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Add builtins for aligning and checking alignment of pointers and integers

Authored by arichardson on Jan 9 2020, 12:48 PM.

Description

Add builtins for aligning and checking alignment of pointers and integers

This change introduces three new builtins (which work on both pointers
and integers) that can be used instead of common bitwise arithmetic:
builtin_align_up(x, alignment), builtin_align_down(x, alignment) and
__builtin_is_aligned(x, alignment).

I originally added these builtins to the CHERI fork of LLVM a few years ago
to handle the slightly different C semantics that we use for CHERI [1].
Until recently these builtins (or sequences of other builtins) were
required to generate correct code. I have since made changes to the default
C semantics so that they are no longer strictly necessary (but using them
does generate slightly more efficient code). However, based on our experience
using them in various projects over the past few years, I believe that adding
these builtins to clang would be useful.

These builtins have the following benefit over bit-manipulation and casts
via uintptr_t:

  • The named builtins clearly convey the semantics of the operation. While checking alignment using builtin_is_aligned(x, 16) versus ((x & 15) == 0) is probably not a huge win in readably, I personally find builtin_align_up(x, N) a lot easier to read than (x+(N-1))&~(N-1).
  • They preserve the type of the argument (including const qualifiers). When using casts via uintptr_t, it is easy to cast to the wrong type or strip qualifiers such as const.
  • If the alignment argument is a constant value, clang can check that it is a power-of-two and within the range of the type. Since the semantics of these builtins is well defined compared to arbitrary bit-manipulation, it is possible to add a UBSAN checker that the run-time value is a valid power-of-two. I intend to add this as a follow-up to this change.
  • The builtins avoids int-to-pointer casts both in C and LLVM IR. In the future (i.e. once most optimizations handle it), we could use the new llvm.ptrmask intrinsic to avoid the ptrtoint instruction that would normally be generated.
  • They can be used to round up/down to the next aligned value for both integers and pointers without requiring two separate macros.
  • In many projects the alignment operations are already wrapped in macros (e.g. roundup2 and rounddown2 in FreeBSD), so by replacing the macro implementation with a builtin call, we get improved diagnostics for many call-sites while only having to change a few lines.
  • Finally, the builtins also emit assume_aligned metadata when used on pointers. This can improve code generation compared to the uintptr_t casts.

[1] In our CHERI compiler we have compilation mode where all pointers are
implemented as capabilities (essentially unforgeable 128-bit fat pointers).
In our original model, casts from uintptr_t (which is a 128-bit capability)
to an integer value returned the "offset" of the capability (i.e. the
difference between the virtual address and the base of the allocation).
This causes problems for cases such as checking the alignment: for example, the
expression if ((uintptr_t)ptr & 63) == 0 is generally used to check if the
pointer is aligned to a multiple of 64 bytes. The problem with offsets is that
any pointer to the beginning of an allocation will have an offset of zero, so
this check always succeeds in that case (even if the address is not correctly
aligned). The same issues also exist when aligning up or down. Using the
alignment builtins ensures that the address is used instead of the offset. While
I have since changed the default C semantics to return the address instead of
the offset when casting, this offset compilation mode can still be used by
passing a command-line flag.

Reviewers: rsmith, aaron.ballman, theraven, fhahn, lebedev.ri, nlopes, aqjune
Reviewed By: aaron.ballman, lebedev.ri
Differential Revision: https://reviews.llvm.org/D71499