ISLE opts for x ± a == y ± b

Author: scottmcm

cranelift/codegen/src/opts/cprop.isle

@@ -172,6 +172,20 @@
 (rule (simplify (select ty (iconst_u _ 0) _ y))
       (subsume y))
 
+;; Similarly, move constants to the right side of an equality comparison.
+;; `x + C == y` --> `x == y - C`
+(rule (simplify (eq ty1 (iadd ty2 x k@(iconst _ _)) y))
+      (eq ty1 x (isub ty2 y k)))
+;; `x - C == y` --> `x == y + C`
+(rule (simplify (eq ty1 (isub ty2 x k@(iconst _ _)) y))
+      (eq ty1 x (iadd ty2 y k)))
+;; `x + C != y` --> `x != y - C`
+(rule (simplify (ne ty1 (iadd ty2 x k@(iconst _ _)) y))
+      (ne ty1 x (isub ty2 y k)))
+;; `x - C != y` --> `x != y + C`
+(rule (simplify (ne ty1 (isub ty2 x k@(iconst _ _)) y))
+      (ne ty1 x (iadd ty2 y k)))
+
 ;; Replace subtraction by a "negative" constant with addition.
 ;; Notably, this gives `x - (-1) == x + 1`, so other patterns don't have to
 ;; match the subtract-negative-one version too.

cranelift/codegen/src/opts/icmp.isle

@@ -13,6 +13,18 @@
 (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)))
 
+;; For integers, adding or subtracting the same thing on both sides of an
+;; equality check (or inequality check) doesn't change the result
+
+(rule (simplify (eq ty (iadd _ a k) (iadd _ b k)))
+      (eq ty a b))
+(rule (simplify (eq ty (isub _ a k) (isub _ b k)))
+      (eq ty a b))
+(rule (simplify (ne ty (iadd _ a k) (iadd _ b k)))
+      (ne ty a b))
+(rule (simplify (ne ty (isub _ a k) (isub _ b k)))
+      (ne ty a b))
+
 ;; Optimize icmp-of-icmp.
 ;; ne(icmp(ty, cc, x, y), 0) == icmp(ty, cc, x, y)
 ;; e.g. neq(ugt(x, y), 0) == ugt(x, y)

cranelift/filetests/filetests/egraph/cprop.clif

@@ -231,6 +231,32 @@ block0:
 ; check: v3 = iconst.i8 0
 ; nextln: return v3
 
+function %icmp_sums_same_addend(i32, i32, i32) -> i8 {
+block0(v0: i32, v1: i32, v2: i32):
+    v3 = iadd v0, v2
+    v4 = iadd v1, v2
+    v5 = icmp eq v3, v4
+    return v5
+}
+
+; check: v6 = icmp eq v0, v1
+; nextln: return v6
+
+function %icmp_sums_const_addends(i32, i32) -> i8 {
+block0(v0: i32, v1: i32):
+    v3 = iconst.i32 123
+    v4 = iconst.i32 456
+    v5 = iadd v0, v3
+    v6 = iadd v1, v4
+    v7 = icmp eq v5, v6
+    return v7
+}
+
+; check: v11 = iconst.i32 333
+; check: v12 = iadd v1, v11
+; check: v15 = icmp eq v0, v12
+; nextln: return v15
+
 function %ireduce_iconst() -> i8 {
 block0:
     v1 = iconst.i16 -10