From the Dev:

@quadhund If I make 1 attack with a longsword as a part of casting Green Flame Blade, while having the Hex spell affecting the target, and then I use Divine Smite once the attack hits, how many damage rolls are there?
@JeremyECrawford When something in the game (Sneak Attack, Divine Smite, hex, etc.) causes your attack/spell/etc. to deal extra dice of damage, those dice are added to the damage the effect is already dealing, if any. It's one big damage roll, extra damage included. 
@DaveWil33 Would only be a second damage roll if there was something locked behind a save like poison damage, yes?
@JeremyECrawford If an attack has a damage roll but also a second damage roll (not extra damage) that is contingent on a saving throw, the damage of that second source is a different damage roll from the first.
@RubiksMoose how do you distinguish between second damage roll and extra damage?
@JeremyECrawford Separate damage rolls are delivered by separate attack rolls, saving throws, or other processes. Extra/bonus/additional damage is called extra/bonus/additional damage.

Quote from TarodNet >>

From the Dev:

@quadhund If I make 1 attack with a longsword as a part of casting Green Flame Blade, while having the Hex spell affecting the target, and then I use Divine Smite once the attack hits, how many damage rolls are there?
@JeremyECrawford When something in the game (Sneak Attack, Divine Smite, hex, etc.) causes your attack/spell/etc. to deal extra dice of damage, those dice are added to the damage the effect is already dealing, if any. It's one big damage roll, extra damage included. 
@DaveWil33 Would only be a second damage roll if there was something locked behind a save like poison damage, yes?
@JeremyECrawford If an attack has a damage roll but also a second damage roll (not extra damage) that is contingent on a saving throw, the damage of that second source is a different damage roll from the first.
@RubiksMoose how do you distinguish between second damage roll and extra damage?
@JeremyECrawford Separate damage rolls are delivered by separate attack rolls, saving throws, or other processes. Extra/bonus/additional damage is called extra/bonus/additional damage.

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