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[ARM] Follow AACPS standard for volatile bit-fields access width

Authored by Diogo Sampaio <diogo.sampaio@arm.com> on Jan 21 2020, 7:13 AM.

Description

[ARM] Follow AACPS standard for volatile bit-fields access width

Summary:
This patch resumes the work of D16586.
According to the AAPCS, volatile bit-fields should
be accessed using containers of the widht of their
declarative type. In such case:

struct S1 {
  short a : 1;
}

should be accessed using load and stores of the width
(sizeof(short)), where now the compiler does only load
the minimum required width (char in this case).
However, as discussed in D16586,
that could overwrite non-volatile bit-fields, which
conflicted with C and C++ object models by creating
data race conditions that are not part of the bit-field,
e.g.

struct S2 {
  short a;
  int  b : 16;
}

Accessing S2.b would also access S2.a.

The AAPCS Release 2019Q1.1
(https://static.docs.arm.com/ihi0042/g/aapcs32.pdf)
section 8.1 Data Types, page 35, "Volatile bit-fields -
preserving number and width of container accesses" has been
updated to avoid conflict with the C++ Memory Model.
Now it reads in the note:

This ABI does not place any restrictions on the access widths
of bit-fields where the container overlaps with a non-bit-field member.
 This is because the C/C++ memory model defines these as being separate
memory locations, which can be accessed by two threads
 simultaneously. For this reason, compilers must be permitted to use a
narrower memory access width (including splitting the access
 into multiple instructions) to avoid writing to a different memory location.

I've updated the patch D16586 to follow such behavior by verifying that we
only change volatile bit-field access when:

  • it won't overlap with any other non-bit-field member
  • we only access memory inside the bounds of the record

Regarding the number of memory accesses, that should be preserved, that will
be implemented by D67399.

Reviewers: rsmith, rjmccall, eli.friedman, ostannard

Subscribers: ostannard, kristof.beyls, cfe-commits, carwil, olista01

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72932