Disable FMA by default. Use -Ofma or jit.opt.start("+fma") to enable.

See the discussion in #918 for the rationale.

src/lj_asm_arm64.h

@@ -337,7 +337,8 @@ static int asm_fusemadd(ASMState *as, IRIns *ir, A64Ins ai, A64Ins air)
 {
   IRRef lref = ir->op1, rref = ir->op2;
   IRIns *irm;
-  if (lref != rref &&
+  if ((as->flags & JIT_F_OPT_FMA) &&
+      lref != rref &&
       ((mayfuse(as, lref) && (irm = IR(lref), irm->o == IR_MUL) &&
        ra_noreg(irm->r)) ||
        (mayfuse(as, rref) && (irm = IR(rref), irm->o == IR_MUL) &&