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

See the discussion in #918 for the rationale.

src/lj_asm_arm.h

@@ -313,7 +313,11 @@ static void asm_fusexref(ASMState *as, ARMIns ai, Reg rd, IRRef ref,
 }
 
 #if !LJ_SOFTFP
-/* Fuse to multiply-add/sub instruction. */
+/*
+** Fuse to multiply-add/sub instruction.
+** VMLA rounds twice (UMA, not FMA) -- no need to check for JIT_F_OPT_FMA.
+** VFMA needs VFPv4, which is uncommon on the remaining ARM32 targets.
+*/
 static int asm_fusemadd(ASMState *as, IRIns *ir, ARMIns ai, ARMIns air)
 {
   IRRef lref = ir->op1, rref = ir->op2;