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Common Subexpression Elimination and Copy Propagation in Titanium

Common Subexpression Elimination and Copy Propagation in Titanium. Johnathon Jamison David Marin CS265 S. Graham. Global Copy Propagation. Given a = b; … x = a + 1; We want to replace a with b on the last line, but we need to know that a and b are unchanged

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Common Subexpression Elimination and Copy Propagation in Titanium

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  1. Common Subexpression Elimination and Copy Propagation in Titanium Johnathon Jamison David Marin CS265 S. Graham

  2. Global Copy Propagation • Given a = b; … x = a + 1; • We want to replace a with b on the last line, but we need to know that a and b are unchanged • Def/use analysis isn’t quite enough (why?)

  3. Inserting Fake Defs and Uses • Add fake defs and uses so that def/use analysis gives us the info we need b = b; a = b; … newtemp = b; x = a + 1; • We can use this technique to enable CSE too.

  4. Interaction with Copy Propagation • Any temps introduced are placed after the calculation, so that copy propagation can remove them a = f * i a = f * i temp_1 = a … … b = f * i b = temp_1 temp_2 = b … … c = f * i c = temp_2

  5. Local CSE • Used Muchnick’s algorithm (described in class) • Used defs to find what was killed • Fully implemented • Except method calls • Since we are using defs already, why not so something more substantial?

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