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Translate builtin_assume and unaligned SIMD intrinsics #1132

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Translate builtin_assume and unaligned SIMD intrinsics #1132

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3 changes: 3 additions & 0 deletions c2rust-transpile/src/translator/builtins.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -613,6 +613,9 @@ impl<'c> Translation<'c> {
)
})
}

// TODO: provide proper implementation, this is just to get stuff building
"__builtin_assume" => Ok(self.convert_expr(ctx.used(), args[0])?),
Comment on lines +617 to +618
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When you do implement this, note that Rust (as of 1.81.0) now has core::hint::assert_unchecked, which should be the equivalent of __builtin_assume.

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@kkysen do you want to add support for __builtin_assume into a separate PR with a test?

It seems my PR is trying to do two things and only one is blocked on what rust provides, so that's messy.

WDYT about landing the rest of this PR? Should the warning reference the PR folkert opened?

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do you want to add support for __builtin_assume into a separate PR with a test?

We're still stuck on the old nightly, so adding support for newer features like this is tricky. That said, we could merge #1227, which would use stable for c2rust-transpile and let us start targeting newer Rust versions for just the transpiler. #1227 didn't work before, but now that CI is on GitHub Actions, it works without much modifications, so it wouldn't be too much work to review and merge that. If we do that, I can add support for it.

// There's currently no way to replicate this functionality in Rust, so we just
// pass the ptr input param in its place.
"__builtin_assume_aligned" => Ok(self.convert_expr(ctx.used(), args[0])?),
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19 changes: 18 additions & 1 deletion c2rust-transpile/src/translator/simd.rs
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Expand Up @@ -91,7 +91,9 @@ impl<'c> Translation<'c> {
pub fn import_simd_typedef(&self, name: &str) -> TranslationResult<bool> {
Ok(match name {
// Public API SIMD typedefs:
"__m128i" | "__m128" | "__m128d" | "__m64" | "__m256" | "__m256d" | "__m256i" => {
"__m64" | "__m128i" | "__m128i_u" | "__m128" | "__m128d" | "__m128_u" | "__m256"
| "__m256d" | "__m256_u" | "__m256i" | "__m256i_u" | "__m512 " | "__m512d"
| "__m512i" | "__m512i_u" | "__m512_u" => {
// __m64 and MMX support were removed from upstream Rust.
// See https://github.com/immunant/c2rust/issues/369
if name == "__m64" {
Expand All @@ -100,6 +102,21 @@ impl<'c> Translation<'c> {
).into());
}

// Drop the `_u` suffix for unaligned SIMD types as they have
// no equivalents in Rust. Warn the user about possible probems.
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Can we use this for unaligned SIMD types?

Godbolt

use core::arch::x86_64::__m256;
use core::mem;

#[unsafe(no_mangle)]
pub fn aligned() -> usize {
    mem::align_of::<__m256>()
}

#[repr(packed)]
pub struct UnalignedM256(pub __m256);

#[unsafe(no_mangle)]
pub fn unaligned() -> usize {
    mem::align_of::<UnalignedM256>()
}

#[repr(packed)] lets you override the alignment and unalign a type.

let name = if name.ends_with("_u") {
warn!(
"Unaligned SIMD type {} has no equivalent in Rust. \
Emitting {} instead. This likely means the potential \
for miscompilation has been introduced.",
name,
&name[..name.len() - 2]
);
&name[..name.len() - 2]
} else {
name
};

self.with_cur_file_item_store(|item_store| {
add_arch_use(item_store, "x86", name);
add_arch_use(item_store, "x86_64", name);
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28 changes: 27 additions & 1 deletion c2rust-transpile/src/translator/structs.rs
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Expand Up @@ -377,7 +377,33 @@ impl<'a> Translation<'a> {

field_entries.push(field);
}
FieldType::Regular { field, .. } => field_entries.push(*field),
FieldType::Regular {
mut field, name, ..
} => {
if let crate::translator::Type::Path(segments, ..) = &field.ty {
for seg in &segments.path.segments {
let tynam = seg.ident.to_string();
match tynam.as_str() {
"__m128i_u" | "__m128_u" | "__m256i_u" | "__m256_u"
| "__m512_u" | "__m512i_u" => {
// TODO: We might already have emitted a warning in `import_simd_typedef`,
// is it always safe to skip the warning here?
log::warn!("Unaligned SIMD type {} in field {} has no equivalent in Rust. \
Emitting {} instead. This likely means the potential \
for miscompilation has been introduced.",
tynam,
name,
&tynam[..tynam.len() - 2],
);
field.ty = *mk().ident_ty(&tynam[..tynam.len() - 2]);
break;
}
_ => {}
}
}
}
field_entries.push(*field)
}
}
}
Ok((field_entries, contains_va_list))
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