coverage: Eliminate more counters by giving them to unreachable nodes #136957
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When preparing a function's coverage counters and metadata during codegen, any part of the original coverage graph that was removed by MIR optimizations can be treated as having an execution count of zero. Somewhat counter-intuitively, if we give those unreachable nodes a _higher_ priority for receiving physical counters (instead of counter expressions), that ends up reducing the total number of physical counters needed. This works because if a node is unreachable, we don't actually create a physical counter for it. Instead that node gets a fixed zero counter, and any other node that would have relied on that physical counter in its counter expression can just ignore that term completely.
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Some changes occurred to MIR optimizations cc @rust-lang/wg-mir-opt |
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I was originally planning to combine this with some larger changes to |
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coverage: Eliminate more counters by giving them to unreachable nodes When preparing a function's coverage counters and metadata during codegen, any part of the original coverage graph that was removed by MIR optimizations can be treated as having an execution count of zero. Somewhat counter-intuitively, if we give those unreachable nodes a _higher_ priority for receiving physical counters (instead of counter expressions), that ends up reducing the total number of physical counters needed. This works because if a node is unreachable, we don't actually create a physical counter for it. Instead that node gets a fixed zero counter, and any other node that would have relied on that physical counter in its counter expression can just ignore that term completely.
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…kingjubilee Rollup of 11 pull requests Successful merges: - rust-lang#136863 (rework rigid alias handling ) - rust-lang#136869 (Fix diagnostic when using = instead of : in let binding) - rust-lang#136895 (debuginfo: Set bitwidth appropriately in enum variant tags) - rust-lang#136928 (eagerly prove WF when resolving fully qualified paths) - rust-lang#136941 (Move `llvm.ccache` to `build.ccache`) - rust-lang#136950 (rustdoc: use better, consistent SVG icons for scraped examples) - rust-lang#136957 (coverage: Eliminate more counters by giving them to unreachable nodes) - rust-lang#136960 (Compiletest should not inherit all host RUSTFLAGS) - rust-lang#136962 (unify LLVM version finding logic) - rust-lang#136970 (ci: move `x86_64-gnu-debug` job to the free runner) - rust-lang#136973 (Fix `x test --stage 1 ui-fulldeps` on macOS (until the next beta bump)) r? `@ghost` `@rustbot` modify labels: rollup
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Rollup merge of rust-lang#136957 - Zalathar:counters, r=oli-obk coverage: Eliminate more counters by giving them to unreachable nodes When preparing a function's coverage counters and metadata during codegen, any part of the original coverage graph that was removed by MIR optimizations can be treated as having an execution count of zero. Somewhat counter-intuitively, if we give those unreachable nodes a _higher_ priority for receiving physical counters (instead of counter expressions), that ends up reducing the total number of physical counters needed. This works because if a node is unreachable, we don't actually create a physical counter for it. Instead that node gets a fixed zero counter, and any other node that would have relied on that physical counter in its counter expression can just ignore that term completely.
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When preparing a function's coverage counters and metadata during codegen, any part of the original coverage graph that was removed by MIR optimizations can be treated as having an execution count of zero.
Somewhat counter-intuitively, if we give those unreachable nodes a higher priority for receiving physical counters (instead of counter expressions), that ends up reducing the total number of physical counters needed.
This works because if a node is unreachable, we don't actually create a physical counter for it. Instead that node gets a fixed zero counter, and any other node that would have relied on that physical counter in its counter expression can just ignore that term completely.