Work around loop identification issues with CBMC #2181
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CBMC has a heuristic to identify loops that identifies any jump to a previously declared basic block as the back edge of a loop. For rust, this happens often due to drop elaboration. See https://rustc-dev-guide.rust-lang.org/mir/drop-elaboration.html#drop-elaboration-1 for more details. To avoid that issue, we now codegen basic blocks in topological order.
tedinski
approved these changes
Feb 2, 2023
| @@ -88,7 +89,7 @@ impl<'tcx> GotocCtx<'tcx> { | |||
| self.codegen_function_prelude(); | |||
| self.codegen_declare_variables(); | |||
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| mir.basic_blocks.iter_enumerated().for_each(|(bb, bbd)| self.codegen_block(bb, bbd)); | |||
| reverse_postorder(mir).for_each(|(bb, bbd)| self.codegen_block(bb, bbd)); | |||
zhassan-aws
approved these changes
Feb 2, 2023
I don't see a reason to remove this. I don't think there is any cost here, since rustc caches this information. Even if we have to rerun this, it will be a simple DFS on the basic blocks. On the plus side, besides working around the CBMC issue, it tends to simplify the CFG for the code we generate which makes it easier to debug later. |
adpaco-aws
reviewed
Feb 2, 2023
It looks like CBMC is no longer able to identify the loop bound and requires an unwind annotation for this test. This is a more reasonable behavior than the one this PR fixes, so I'm going ahead and just updating this harness.
It only fails on linux now, so for the fixme test, I am forcing a failure on MacOS. For the regular test, we just skip it for Linux.
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Description of changes:
CBMC has a heuristic to identify loops that identifies any jump to a previously declared basic block as the back edge of a loop.
For rust, this happens often due to drop elaboration. See https://rustc-dev-guide.rust-lang.org/mir/drop-elaboration.html#drop-elaboration-1 for more details.
To avoid that issue, we now codegen basic blocks in topological order.
Resolved issues:
Resolves #2164
Related RFC:
Call-outs:
Testing:
How is this change tested? New tests
Is this a refactor change? No
Checklist
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.