What is the significance of memory allocation with regards to compilers

Show 6 more comments. Active Oldest Votes. Improve this answer. AnT AnT k 39 39 gold badges silver badges bronze badges. Rahul: C does not support static VLAs. If n is a run-time value, then static int k[n] is not allowed. But even if it were allowed, it wouldn't allocate a new memory block every time. So, there's no similarity to malloc here even with static. There's plenty of confusion about pointers and arrays out and about as is.

Rahul: VLA were introduced in C99 standard. Formally, they have been around for about 18 years now. Of course, compiler support took some time. However, alloca allocates memory with "function" lifetime: the memory persists until the function exits.

Meanwhile VLA has normal block-based lifetime. In this regard VLA is very different from alloca. However, it is worth nothing that since C11 VLA is an optional feature of the language. Show 12 more comments. Peter Peter Modern operating systems and even old and embedded OS's in some cases allow user configuration of the stack size — M. Add a comment. L3 ; load pointer to format string for scanf ; into r0 bl scanf ; call scanf arguments in r0 and r1 ldr r2, [fp, -8] ; load r2 with value of n ldr r0,.

Mats Petersson Mats Petersson k 13 13 gold badges silver badges bronze badges. You can also use std::vector for many other purposes. So there is a close substitute. Davislor although this wont be on the stack. But if you really want a std::vector that calls alloca , you can get one.

They try successively smaller new : godbolt. But real compilers don't. This is called out in C11 7. Linkon Linkon 9 9 silver badges 15 15 bronze badges.

Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Search for: Search. The data these two components work with have different lifetimes; therefore, conceptually, the Compiler Memory Manager provides the ability to allocate two different kinds of memory: Scratch Memory.

This refers to memory required to perform the compilation eg. This memory is allocated by the JIT Compiler component, and is released at the end of a compilation. There is a Scratch Space Limit specified for each compilation; reaching this limit causes the compilation to abort, possibly retried at a lower optimization level.

There are two sub-types of Scratch Memory: Heap Memory is just dynamically allocated memory, analogous to new or malloc. Persistent Memory. This refers to memory that persists throughout the lifetime of the JVM eg.

Class Hierarchy Table entries, IProfiler data, etc. This memory is generally allocated by the JIT Runtime component. Allocating Scratch Memory There are four main ways of allocating Scratch Memory: Compilation Allocator This approach is used to allocate memory from the pool of memory that will exist until the end of the compilation.

Heap Memory Region This approach is also used to allocate memory from the pool of memory that exists until the end of the compilation. Like this: Like Loading Next Next post: gcpolicy:nogc. Leave a Reply Cancel reply. However, it may be that there is sufficient memory, but not available in one contiguous chunk that can satisfy the allocation request.

This situation is called memory fragmentation. The best way to understand memory fragmentation is to look at an example. For this example, it is assumed hat there is a 10K heap. First, an area of 3K is requested, thus:. This results in a failure — NULL is returned into p1 — because, even though 6K of memory is available, there is not a 4K contiguous block available. This is memory fragmentation. It would seem that an obvious solution would be to de-fragment the memory, merging the two 3K blocks to make a single one of 6K.

However, this is not possible because it would entail moving the 4K block to which p2 points. Moving it would change its address, so any code that has taken a copy of the pointer would then be broken. This is only possible because these languages do not support direct pointers, so moving the data has no adverse effect upon application code. This defragmentation may occur when a memory allocation fails or there may be a periodic garbage collection process that is run.

In either case, this would severely compromise real time performance and determinism. A real time operating system may provide a service which is effectively a reentrant form of malloc.

However, it is unlikely that this facility would be deterministic. Memory management facilities that are compatible with real time requirements — i. This creates a partition pool with the descriptor MyPool, containing bytes of memory, filled with partitions of size 40 bytes i. The pool is located at address 0xB The pool is configured such that, if a task attempts to allocate a block, when there are none available, and it requests to be suspended on the allocation API call, suspended tasks will be woken up in a first-in, first-out order.

The other option would have been task priority order. Another API call is available to request allocation of a partition. Here is an example using Nucleus OS:. This requests the allocation of a partition from MyPool. When successful, a pointer to the allocated block is returned in ptr. If a task of higher priority was suspended pending availability of a partition, it would now be run. There is no possibility for fragmentation, as only fixed size blocks are available.

The only failure mode is true resource exhaustion, which may be controlled and contained using task suspend, as shown. Additional API calls are available which can provide the application code with information about the status of the partition pool — for example, how many free partitions are currently available. Care is required in allocating and de-allocating partitions, as the possibility for the introduction of memory leaks remains.

The potential for programmer error resulting in a memory leak when using partition pools is recognized by vendors of real time operating systems. Typically, a profiler tool is available which assists with the location and rectification of such bugs.

Having identified a number of problems with dynamic memory behavior in real time systems, some possible solutions and better approaches can be proposed. It is possible to use partition memory allocation to implement malloc in a robust and deterministic fashion. The idea is to define a series of partition pools with block sizes in a geometric progression; e.

A malloc function may be written to deterministically select the correct pool to provide enough space for a given allocation request. This approach takes advantage of the deterministic behavior of the partition allocation API call, the robust error handling e.

Dynamic memory includes stack and heap. Dynamic behavior in embedded real time systems is generally a source of concern, as it tends to be non-deterministic and failure is hard to contain.

Using the facilities provided by most real time operating systems, a dynamic memory facility may be implemented which is deterministic, immune from fragmentation and with good error handling. No portion of this site may be copied, retransmitted, reposted, duplicated or otherwise used without the express written permission of Design And Reuse.