HomePhabricator

[ARM] Follow AACPS for preserving number of loads/stores of volatile bit-fields

Authored by Diogo Sampaio <diogo.sampaio@arm.com> on Feb 7 2020, 2:03 AM.

Description

[ARM] Follow AACPS for preserving number of loads/stores of volatile bit-fields

Summary:
Following the AAPCS, every store to a volatile bit-field requires to generate one load of that field, even if all the bits are going to be replaced.
This patch allows the user to opt-in in following such rule, whenever the a.

AAPCS Release 2019Q1.1 (https://static.docs.arm.com/ihi0042/g/aapcs32.pdf)
section 8.1 Data Types, page 35, paragraph: Volatile bit-fields – preserving number and width of container accesses

When a volatile bit-field is written, and its container does not overlap with any non-bit-field member, its
container must be read exactly once and written exactly once using the access width appropriate to the
type of the container. The two accesses are not atomic.

Reviewers: lebedev.ri, ostannard, jfb, eli.friedman

Reviewed By: jfb

Subscribers: rsmith, rjmccall, dexonsmith, kristof.beyls, jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67399