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Loop Invariant Computation and Code Motion

Loop Invariant Computation and Code Motion. Loop-invariant computation Algorithms for code motion Summary: 13.1.2 (global CSE), 13.2. Loop-Invariant Computation and Code Motion. Loop invariant computation A computation whose value does not change as long as control stays within the loop

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Loop Invariant Computation and Code Motion

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  1. Loop Invariant Computation and Code Motion Loop-invariant computation Algorithms for code motion Summary: 13.1.2 (global CSE), 13.2

  2. Loop-Invariant Computation and Code Motion • Loop invariant computation • A computationwhose value does not change as long as control stays within the loop • Loop invariant code motion (LICM) • Move a loop invariant statement within a loop to the pre-header of the loop ?

  3. Example • Which statement is a loop-invariant? • Which statement can we move to the preheader? I I I

  4. LICM Algorithm • Observations • Loop invariant: operands are defined outside loop or are defined by loop invariants • Code motion: not all invariant statements can be moved to the pre-header • Algorithm • Detect loop invariant computations • Check conditions for code motion • Code transformation

  5. Detecting loop invariant computation • Compute reaching definitions • Repeat:markA=B+C as invariant if • All reaching definitions of B are outside of the loop, or there is exactly one reaching definition for B and it is from a loop-invariant statement inside the loop • Check similarly for C • Until no changes to the set of loop-invariant statements occur

  6. Example I(1) I(1) I(1) I(2)

  7. Conditions for Code Motion • Correctness: movement does not change the program semantics • Performance: code should not be slowed down Need information on • Control flow of the loop: dominate all the exits • Other definitions: no other definitions of A • Other uses: all uses of A are dominated by block b OK? I

  8. Code Motion Algorithm Given a set of nodes in a loop • Compute reaching definitions • Compute loop invariant computation • Compute dominators • Find the exits of the loop, which are successor nodes located outside loop

  9. Code Motion Details • Candidate statement for code motion • Loop invariant • Located in a block that dominates all the exits of the loop • Assign to a variable not assigned to elsewhere in the loop • Located in a block that dominate all blocks in the loop that contain the use of the variable • Perform a depth-first search of the blocks • Move the candidate statement to pre-header if all the invariant operations it depends on have been moved

  10. Examples I I I I I I outside loop

  11. More Aggressive Optimizations • Gamble on: most loops get executed • Can we relax constraint of dominating all exits? • If liveness check and exception check is satisfied

  12. Landing Pads Code for most loops is generated in a test-repeat-until form to simplify dominance

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