Stabilize -Cmin-function-alignment by folkertdev · Pull Request #142824 · rust-lang/rust (original) (raw)

tracking issue: #82232
split out from: #140261

Request for Stabilization

Summary

The -Cmin-function-alignment=<align> flag specifies the minimum alignment of functions for which code is generated.
The align value must be a power of 2, other values are rejected.

Note that -Zbuild-std (or similar) is required to apply this minimum alignment to standard library functions.
By default, these functions come precompiled and their alignments won't respect the min-function-alignment flag.

This flag is equivalent to:

• -fmin-function-alignment for GCC
• -falign-functions for Clang

The specified alignment is a minimum. A higher alignment can be specified for specific functions by annotating the function with a #[align(<align>)] attribute.
The attribute's value is ignored when it is lower than the value passed to min-function-alignment.

There are two additional edge cases for this flag:

• targets have a minimum alignment for functions (e.g. on x86_64 the lowest that LLVM generates is 16 bytes).
  A min-function-alignment value lower than the target's minimum has no effect.
• the maximum alignment supported by this flag is 8192. Trying to set a higher value results in an error.

Testing

• tests/codegen/min-function-alignment.rs tests the behavior of the -Zmin-function-alignment flag and how it interacts with #[align(N)].
• tests/ui/invalid-compile-flags/min-function-alignment.rs tests that incorrect alignments error

History

• add -Zmin-function-alignment #134030

The -Zmin-function-alignment flag was requested by rust-for-linux #128830. It will be used soon (see #t-compiler/help > ✔ Alignment for function addresses).

Miri supports function alignment since #140072. In const-eval there is no way to observe the address of a function pointer, so no special attention is needed there (see #t-compiler/const-eval > function address alignment).

Originally, the maximum allowed alignment was 1 << 29, because this is the highest value the LLVM API accepts. However, on COFF the highest supported alignment is only 8192 (see #142638). Practically speaking, that seems more than sufficient for all known use cases. So for simplicity, for now, we limit the alignment to 8192. The value can be increased on platforms that support it if the need arises.

r? @workingjubilee

the first commit can be split out if that is more convenient.