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Start block is not allowed to have basic block predecessors#88968

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tmiasko:start-block-no-predecessors
Sep 19, 2021
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Start block is not allowed to have basic block predecessors#88968
bors merged 4 commits intorust-lang:masterfrom
tmiasko:start-block-no-predecessors

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@tmiasko tmiasko commented Sep 15, 2021

  • The MIR validator is extended to detect potential violations.
  • The start block has no predecessors after building MIR, so no changes are required there.
  • The SimplifyCfg could previously violate this requirement when collapsing goto chains, so transformation is disabled for the start block, which also substantially simplifies the implementation.
  • The LLVM function entry block also must not have basic block predecessors. Previously, to ensure that code generation had to perform necessary adjustments. This now became unnecessary.

The motivation behind the change is to align with analogous requirement in LLVM, and to avoid potential latent bugs like the one reported in #88043.

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rust-highfive commented Sep 15, 2021

r? @davidtwco

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@rust-highfive rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 15, 2021
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davidtwco approved these changes Sep 15, 2021
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LGTM!

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davidtwco commented Sep 15, 2021

@bors r+

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bors commented Sep 15, 2021

📌 Commit a923975d2cf29c8a73da2334e35f12338367cd9a has been approved by davidtwco

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 15, 2021
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Mark-Simulacrum commented Sep 15, 2021

@bors rollup=never -- codegen/MIR changes, may be perf sensitive

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bors commented Sep 18, 2021

⌛ Testing commit a923975d2cf29c8a73da2334e35f12338367cd9a with merge 453ff06ca7e1d2f4764f6e383cdea63bd24a3f46...

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bors commented Sep 18, 2021

💔 Test failed - checks-actions

@bors bors added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Sep 18, 2021
@tmiasko
Start block is not allowed to have basic block predecessors
5118dd5
@tmiasko
Do not collapse goto chains beginning with the start block
4d614e1 
If any block on a goto chain has more than one predecessor, then the new
start block would have basic block predecessors.

Skip the transformation for the start block altogether, to avoid
violating the new invariant that the start block does not have any basic
block predecessors.
@tmiasko
Remove support for reentrant start blocks from codegen
3489ba3 
The start block is guaranteed not to have any basic block predecessors.
@tmiasko
./x.py test --bless
f1f1c8f
@tmiasko tmiasko force-pushed the start-block-no-predecessors branch from a923975 to f1f1c8f Compare September 18, 2021 06:09
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tmiasko commented Sep 18, 2021

Rebased and reblessed mir opt coverage tests.

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jackh726 commented Sep 18, 2021

@bors r=davidtwco

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bors commented Sep 18, 2021

📌 Commit f1f1c8f has been approved by davidtwco

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 18, 2021
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bors commented Sep 19, 2021

⌛ Testing commit f1f1c8f with merge 10967a1...

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bors commented Sep 19, 2021

☀️ Test successful - checks-actions
Approved by: davidtwco
Pushing 10967a1 to master...

@bors bors added the merged-by-bors This PR was explicitly merged by bors. label Sep 19, 2021
@bors bors merged commit 10967a1 into rust-lang:master Sep 19, 2021
@rustbot rustbot added this to the 1.57.0 milestone Sep 19, 2021
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rust-timer commented Sep 19, 2021

Finished benchmarking commit (10967a1): comparison url.

Summary: This benchmark run did not return any relevant changes.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

@tmiasko tmiasko deleted the start-block-no-predecessors branch September 19, 2021 12:53
@tmiasko tmiasko mentioned this pull request Mar 5, 2025
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