0becc89d4a
alignment of `byval` on x86 in the process.
Commit 88e4d2c291 from five years ago removed
support for alignment on indirectly-passed arguments because of problems with
the `i686-pc-windows-msvc` target. Unfortunately, the `memcpy` optimizations I
recently added to LLVM 16 depend on this to forward `memcpy`s. This commit
attempts to fix the problems with `byval` parameters on that target and now
correctly adds the `align` attribute.
The problem is summarized in [this comment] by @eddyb. Briefly, 32-bit x86 has
special alignment rules for `byval` parameters: for the most part, their
alignment is forced to 4. This is not well-documented anywhere but in the Clang
source. I looked at the logic in Clang `TargetInfo.cpp` and tried to replicate
it here. The relevant methods in that file are
`X86_32ABIInfo::getIndirectResult()` and
`X86_32ABIInfo::getTypeStackAlignInBytes()`. The `align` parameter attribute
for `byval` parameters in LLVM must match the platform ABI, or miscompilations
will occur. Note that this doesn't use the approach suggested by eddyb, because
I felt it was overkill to store the alignment in `on_stack` when special
handling is really only needed for 32-bit x86.
As a side effect, this should fix #80127, because it will make the `align`
parameter attribute for `byval` parameters match the platform ABI on LLVM
x86-64.
[this comment]: https://github.com/rust-lang/rust/pull/80822#issuecomment-829985417
62 lines
1.2 KiB
Rust
62 lines
1.2 KiB
Rust
// Issue #80127: Passing structs via FFI should work with explicit alignment.
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use std::ffi::CString;
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use std::ptr::null_mut;
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#[derive(Clone, Copy, Debug, PartialEq)]
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#[repr(C)]
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#[repr(align(16))]
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pub struct TwoU64s {
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pub a: u64,
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pub b: u64,
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}
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#[repr(C)]
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#[derive(Debug, Copy, Clone)]
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pub struct BoolAndU32 {
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pub a: bool,
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pub b: u32,
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}
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#[link(name = "test", kind = "static")]
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extern "C" {
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fn many_args(
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a: *mut (),
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b: *mut (),
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c: *const i8,
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d: u64,
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e: bool,
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f: BoolAndU32,
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g: *mut (),
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h: TwoU64s,
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i: *mut (),
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j: *mut (),
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k: *mut (),
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l: *mut (),
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m: *const i8,
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) -> i32;
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}
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fn main() {
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let two_u64s = TwoU64s { a: 1, b: 2 };
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let bool_and_u32 = BoolAndU32 { a: true, b: 3 };
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let string = CString::new("Hello world").unwrap();
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unsafe {
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many_args(
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null_mut(),
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null_mut(),
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null_mut(),
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4,
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true,
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bool_and_u32,
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null_mut(),
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two_u64s,
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null_mut(),
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null_mut(),
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null_mut(),
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null_mut(),
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string.as_ptr(),
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);
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}
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}
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