[Branch Hinting] Add branch hint handling in RemoveUnusedBrs #7706
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| /* | ||
| * Copyright 2025 WebAssembly Community Group participants | ||
| * | ||
| * Licensed under the Apache License, Version 2.0 (the "License"); | ||
| * you may not use this file except in compliance with the License. | ||
| * You may obtain a copy of the License at | ||
| * | ||
| * http://www.apache.org/licenses/LICENSE-2.0 | ||
| * | ||
| * Unless required by applicable law or agreed to in writing, software | ||
| * distributed under the License is distributed on an "AS IS" BASIS, | ||
| * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | ||
| * See the License for the specific language governing permissions and | ||
| * limitations under the License. | ||
| */ | ||
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| #ifndef wasm_ir_branch_hint_h | ||
| #define wasm_ir_branch_hint_h | ||
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| #include "wasm.h" | ||
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| // | ||
| // Branch hint utilities to get them, set, flip, etc. | ||
| // | ||
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| namespace wasm::BranchHints { | ||
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| // Get the branch hint for an expression. | ||
| inline std::optional<bool> get(Expression* expr, Function* func) { | ||
| auto iter = func->codeAnnotations.find(expr); | ||
| if (iter == func->codeAnnotations.end()) { | ||
| // No annotations at all. | ||
| return {}; | ||
| } | ||
| return iter->second.branchLikely; | ||
| } | ||
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| // Set the branch hint for an expression, trampling anything existing before. | ||
| inline void set(Expression* expr, std::optional<bool> likely, Function* func) { | ||
| // When we are writing an empty hint, do not create an empty annotation if one | ||
| // did not exist. | ||
| if (!likely && !func->codeAnnotations.count(expr)) { | ||
| return; | ||
| } | ||
| func->codeAnnotations[expr].branchLikely = likely; | ||
| } | ||
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| // Clear the branch hint for an expression. | ||
| inline void clear(Expression* expr, Function* func) { | ||
| // Do not create an empty annotation if one did not exist. | ||
| auto iter = func->codeAnnotations.find(expr); | ||
| if (iter == func->codeAnnotations.end()) { | ||
| return; | ||
| } | ||
| iter->second.branchLikely = {}; | ||
| } | ||
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| // Copy the branch hint for an expression to another, trampling anything | ||
| // existing before for the latter. | ||
| inline void copyTo(Expression* from, Expression* to, Function* func) { | ||
| auto fromLikely = get(from, func); | ||
| set(to, fromLikely, func); | ||
| } | ||
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| // Flip the branch hint for an expression (if it exists). | ||
| inline void flip(Expression* expr, Function* func) { | ||
| if (auto likely = get(expr, func)) { | ||
| set(expr, !*likely, func); | ||
| } | ||
| } | ||
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| // Copy the branch hint for an expression to another, flipping it while we do | ||
| // so. | ||
| inline void copyFlippedTo(Expression* from, Expression* to, Function* func) { | ||
| copyTo(from, to, func); | ||
| flip(to, func); | ||
| } | ||
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| // Given two expressions to read from, apply the AND hint to a target. That is, | ||
| // the target will be true when both inputs are true. |to| may be equal to | ||
| // |from1| or |from2|. The hint of |to| is trampled. | ||
| inline void applyAndTo(Expression* from1, | ||
| Expression* from2, | ||
| Expression* to, | ||
| Function* func) { | ||
| // If from1 and from2 are both likely, then from1 && from2 is slightly less | ||
| // likely, but we assume our hints are nearly certain, so we apply it. And, | ||
| // converse, if from1 and from2 and both unlikely, then from1 && from2 is even | ||
| // less likely, so we can once more apply a hint. If the hints differ, then | ||
| // one is unlikely or unknown, and we can't say anything about from1 && from2. | ||
| auto from1Hint = BranchHints::get(from1, func); | ||
| auto from2Hint = BranchHints::get(from2, func); | ||
| if (from1Hint == from2Hint) { | ||
| set(to, from1Hint, func); | ||
| } else { | ||
| // The hints do not even match. | ||
| BranchHints::clear(to, func); | ||
| } | ||
| } | ||
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| // As |applyAndTo|, but now the condition on |to| the OR of |from1| and |from2|. | ||
| inline void applyOrTo(Expression* from1, | ||
| Expression* from2, | ||
| Expression* to, | ||
| Function* func) { | ||
| // If one is likely then so is the from1 || from2. If both are unlikely then | ||
| // from1 || from2 is slightly more likely, but we assume our hints are nearly | ||
| // certain, so we apply it. | ||
| auto from1Hint = BranchHints::get(from1, func); | ||
| auto from2Hint = BranchHints::get(from2, func); | ||
| if ((from1Hint && *from1Hint) || (from2Hint && *from2Hint)) { | ||
| set(to, true, func); | ||
| } else if (from1Hint && from2Hint) { | ||
| // We ruled out that either one is present and true, so if both are present, | ||
| // both must be false. | ||
| assert(!*from1Hint && !*from2Hint); | ||
| set(to, false, func); | ||
| } else { | ||
| // We don't know. | ||
| BranchHints::clear(to, func); | ||
| } | ||
| } | ||
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| } // namespace wasm::BranchHints | ||
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| #endif // wasm_ir_branch_hint_h | ||
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From the name and doc comment, I would have expected this to set
toas likely iffrom1is likely orfrom2is likely. I would not have expected it to reason about branch probabilities at all. Can we move this reasoning up into the callers so we don't have such a misleading method name?There was a problem hiding this comment.
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Moving the logic into the callers would require duplicating the large internal comment - it is not trivial how to handle this, so a helper seems natural?
How about
applyCombinedOrConditionTo?