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div to shift for positive power of 2 divs #8474

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div to shift for positive power of 2 divs #8474

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8 changes: 7 additions & 1 deletion cranelift/codegen/src/opts/arithmetic.isle
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Expand Up @@ -85,7 +85,13 @@
(iconst_u ty 1)))
(subsume x))

;; TODO: strength reduction: div to shifts
;; TODO: strength reduction: div to shifts when negative power of two
;; x/c == x>>log2(c) when c is a power of two.
(rule (simplify (udiv ty x (iconst _ (imm64_power_of_two c))))
(ushr ty x (iconst ty (imm64 c))))

(rule (simplify (sdiv ty x (iconst _ (imm64_power_of_two c))))
(sshr ty x (iconst ty (imm64 c))))
;; TODO: div/rem by constants -> magic multiplications

;; x*2 == x+x.
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