Increase alignment of AArch64AbsLongThunk to 8 #244
Closed
Conversation
This file contains hidden or 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
This permits an AArch64AbsLongThunk to be used in an environment where unaligned accesses are disabled. The AArch64AbsLongThunk does a load of an 8-byte address. When unaligned accesses are disabled this address must be 8-byte aligned. The vast majority of AArch64 systems will have unaligned accesses enabled in userspace. However, after a reset, before the MMU has been enabled, all memory accesses are to "device" memory, which requires aligned accesses. In systems with multi-stage boot loaders a thunk may be required to a later stage before the MMU has been enabled. As we only want to increase the alignment when the ldr is used we delay the increase in thunk alignment until we know we are going to write an ldr. We also need to account for the ThunkSection alignment increase when this happens. In some of the test updates, particularly those with shared CHECK lines with position independent thunks it was easier to ensure that the thunks started at an 8-byte aligned address in all cases. This is a cherry-pick of upstream llvm-project PR 133738 to the arm-software release/20.x branch. Downstream issue: arm#243
statham-arm
approved these changes
Apr 1, 2025
|
Will abandon this in favour of a patch file based update like #216 |
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Outdated suggestions cannot be applied.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.
This permits an AArch64AbsLongThunk to be used in an environment where unaligned accesses are disabled.
The AArch64AbsLongThunk does a load of an 8-byte address. When unaligned accesses are disabled this address must be 8-byte aligned.
The vast majority of AArch64 systems will have unaligned accesses enabled in userspace. However, after a reset, before the MMU has been enabled, all memory accesses are to "device" memory, which requires aligned accesses. In systems with multi-stage boot loaders a thunk may be required to a later stage before the MMU has been enabled.
As we only want to increase the alignment when the ldr is used we delay the increase in thunk alignment until we know we are going to write an ldr. We also need to account for the ThunkSection alignment increase when this happens.
In some of the test updates, particularly those with shared CHECK lines with position independent thunks it was easier to ensure that the thunks started at an 8-byte aligned address in all cases.
This is a cherry-pick of upstream llvm-project PR 133738 to the arm-software release/20.x branch.
Downstream issue: #243