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Add missing subsume uses in egraph rules #7879

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merged 2 commits into from
Feb 6, 2024
Merged

Add missing subsume uses in egraph rules #7879

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34104ff
Fix a few egraph rules that needed `subsume`
elliottt Feb 6, 2024
37d21c6
Augment egraph rule guideline 2 to talk about constants
elliottt Feb 6, 2024
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6 changes: 6 additions & 0 deletions cranelift/codegen/src/opts/README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -38,6 +38,12 @@ of it boils down to the fact that, unlike traditional e-graphs, our rules are
place of `x` in such cases would introduce uses of `y` where it is not
defined.

An exception to this rule is discarding constants, as they can be
rematerialized anywhere without introducing correctness issues. For example,
the (admittedly silly) rule `(select 1 x (iconst_u _)) => x` would be a good
candidate for not using `subsume`, as it does not discard any non-constant
values introduced in its LHS.

3. Avoid overly general rewrites like commutativity and associativity. Instead,
prefer targeted instances of the rewrite (for example, canonicalizing adds
where one operand is a constant such that the constant is always the add's
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4 changes: 2 additions & 2 deletions cranelift/codegen/src/opts/extends.isle
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Expand Up @@ -29,12 +29,12 @@
(slt ty
(uextend $I64 x @ (value_type $I32))
(iconst_u _ 0)))
(iconst_u ty 0))
(subsume (iconst_u ty 0)))
(rule (simplify
(sge ty
(uextend $I64 x @ (value_type $I32))
(iconst_u _ 0)))
(iconst_u ty 1))
(subsume (iconst_u ty 1)))

;; Sign-extending can't change whether a number is zero nor how it signed-compares to zero
(rule (simplify (eq _ (sextend _ x@(value_type ty)) (iconst_s _ 0)))
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20 changes: 10 additions & 10 deletions cranelift/codegen/src/opts/icmp.isle
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Expand Up @@ -2,16 +2,16 @@

;; `x == x` is always true for integers; `x != x` is false. Strict
;; inequalities are false, and loose inequalities are true.
(rule (simplify (eq (ty_int ty) x x)) (iconst_u ty 1))
(rule (simplify (ne (ty_int ty) x x)) (iconst_u ty 0))
(rule (simplify (ugt (ty_int ty) x x)) (iconst_u ty 0))
(rule (simplify (uge (ty_int ty) x x)) (iconst_u ty 1))
(rule (simplify (sgt (ty_int ty) x x)) (iconst_u ty 0))
(rule (simplify (sge (ty_int ty) x x)) (iconst_u ty 1))
(rule (simplify (ult (ty_int ty) x x)) (iconst_u ty 0))
(rule (simplify (ule (ty_int ty) x x)) (iconst_u ty 1))
(rule (simplify (slt (ty_int ty) x x)) (iconst_u ty 0))
(rule (simplify (sle (ty_int ty) x x)) (iconst_u ty 1))
(rule (simplify (eq (ty_int ty) x x)) (subsume (iconst_u ty 1)))
(rule (simplify (ne (ty_int ty) x x)) (subsume (iconst_u ty 0)))
(rule (simplify (ugt (ty_int ty) x x)) (subsume (iconst_u ty 0)))
(rule (simplify (uge (ty_int ty) x x)) (subsume (iconst_u ty 1)))
(rule (simplify (sgt (ty_int ty) x x)) (subsume (iconst_u ty 0)))
(rule (simplify (sge (ty_int ty) x x)) (subsume (iconst_u ty 1)))
(rule (simplify (ult (ty_int ty) x x)) (subsume (iconst_u ty 0)))
(rule (simplify (ule (ty_int ty) x x)) (subsume (iconst_u ty 1)))
(rule (simplify (slt (ty_int ty) x x)) (subsume (iconst_u ty 0)))
(rule (simplify (sle (ty_int ty) x x)) (subsume (iconst_u ty 1)))

;; Optimize icmp-of-icmp.
(rule (simplify (ne ty
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