[PATCH] D36562: [Bitfield] Make the bitfield a separate location if it has width of legal integer type and its bit offset is naturally aligned for the type

Xinliang David Li via llvm-commits llvm-commits at lists.llvm.org
Wed Aug 9 20:14:05 PDT 2017

On Wed, Aug 9, 2017 at 7:42 PM, Chandler Carruth via Phabricator <
reviews at reviews.llvm.org> wrote:

> chandlerc added a comment.
> This has been discussed before and I still pretty strongly disagree with
> it.
> This cripples the ability of TSan to find race conditions between accesses
> to consecutive bitfields -- and these bugs have actually come up.

Can you elaborate on this? Do you mean race conditions due to widening?

> We also have had cases in the past where LLVM missed significant bitfield
> combines and optimizations due to loading them as separate integers. Those
> would become problems again, and I think they would be even harder to solve
> than narrowing the access is going to be because we will have strictly less
> information to work with.
 Can you elaborate here too? If there were missed optimization that later
got fixed, there should be regression tests for them, right?  And what
information is missing?



> Ultimately, while I understand the appeal of this approach, I don't think
> it is correct and I think we should instead figure out how to optimize
> these memory accesses well in LLVM. That approach will have the added
> benefit of optimizing cases where the user has manually used a large
> integer to simulate bitfields, and making combining and canonicalization
> substantially easier.
> Repository:
>   rL LLVM
> https://reviews.llvm.org/D36562
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