[opt] Prevent select lifting from pessimizing constant shifts to variable #3007
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…able Modified `ProfitabilityGuardForLiftedOp` in `select_lifting_pass.cc` to avoid lifting a select over constant shift amounts (e.g., `sel(s, x<<1, x<<2)`) into a variable shift (e.g., `x << sel(s, 1, 2)`). This transformation is almost always unprofitable, since constant shifts can be rewritten to slices & concats with literals (approximately free). This heuristic is only applied when no delay model is provided. If a delay model is present, the existing delay-based profitability analysis is used. Added new test cases in `select_lifting_pass_test.cc` to cover: * Not lifting constant shifts without a delay model. * Lifting when a fake delay model indicates profitability. * Not lifting when a fake delay model indicates unprofitability. #geminiassisted PiperOrigin-RevId: 802324592
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[opt] Prevent select lifting from pessimizing constant shifts to variable
Modified
ProfitabilityGuardForLiftedOpinselect_lifting_pass.ccto avoid lifting a select over constant shift amounts (e.g.,sel(s, x<<1, x<<2)) into a variable shift (e.g.,x << sel(s, 1, 2)). This transformation is almost always unprofitable, since constant shifts can be rewritten to slices & concats with literals (approximately free). This heuristic is only applied when no delay model is provided. If a delay model is present, the existing delay-based profitability analysis is used.Added new test cases in
select_lifting_pass_test.ccto cover:#geminiassisted