Cranelift: Rewrite or(and(x, y), not(y)) => or(x, not(y)) again (#5684)

This rewrite was introduced in #5676 and then reverted in #5682 due to a footgun
where we accidentally weren't actually checking the `y == !z` precondition. This
commit fixes the precondition check. It also fixes the arithmetic to be
correctly masked to the value type's width.

This reverts commit 268f6bfc1d.

cranelift/codegen/src/isle_prelude.rs

@@ -109,6 +109,11 @@ macro_rules! isle_common_prelude_methods {
             !x
         }
 
+        #[inline]
+        fn u64_eq(&mut self, x: u64, y: u64) -> bool {
+            x == y
+        }
+
         #[inline]
         fn u64_is_zero(&mut self, value: u64) -> bool {
             0 == value

cranelift/codegen/src/opts/algebraic.isle

@@ -138,6 +138,34 @@
 (rule (simplify (bnot ty (band t x y)))
       (bor ty (bnot ty x) (bnot ty y)))
 
+;; `or(and(x, y), not(y)) == or(x, not(y))`
+(rule (simplify (bor ty
+                     (band ty x y)
+                     z @ (bnot ty y)))
+      (bor ty x z))
+;; Duplicate the rule but swap the `bor` operands because `bor` is
+;; commutative. We could, of course, add a `simplify` rule to do the commutative
+;; swap for all `bor`s but this will bloat the e-graph with many e-nodes. It is
+;; cheaper to have additional rules, rather than additional e-nodes, because we
+;; amortize their cost via ISLE's smart codegen.
+(rule (simplify (bor ty
+                     z @ (bnot ty y)
+                     (band ty x y)))
+      (bor ty x z))
+
+;; `or(and(x, y), not(y)) == or(x, not(y))` specialized for constants, since
+;; otherwise we may not know that `z == not(y)` since we don't generally expand
+;; constants in the e-graph.
+;;
+;; (No need to duplicate for commutative `bor` for this constant version because
+;; we move constants to the right.)
+(rule (simplify (bor ty
+                     (band ty x (iconst ty (u64_from_imm64 y)))
+                     z @ (iconst ty (u64_from_imm64 zk))))
+      (if-let $true (u64_eq (u64_and (ty_mask ty) zk)
+                            (u64_and (ty_mask ty) (u64_not y))))
+      (bor ty x z))
+
 ;; x*2 == 2*x == x+x.
 (rule (simplify (imul ty x (iconst _ (simm32 2))))
       (iadd ty x x))

cranelift/codegen/src/prelude.isle

@@ -141,6 +141,9 @@
 (decl pure u64_not (u64) u64)
 (extern constructor u64_not u64_not)
 
+(decl pure u64_eq (u64 u64) bool)
+(extern constructor u64_eq u64_eq)
+
 (decl pure u64_sextend_u32 (u64) u64)
 (extern constructor u64_sextend_u32 u64_sextend_u32)