The idea that troops were knowingly sent in to be killed the moment they landed until (hopefully) enough bodies piled up to gain a foothold and capture it.

The idea that Operation Overlord was planned in any way similar to the popular conception of mass assault tactics from WW1 is quite falacious, and Allied planners went to quite extensive lengths to plan for every potential obstacle and situation in order to minimise casualties. While these plans didn't always come to fruition in the action of the day, it should be noted they existed, and when they did they were highly effective.

While the Operation Neptune landings didn't meet all of their intended objectives, and casualties on some beaches were high, particularly on Omaha Beach, casualties overall were actually far lower than had been predicted for the operation overall. Casualties on Omaha Beach were actually higher than the other beaches combined; US forces landing at Utah Beach faced light resistance and only suffered around 200 casualties. Canadian casualties on Juno beach were around 900, largely because the choppy seas meant that the first wave of infantry arrived before the supporting armour. Despite that, Canadian forces were able to advance off the beach area quite rapidly, and certainly before the German guns were overwhelmed by mountains of the dead.

Allied planners scouted the Normandy beaches in exhaustive detail, and field officers were aware of the location of practically every last emplacement, bunker, ditch and minefield. The landings were supported by prior air and naval bombardment which, while not always successful, were designed to knock out German emplacements and inland artillery to minimize their response to the landings, while the parachute landings inland aimed to cut bridges and road links and prevent rapid German responses to the landings. Some of these, like the capture of Pegasus Bridge, were more successful than others.

A number of armoured innovations were also designed to assist the landings, particularly in the British sectors. The most famous of "Hobart's Funnies" is probably the Sherman Duplex Drive 'swimming tank'. These were the tanks which were delayed supporting the Canadians at Juno Beach. At Sword Beach, however, 21 of 25 DD tanks reached the beach in the first wave and made rapid progress in clearing the beach and moving inland. A variety of other 'funnies' were built on the Churchill platform, including the 'Crocodile' flamethrower tank and the Assault Vehicle Royal Engineers (AVRE) which carried a giant spiggot mortar and was used to demolish buildings and bunkers. Other variants included bridge-layers, Shermans fitted with anti-mine flails, and tanks carrying vast fascines to fill German trenches and ditches. The Churchill AVRE was particularly effective on D-Day, not only for its practical role, but also for the morale value for Allied troops seeing its enormous weapon in action, and similarly prompting German forces to surrender.

The idea that troops were knowingly sent in to be killed the moment they landed until (hopefully) enough bodies piled up to gain a foothold and capture it.

They weren't sent in to be killed the moment they landed. They were sent in to make it across the beach, it'd have been pointless to send men in just to die and after the ill-fated raid on Dieppe a lot of work was done specifically so that men wouldn't get cut down on the beaches. Men weren't sent in to die on the beaches, they were sent in to take the beaches with the expectation that some of them would be wounded or killed by the enemy attempting to stop them but that this could be reduced by proper preparation and support.

they were essentially walking into a slaughterhouse

I'm not sure how this differs from being asked to make any other kind of attack, you could characterise pretty much any sort of military enlistment in this way.

Operation Overlord included a vast support operation involving hundreds of warships (destroyers, cruisers and battleships) providing fire support and a sustained aerial bombardment, as well as paratrooper airdrops to seize key locations and cut off reinforcements.

On the beaches themselves, the British deployed a series of specialised armoured vehicles under the umbrella of the 79th Armoured Division commanded by Percy Hobart (dubbed 'Hobart's funnies') to aid the landings, overcome obstacles and reduce casualties. Methods were devised for landing these vehicles quickly, either by creating amphibious 'Duplex Drive' tanks that could 'swim' ashore to give support or by use of the Landing Craft, Tank to deliver them to the beach.

In addition, the methods for infantry assaults on fortified positions had been well developed in the 30 years between 1914 and 1944, and some of the basics can be found in Infantry Training Part VIII. - Fieldcraft, Battle Drill, Section and Platoon Tactics 1944. Page 21 explains the expectations of an attacker against a position bombarded by artillery as

Apart from direct hits from shells or bombs, few if any of the enemy will be killed by your covering fire. Do not therefore expect to find only dead men when you assault. A well-trained enemy with good morale will begin firing again as soon as your own supporting fire ceases for safety reasons.

showing that the expectation was set that the bombardment would protect them from (some) fire on the way in, but that they would have to fight the defenders with regular infantry tactics once they were in close. The manual then continues with an explanation of how to fight these positions using the weapons an infantry unit would have, how to move using cover and to cross obstacles.

If you want to know more about the specialist armoured vehicles used, the Tank Museum in Bovington has a video series on their examples. The DD sherman video can be found here hosted by historian David Fletcher (MBE), and a video by museum curator David Willey on the Funnies in general can be found here.

