cranelift: simplify fneg(fneg(x)) to x (#6034)

cranelift/codegen/src/opts/algebraic.isle

@@ -527,6 +527,9 @@
 ;; with the `bitselect` instruction, but the pattern is a bit more complicated
 ;; due to most bitselects-over-floats having bitcasts.
 
+;; fneg(fneg(x)) == x.
+(rule (simplify (fneg ty (fneg ty x))) (subsume x))
+
 ;; If both of the multiplied arguments to an `fma` are negated then remove
 ;; both of them since they cancel out.
 (rule (simplify (fma ty (fneg ty x) (fneg ty y) z))

cranelift/filetests/filetests/egraph/algebraic.clif

@@ -492,6 +492,15 @@ block0(v1: i16):
 ; check: v4 = sextend.i64 v1
 ; check: return v4
 
+function %double_fneg(f32) -> f32 {
+block0(v1: f32):
+    v2 = fneg v1
+    v3 = fneg v2
+    return v3
+}
+
+; check: return v1
+
 function %fma_double_fneg(f32, f32, f32) -> f32 {
 block0(v1: f32, v2: f32, v3: f32):
     v4 = fneg v1