protected final RegisterSpec getDefinitionSpecForSsaReg(int reg) {
    SsaInsn definition = ssaMeth.getDefinitionForRegister(reg);

    return definition == null ? null : definition.getResult();
  }
Returns the RegisterSpec of the definition of the register.

@param reg {@code >= 0;} SSA register
@return definition spec of the register or null if it is never defined (for the case of
    "version 0" SSA registers)
android/android_framework/dex-maker/src/main/java/com/android/dx/ssa/back/RegisterAllocator.java