Data Hazards and stalls

I am studying for my exam tomorrow and I am having difficulty in the below code :

sub $  2, $  1, $  3 and $  12, $  2, $  5 or $  13, $  6, $  2 add $  14, $  2, $  2 sw $  15, 100($  2) 

Due to the ALU-ALU dependency here on Register $ 2 , The sub instruction does not write its result until the fifth stage, meaning that we would have to waste three clock cycles in the pipeline. My question is why 3 clock cycles ? This dependency can be solved by inserting two nops and therefore we are wasting 2 clock cycles ? Please clarify it to me as I am trying to relate the nops to the wasted cycles and I am sure that I have a huge misunderstanding here .

Would the spell Purify Food and Drink remove physical hazards like bone shards from otherwise normal food?

In a session of the Hoard of the Dragon Queen adventure, our characters were sitting down to a campsite meal provided by the chuck wagon. Being aware that they were being hunted/stalked, the cleric of the party (a dwarf for what it’s worth) cast Purify Food and Drink on their meal. They were getting ready to chow down when an NPC approached and advised them not to eat. This NPC then proceeded to fish or filter (I can’t recall which now) several sharp bone that would have (we were informed) been quite deleterious to our health.

The DM ruled that since bone is not poisonous, it would not be affected by the spell. I say that, RAW, by the very definition of “purify”, any harmful substance would be removed.

I researched the site and found a similar sort of question in: How great is the purifying power of Purify Food and Drink?, but it didn’t precisely speak to the question at hand. Our DM requested that I post the question here seeking disinterested third-party opinions.

The description of the Purify Food and Drink spell says:

All nonmagical food and drink within a 5-foot-radius sphere centered on a point of your choice within range is purified and rendered free of poison and disease.

From the definition of “purified” on OxfordDictionaries.com:

  1. having had contaminants removed; cleansed.

The definition of “purify” on Dictionary.com:

  1. to make pure; free from anything that debases, pollutes, adulterates, or contaminates: to purify metals.
  2. to free from foreign, extraneous, or objectionable elements

The Merriam-Webster definition of purify:

: to make pure: such as

a : to clear from material defilement or imperfection

And of “pure”:

1 a (1) : unmixed with any other matter
(2) : free from dust, dirt, or taint

“Purify” in the Cambridge English Dictionary:

(NOT MIXED) ​ to remove bad substances from something to make it pure:

[…]

(MAKE NOT MIXED) ​ to rid something of dirty or harmful substances

Whose interpretation, RAW, holds more water? Or, more to the point, would the spell Purify Food and Drink remove physical hazards like bone shards from otherwise normal food?

Please note the school of transmutation. To transmute is to change in form, nature, or substance.

Why are tactile ground surface indicators potential slip hazards?

For those who are not familiar with the technical sounding term for something that you see in normal everyday life in the city, tactile pavings, also known as tactile ground surface indicators (TGSI) are:

a system of textured ground surface indicator found on footpaths, stairs and train station platforms to assist pedestrians who are visually impaired

https://en.wikipedia.org/wiki/Tactile_paving

My experience with some of these tactile pavings is that if they are made from certain types of water resistance material (e.g. metal or plastic-like material) then they often become somewhat of a slip hazard when there is a lot of water on the surface (such as when it is raining).

It would seem rather curious that something created to assist pedestrians who are visually impaired would be a potential slip hazard for those who are not visually impaired. This is despite the fact that there are various building or design standards that require the testing of TGSI. However, as can be seen in some discussions online, this is not always the case due to a number of factors.

Is this something that is taken into consideration in the design and development of tactile ground surface indicators? How come people who are visually impaired don’t seem to have a problem with this (or maybe they do but are less affected by it)?

lw and sw hazards example MIPS

The below example confused me:

lw $  r0, 4($  r0) sw $  r0, 4($  r0) add $  r0, $  r0, $  r0 

Using MIPS 5 stage execution what are the hazards we have 1) without forwarding 2) with forwarding only in the stage of execution (exe or alu) 3) with forwarding. Add nops to eliminate hazards.