Cranelift: Rewrite conditional branches with constant conditions into unconditional jumps#13267
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fitzgen wants to merge 4 commits intobytecodealliance:mainfrom
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Cranelift: Rewrite conditional branches with constant conditions into unconditional jumps#13267fitzgen wants to merge 4 commits intobytecodealliance:mainfrom
fitzgen wants to merge 4 commits intobytecodealliance:mainfrom
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… unconditional jumps This requires two tweaks to the egraph pass's machinery: 1. We maintain a set of reachable blocks. This is initialized to the entry block. After optimizing a block's intructions, we look at the terminator and mark any other block it branches to as reachable. When visiting blocks to optimize their instructions, we skip unreachable blocks and remove them and their instructions from the function's layout. 2. We always visit blocks in reverse-post order, rather than dominator-tree order (which is related but slightly weaker). This ensures that we always visit predecessors before successors, and therefore that if a block is not marked reachable, then it really is unreachable and is safe to remove. See code comments for the details of when dominator-tree order breaks down.
Lowering expects them
cfallin
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May 4, 2026
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Nice -- I'm happy to see how this is actually pretty straightforward given all of the primitives that we have! The RPO traversal change makes sense; just a comment below on how we describe/justify it. And a small request for testing. Otherwise LGTM!
cfallin
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And include a proof of why the new traversal is correct.
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This requires two tweaks to the egraph pass's machinery:
We maintain a set of reachable blocks. This is initialized to the entry block. After optimizing a block's intructions, we look at the terminator and mark any other block it branches to as reachable. When visiting blocks to optimize their instructions, we skip unreachable blocks and remove them and their instructions from the function's layout.
We always visit blocks in reverse-post order, rather than dominator-tree order (which is related but slightly weaker). This ensures that we always visit predecessors before successors, and therefore that if a block is not marked reachable, then it really is unreachable and is safe to remove. See code comments for the details of when dominator-tree order breaks down.