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    Syntax-Directed Translation and Intermediate Representations Short Notes for GATE CS

    Syntax-Directed Translation and Intermediate Representations short notes for GATE CS: 5 study cards covering concepts, formulas, shortcuts and exam traps, plu

    syntax directed translation and intermediate representations short notes

    Summary: The Backpatching Cheat Sheet

    Backpatching Cheat Sheet

    Core Functions
    • makelist(i): New list.
    • merge(p1, p2): Combine.
    • backpatch(p, i): Fill blanks.
    Short-Circuit
    • OR: Merge truelists. Backpatch false to .
    • AND: Merge falselists. Backpatch true to .
    Control Structures
    Structure True List False List Special
    if (B) S Start of Nextlist -
    if-else Start of Start of Emit goto after
    while Start of Exit Emit goto begin
    Golden Rule: Execution must never accidentally fall through from one mutually exclusive branch into another. Always emit unconditional jumps to enforce boundaries.

    Summary of Classification and Evaluation

    Classification and Evaluation

    S-attributed means only synthesized attributes, evaluated bottom-up. L-attributed means inherited attributes depend only on the parent or left siblings, evaluated top-down or left-to-right. If an inherited attribute looks right, it is neither.

    Ranker Inspection

    Scan the semantic rules for any inherited attribute that references a non-terminal to its right. If found, the definition is immediately neither S-attributed nor L-attributed. If all inherited attributes reference only the parent or left siblings, it is L-attributed. If there are no inherited attributes at all, it is S-attributed.

    Explain this more simply
    Think of a one-way street system. S-attributed is a system where all streets go up (synthesized). L-attributed is a system where streets go up, or down/sideways, but only to the left. If you find a street going to the right, the system is broken.

    Summary of Evaluation Orders and Traps

    Trap
    Why It Feels Right
    Three-Second Check
    Confusing L-attributed with S-attributed when inherited attributes only depend on parent
    Parent-only inheritance feels like safe downward flow
    Any inherited attribute present means not S-attributed. Period.
    Evaluating inherited attributes from right siblings
    Dependency graph may show a valid topological sort ignoring left-to-right constraint
    L-attributed strictly requires left siblings only. Right-to-left inheritance is invalid.
    Assuming depth-first evaluation always works for L-attributed
    DFS is a standard topological sort that works for S-attributed
    Scan for inherited attribute referencing a right sibling. If found, DFS is dead.
    Explain this more simply
    Think of a relay race. You cannot pass the baton if the next runner is not ready. A cycle is like two runners waiting for each other. The race never starts. Topological sort ensures the runners are lined up in the exact order they will receive the baton.

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    Syntax-Directed Translation and Intermediate Representations Short Notes for GATE CS

    Syntax-Directed Translation and Intermediate Representations short notes for GATE CS: 5 study cards covering concepts, formulas, shortcuts and exam traps, plus solved practice questions.

    Summary: The Backpatching Cheat Sheet

    Backpatching Cheat Sheet

    Core Functions
    • makelist(i): New list.
    • merge(p1, p2): Combine.
    • backpatch(p, i): Fill blanks.
    Short-Circuit
    • OR: Merge truelists. Backpatch false to .
    • AND: Merge falselists. Backpatch true to .
    Control Structures
    Structure True List False List Special
    if (B) S Start of Nextlist -
    if-else Start of Start of Emit goto after
    while Start of Exit Emit goto begin
    Golden Rule: Execution must never accidentally fall through from one mutually exclusive branch into another. Always emit unconditional jumps to enforce boundaries.

    Summary of Classification and Evaluation

    Classification and Evaluation

    S-attributed means only synthesized attributes, evaluated bottom-up. L-attributed means inherited attributes depend only on the parent or left siblings, evaluated top-down or left-to-right. If an inherited attribute looks right, it is neither.

    Ranker Inspection

    Scan the semantic rules for any inherited attribute that references a non-terminal to its right. If found, the definition is immediately neither S-attributed nor L-attributed. If all inherited attributes reference only the parent or left siblings, it is L-attributed. If there are no inherited attributes at all, it is S-attributed.

    Explain this more simply
    Think of a one-way street system. S-attributed is a system where all streets go up (synthesized). L-attributed is a system where streets go up, or down/sideways, but only to the left. If you find a street going to the right, the system is broken.

    Summary of Evaluation Orders and Traps

    Trap
    Why It Feels Right
    Three-Second Check
    Confusing L-attributed with S-attributed when inherited attributes only depend on parent
    Parent-only inheritance feels like safe downward flow
    Any inherited attribute present means not S-attributed. Period.
    Evaluating inherited attributes from right siblings
    Dependency graph may show a valid topological sort ignoring left-to-right constraint
    L-attributed strictly requires left siblings only. Right-to-left inheritance is invalid.
    Assuming depth-first evaluation always works for L-attributed
    DFS is a standard topological sort that works for S-attributed
    Scan for inherited attribute referencing a right sibling. If found, DFS is dead.
    Explain this more simply
    Think of a relay race. You cannot pass the baton if the next runner is not ready. A cycle is like two runners waiting for each other. The race never starts. Topological sort ensures the runners are lined up in the exact order they will receive the baton.

    Summary of Parser Compatibility

    Parser Compatibility

    S-attributed definitions work with any bottom-up parser, like LR. L-attributed definitions work with top-down parsers, like LL, and can be adapted for LR parsers using marker non-terminals or stack manipulation. If the definition is neither, it requires a multi-pass approach or full dependency graph evaluation.

    Ranker Inspection for LR Compatibility

    Verify that all inherited attributes can be computed using only the symbols currently on the parser stack. This strictly enforces the L-attributed left-to-right constraint. If an inherited attribute requires a symbol not yet on the stack, the definition cannot be evaluated during a single bottom-up pass.

    Explain this more simply
    Bottom-up parsers build the tree from the leaves up, so they naturally compute synthesized attributes. Top-down parsers build from the root down, naturally passing inherited attributes. Matching the attribute flow to the parser's construction direction is the key to compatibility.

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