted { HINT_THRESHOLD } else { DEFAULT_THRESHOLD };

// Significantly lower the threshold for inlining cold functions
Expand Down