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yusufcanislek
45 lines
2.4 KiB
Markdown
45 lines
2.4 KiB
Markdown
# LLVM API Cheat Sheet for Mergen Passes
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Quick reference for LLVM APIs used in GEPLoadPass, PromotePseudoStackPass,
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ReplaceTruncWithLoadPass, and PromotePseudoMemory.
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1. **`Value::replaceAllUsesWith(Value* newVal)`**
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Replaces every use of this value with `newVal`. Does NOT touch this instruction's own operands.
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*Gotcha*: After RAUW the instruction is dead but still holds operand references — erase it or it leaks.
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2. **`Instruction::eraseFromParent()`**
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Removes from the parent BasicBlock ilist and deallocates. Pointer is invalid after the call.
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*Gotcha*: Asserts `use_empty()`. RAUW first, then erase — never the reverse.
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3. **`Value::use_empty()`**
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Returns true when nothing references this value.
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*Gotcha*: Check AFTER erasing/replacing all users, not before. A stale user makes this return false.
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4. **`GetElementPtrInst::getPointerOperand()`**
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Returns operand 0 — the base pointer. For `gep i8, ptr %memory, i64 <off>` this is `%memory`.
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*Gotcha*: Prefer over `getOperand(getNumOperands()-2)` which breaks on multi-index GEPs.
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5. **`Type::getIntegerBitWidth()`**
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Returns the bit width of an `iN` type.
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*Gotcha*: Asserts if `!isIntegerTy()`. Always guard: `if (Ty->isIntegerTy()) { ... getBitWidth ... }`.
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6. **`Type::isIntegerTy()`**
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Returns true only for `iN` types. False for float, vector, pointer, void, struct, array.
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*Gotcha*: Pointer types are NOT integer types — test before calling `getIntegerBitWidth()` or `computeKnownBits`.
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7. **BasicBlock iteration with erasure**
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Pattern: `for (auto it = BB.begin(); it != BB.end();) { auto* I = &*it++; /* may erase I */ }`
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*Gotcha*: `it++` must advance BEFORE any erase. A range-for (`for (auto &I : BB)`) crashes on erasure.
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8. **`SmallPtrSet<T*, N>`**
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Inline-storage set for pointer deduplication. Use when collecting Instructions for deferred erasure.
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*Gotcha*: Inserting the same pointer twice is safe (returns false), but erasing a pointer twice is use-after-free. The set prevents that.
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9. **`computeKnownBits(Value*, DataLayout&)`**
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Computes known-zero/known-one bit masks via ValueTracking.
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*Gotcha*: Asserts on non-integer, non-pointer types. Guard callers with `isIntegerTy() || isPointerTy()`.
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10. **`PreservedAnalyses::none()` vs `::all()`**
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Return `none()` if ANY IR was modified; `all()` if the pass was a no-op.
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*Gotcha*: Returning `all()` after mutating IR silently poisons cached analyses for downstream passes.
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