Add function to check buffer alignment #87
Merged
Conversation
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Some protocols require buffers to be aligned: for example - Xen shared memory protocols require alignment to pages - Linux O_DIRECT requires buffers to be aligned to the disk sector size The new function `check_alignment cstr alignment` returns `true` if the data at `cstr.off` has a memory address for which `address mod alignment = 0`. This will allow implementations which require aligned buffers to check for it, and fail appropriately if given unaligned buffers. Note this patch assumes that javascript does not have buffer addresses or memory alignment requirements, so the js stubs always returns an address of 0. Signed-off-by: David Scott <dave.scott@docker.com>
These tests verify that we find the expected number of aligned offsets within a couple of example buffers. Signed-off-by: David Scott <dave.scott@docker.com>
|
|
||
| let check_alignment t alignment = | ||
| let address = address_bigstring t.buffer in | ||
| Int64.(rem (add address (of_int t.off)) (of_int alignment) = 0L) |
There was a problem hiding this comment.
There are an awful lot of minor heap allocations here... if this is in the fast path as an assert it will probably increase GC pressure
|
Yeah good point, I'll respin with a C version. |
Before this patch we returned the buffer address from C and then calculated using OCaml's boxed Int64.t. This patch reduces minor allocations by performing the calculation in the C stubs. This makes the function suitable for use in an assert in a performance sensitive path. Signed-off-by: David Scott <dave.scott@docker.com>
djs55
added a commit
that referenced
this pull request
Mar 12, 2016
Add function to check buffer alignment
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Some protocols require buffers to be aligned to particular boundaries, for example
This PR adds a simple
check_alignmentfunction (similar tocheck_bounds) which returnstrueif the data inside a struct (i.e. the first byte at the offset) has an address which is suitably aligned.This will enable better sanity-checking and error-reporting in other Mirage libraries.