[compiler] Fix for scopes with unreachable fallthroughs #34335
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Fixes #34108. If a scope ends with with a conditional where some/all branches exit via labeled break, we currently compile in a way that works but bypasses memoization. We end up with a shape like ```js let t0; label: { if (changed) { ... if (cond) { t0 = ...; break label; } // we don't save the output if the break happens! t0 = ...; $[0] = t0; } else { t0 = $[0]; } ``` The fix here is to update AlignReactiveScopesToBlockScopes to take account of breaks that don't go to the natural fallthrough. In this case, we take any active scopes and extend them to start at least as early as the label, and extend at least to the label fallthrough. Thus we produce the correct: ```js let t0; if (changed) { label: { ... if (cond) { t0 = ...; break label; } t0 = ...; } // now the break jumps here, and we cache the value $[0] = t0; } else { t0 = $[0]; } ```
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…nctions (#34343) `@enablePreserveExistingMemoizationGuarantees` mode currently does not guarantee memoization of primitive-returning functions. We're often able to infer that a function returns a primitive based on how its result is used, for example `foo() + 1` or `object[getIndex()]`, and by default we do not currently memoize computation that produces a primitive. The reasoning behind this is that the compiler is primarily focused on stopping cascading updates — it's fine to recompute a primitive since we can cheaply compare that primitive and avoid unnecessary downstream recomputation. But we've gotten a lot of feedback that people find this surprising, and that sometimes the computation can be expensive enough that it should be memoized. This PR changes `@enablePreserveExistingMemoizationGuarantees` mode to ensure that primitive-returning functions get memoized. Other modes will not memoize these functions. Separately from this we are considering enabling this mode by default. --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/react/pull/34343). * #34347 * #34346 * __->__ #34343 * #34335
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Fixes #34108. If a scope ends with with a conditional where some/all branches exit via labeled break, we currently compile in a way that works but bypasses memoization. We end up with a shape like ```js let t0; label: { if (changed) { ... if (cond) { t0 = ...; break label; } // we don't save the output if the break happens! t0 = ...; $[0] = t0; } else { t0 = $[0]; } ``` The fix here is to update AlignReactiveScopesToBlockScopes to take account of breaks that don't go to the natural fallthrough. In this case, we take any active scopes and extend them to start at least as early as the label, and extend at least to the label fallthrough. Thus we produce the correct: ```js let t0; if (changed) { label: { ... if (cond) { t0 = ...; break label; } t0 = ...; } // now the break jumps here, and we cache the value $[0] = t0; } else { t0 = $[0]; } ``` --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/react/pull/34335). * #34347 * #34346 * #34343 * __->__ #34335 DiffTrain build for [5d64f74](5d64f74)
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Fixes #34108. If a scope ends with with a conditional where some/all branches exit via labeled break, we currently compile in a way that works but bypasses memoization. We end up with a shape like ```js let t0; label: { if (changed) { ... if (cond) { t0 = ...; break label; } // we don't save the output if the break happens! t0 = ...; $[0] = t0; } else { t0 = $[0]; } ``` The fix here is to update AlignReactiveScopesToBlockScopes to take account of breaks that don't go to the natural fallthrough. In this case, we take any active scopes and extend them to start at least as early as the label, and extend at least to the label fallthrough. Thus we produce the correct: ```js let t0; if (changed) { label: { ... if (cond) { t0 = ...; break label; } t0 = ...; } // now the break jumps here, and we cache the value $[0] = t0; } else { t0 = $[0]; } ``` --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/react/pull/34335). * #34347 * #34346 * #34343 * __->__ #34335 DiffTrain build for [5d64f74](5d64f74)
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…nctions (#34343) `@enablePreserveExistingMemoizationGuarantees` mode currently does not guarantee memoization of primitive-returning functions. We're often able to infer that a function returns a primitive based on how its result is used, for example `foo() + 1` or `object[getIndex()]`, and by default we do not currently memoize computation that produces a primitive. The reasoning behind this is that the compiler is primarily focused on stopping cascading updates — it's fine to recompute a primitive since we can cheaply compare that primitive and avoid unnecessary downstream recomputation. But we've gotten a lot of feedback that people find this surprising, and that sometimes the computation can be expensive enough that it should be memoized. This PR changes `@enablePreserveExistingMemoizationGuarantees` mode to ensure that primitive-returning functions get memoized. Other modes will not memoize these functions. Separately from this we are considering enabling this mode by default. --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/react/pull/34343). * #34347 * #34346 * __->__ #34343 * #34335 DiffTrain build for [735e9ac](735e9ac)
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Fixes facebook#34108. If a scope ends with with a conditional where some/all branches exit via labeled break, we currently compile in a way that works but bypasses memoization. We end up with a shape like ```js let t0; label: { if (changed) { ... if (cond) { t0 = ...; break label; } // we don't save the output if the break happens! t0 = ...; $[0] = t0; } else { t0 = $[0]; } ``` The fix here is to update AlignReactiveScopesToBlockScopes to take account of breaks that don't go to the natural fallthrough. In this case, we take any active scopes and extend them to start at least as early as the label, and extend at least to the label fallthrough. Thus we produce the correct: ```js let t0; if (changed) { label: { ... if (cond) { t0 = ...; break label; } t0 = ...; } // now the break jumps here, and we cache the value $[0] = t0; } else { t0 = $[0]; } ``` --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/react/pull/34335). * facebook#34347 * facebook#34346 * facebook#34343 * __->__ facebook#34335
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…nctions (facebook#34343) `@enablePreserveExistingMemoizationGuarantees` mode currently does not guarantee memoization of primitive-returning functions. We're often able to infer that a function returns a primitive based on how its result is used, for example `foo() + 1` or `object[getIndex()]`, and by default we do not currently memoize computation that produces a primitive. The reasoning behind this is that the compiler is primarily focused on stopping cascading updates — it's fine to recompute a primitive since we can cheaply compare that primitive and avoid unnecessary downstream recomputation. But we've gotten a lot of feedback that people find this surprising, and that sometimes the computation can be expensive enough that it should be memoized. This PR changes `@enablePreserveExistingMemoizationGuarantees` mode to ensure that primitive-returning functions get memoized. Other modes will not memoize these functions. Separately from this we are considering enabling this mode by default. --- [//]: # (BEGIN SAPLING FOOTER) Stack created with [Sapling](https://sapling-scm.com). Best reviewed with [ReviewStack](https://reviewstack.dev/facebook/react/pull/34343). * facebook#34347 * facebook#34346 * __->__ facebook#34343 * facebook#34335
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Fixes #34108. If a scope ends with with a conditional where some/all branches exit via labeled break, we currently compile in a way that works but bypasses memoization. We end up with a shape like
The fix here is to update AlignReactiveScopesToBlockScopes to take account of breaks that don't go to the natural fallthrough. In this case, we take any active scopes and extend them to start at least as early as the label, and extend at least to the label fallthrough. Thus we produce the correct:
Stack created with Sapling. Best reviewed with ReviewStack.