The plans for the Dday landings did not consist of "piling up bodies to gain a foothold." Contrary to what Hollywood has portrayed, most of the 6 beachheads were secured with relatively little resistance. Unfortunately for the US 1st Infantry division besieging Omaha beach, they encountered a whole division of German infantry and the fighting was fierce; it's this part of the landings which is so often represented on film. Yet, even then, to say the landings amounted to hopefulness and "piling up bodies" does a great disservice to the scale and depth of planning behind the undertaking.

As for did the men know, they knew of the invasion and they knew they would be the spear-tip of the Allied advance. The dates and locations of the actual attack were kept secret, but the nature of their training and presence was clear to all. Eisenhower shared this letter with the Allied forces:

"You are about to embark upon the Great Crusade, toward which we have striven these many months. The eyes of the world are upon you. The hopes and prayers of liberty-loving people everywhere march with you. In company with our brave Allies and brothers-in-arms on other Fronts, you will bring about the destruction of the German war machine, the elimination of Nazi tyranny over the oppressed peoples of Europe, and security for ourselves in a free world."

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To address the idea that a better plan could've been formulated:

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Operation Overlord was the largest single military undertaking in the history of the world, before or since. It involved elements from some 14 nations, the bulk of which was made up of British, American & Canadian forces. It utilised over 150,000 allied soldiers, close to 12,000 aircraft and some 7,000 boats. It was the brainchild of the greatest strategic minds available, headed by Dwight Eisenhower & Bernard Montgomery. It was not merely a series of landings, but a combined and sustained effort from the land, sea and air.

A year previously, it had been decided that the invasion would require absolute air superiority. Operation Point Blank was launched. The Allied Combined Bomber Offensive orchestrated a joint effort, utilising both the British & US air services, to disable the Luftwaffe. The bombing campaign focused of airfields and factories producing airplanes, whilst the bombers escorts sought to whittle down German fighter aircraft in a battle of attrition.

Thousands of reconnaissance runs were conducted, carefully mapping the terrain, obstacles and defences arrayed across France's northern coastline. To sow confusion, these efforts were extended throughout much of Europe's northern coast. The scale and expense of this undertaking was vast, yet this was just another small part of the whole.

New technology was brought to bear, perhaps the most iconic of which are the Mulberry Harbours. Rather than waste manpower attempting to seize heavily fortified French ports, the British constructed artificial jetties designed to be towed into place. Further developments were new landing craft and specially developed variations of Churchill & Sherman tanks to better combat the terrain and obstacles of the beachheads, from flame-throwing tanks to destroy bunkers to "mine-flaying" tanks to clear pathways. They even constructed a series of, unfortunately ill-fated, amphibious tanks!

British intelligence operatives had laboriously strewn a trail of misinformation, convincing the German high command that the Allied invasion would strike at Calais, not Normandy. Titled Operation Bodyguard, the lengths the intelligence community went to cannot be understated. They went so far as to construct an entire fake army, complete with tanks, planes and boats, at Kent & Sussex (a likely staging ground for a Calais landing) to trick German air reconnaissance. They even went so far as to lay straw through the streets of Portsmouth on the night of the invasion, so the passing tanks were less likely to alert any witnesses. They sold tales of a planned invasion of Norway and built up a rumour mill surrounding the supposed major offensive on the Port du Calais.

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The invasion itself had several parts.

I’ll surmise Omaha beach here, due to the word-cap, but I’ll comment with a more detailed explanation.

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Prior to the landings, a bombing run had been dispatched, but due to overcast whether; it'd missed. The German emplacements were fully operational and to make matters worse, rough seas meant that of 29 amphibious tanks sent to assist the infantry, 27 failed to make it ashore. Again thanks to rough seas, the infantry landed in scattered and sporadic smatterings, often off target. The resistance was fierce, they'd expected a battalion but were facing a whole German division and the assault was harrowing and casualties were high. The first wave failed to take its objectives.

The second, and larger wave, had greater success but no smaller challenges. As the sustained invasion carried on, the plan was in disarray. Commanders were either absent of killed, supporting trucks and tanks were mostly stuck in the water, and any sense of cohesion was lost. General Omar Bradley, commander of the US first army, even considered evacuating the beach. By days end, the battle for Omaha beach had resulted in anywhere between 3,000 - 5,000 US casualties (with some 1,200 German losses). This was higher than the rest of the beaches combined. Omaha beach was hard fought but was successfully taken as part of the greatest invasion the world has ever seen.

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TL;DR - The Dday landings were the biggest military undertaking the world had ever, or has ever, seen. It was the combined effort of 14 nations, with thousands of contributors, from pilots and engineers, to sailors and spies. It was the brainchild of the greatest military minds of a generation. The depth of planning cannot be overstated, every resource available was brought to bear by the joint Allied command to ensure the success of the operation. 10,000 Allied soldiers were casualties of the first day, but through their sacrifice they brought about "the elimination of Nazi tyranny over the oppressed peoples of Europe, and security for ourselves in a free world."