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Rollup merge of #108681 - nnethercote:needs_process_obligation-comments, r=lqd
Improve comments in `needs_process_obligation`. And a couple of other places. r? `@lqd`
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commit
37bd50eddd
@ -426,6 +426,7 @@ impl<O: ForestObligation> ObligationForest<O> {
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// nodes. Therefore we use a `while` loop.
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let mut index = 0;
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while let Some(node) = self.nodes.get_mut(index) {
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// This test is extremely hot.
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if node.state.get() != NodeState::Pending
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|| !processor.needs_process_obligation(&node.obligation)
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{
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@ -439,6 +440,7 @@ impl<O: ForestObligation> ObligationForest<O> {
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// out of sync with `nodes`. It's not very common, but it does
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// happen, and code in `compress` has to allow for it.
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// This code is much less hot.
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match processor.process_obligation(&mut node.obligation) {
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ProcessResult::Unchanged => {
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// No change in state.
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@ -212,36 +212,44 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
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/// Identifies whether a predicate obligation needs processing.
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///
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/// This is always inlined, despite its size, because it has a single
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/// callsite and it is called *very* frequently.
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/// This is always inlined because it has a single callsite and it is
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/// called *very* frequently. Be careful modifying this code! Several
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/// compile-time benchmarks are very sensitive to even small changes.
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#[inline(always)]
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fn needs_process_obligation(&self, pending_obligation: &Self::Obligation) -> bool {
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// If we were stalled on some unresolved variables, first check whether
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// any of them have been resolved; if not, don't bother doing more work
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// yet.
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match pending_obligation.stalled_on.len() {
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// Match arms are in order of frequency, which matters because this
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// code is so hot. 1 and 0 dominate; 2+ is fairly rare.
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1 => {
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let infer_var = pending_obligation.stalled_on[0];
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self.selcx.infcx.ty_or_const_infer_var_changed(infer_var)
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}
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0 => {
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// In this case we haven't changed, but wish to make a change.
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true
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}
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_ => {
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// This `for` loop was once a call to `all()`, but this lower-level
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// form was a perf win. See #64545 for details.
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(|| {
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for &infer_var in &pending_obligation.stalled_on {
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if self.selcx.infcx.ty_or_const_infer_var_changed(infer_var) {
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return true;
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}
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let stalled_on = &pending_obligation.stalled_on;
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match stalled_on.len() {
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// This case is the hottest most of the time, being hit up to 99%
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// of the time. `keccak` and `cranelift-codegen-0.82.1` are
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// benchmarks that particularly stress this path.
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1 => self.selcx.infcx.ty_or_const_infer_var_changed(stalled_on[0]),
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// In this case we haven't changed, but wish to make a change. Note
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// that this is a special case, and is not equivalent to the `_`
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// case below, which would return `false` for an empty `stalled_on`
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// vector.
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//
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// This case is usually hit only 1% of the time or less, though it
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// reaches 20% in `wasmparser-0.101.0`.
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0 => true,
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// This case is usually hit only 1% of the time or less, though it
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// reaches 95% in `mime-0.3.16`, 64% in `wast-54.0.0`, and 12% in
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// `inflate-0.4.5`.
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//
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// The obvious way of writing this, with a call to `any()` and no
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// closure, is currently slower than this version.
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_ => (|| {
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for &infer_var in stalled_on {
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if self.selcx.infcx.ty_or_const_infer_var_changed(infer_var) {
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return true;
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}
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false
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})()
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}
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}
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false
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})(),
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}
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}
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