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translate-c: packed struct implies align(1) on every field

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Superceeds PR #12735 (now supporting all packed structs in GNU C)
Fixes issue #12733

This stops translating C packed struct as a Zig packed struct.
Instead use a regular `extern struct` with `align(1)`.

This is because (as @Vexu explained) Zig packed structs are really just integers (not structs).

Alignment issue is more complicated. I think @ifreund was the
first to notice it in his comment on PR #12735

Justification of my interpretion of the C(lang) behavior
comes from a careful reading of the GCC docs for type & variable attributes:

(clang emulates gnu's packed attribute here)

The final line of the documentation for __attribute__ ((aligned)) [on types] says:

> When used on a struct, or struct member, *the aligned attribute can only increase the alignment*; in order to decrease it, the packed attribute must be specified as well.

This implies that GCC uses the `packed` attribute for alignment purposes
in addition to eliminating padding.

The documentation for __attribute__((packed)) [on types], states:

> This attribute, attached to a struct, union, or C++ class type definition, specifies that each of its members (other than zero-width bit-fields) is placed to minimize the memory required. **This is equivalent to specifying the packed attribute on each of the members**.

The key is resolving this indirection, and looking at the documentation
for __attribute__((packed)) [on fields (wierdly under "variables" section)]:

> The packed attribute specifies that a **structure member should have the smallest possible alignment** — one bit for a bit-field and one byte otherwise, unless a larger value is specified with the aligned attribute. The attribute does not apply to non-member objects.

Furthermore, alignment is the only effect of the packed attribute mentioned in the GCC docs (for "common" architecture).
Based on this, it seems safe to completely substitute C 'packed' with Zig 'align(1)'.
Target-specific or undocumented behavior potentially changes this.

Unfortunately, the current implementation of `translate-c` translates as
`packed struct` without alignment info.

Because Zig packed structs are really integers (as mentioned above),
they are the wrong interpretation and we should be using 'extern struct'.

Running `translate-c` on the following code:

```c
struct foo {
    char a;
    int b;
} __attribute__((packed));

struct bar {
    char a;
    int b;
    short c;
    __attribute__((aligned(8))) long d;
} __attribute__((packed));
```

Previously used a 'packed struct' (which was not FFI-safe on stage1).

After applying this change, the translated structures have align(1)
explicitly applied to all of their fields AS EXPECTED (unless explicitly overriden).
This makes Zig behavior for `tranlsate-c` consistent with clang/GCC.

Here is the newly produced (correct) output for the above example:

```zig
pub const struct_foo = extern struct {
    a: u8 align(1),
    b: c_int align(1),
};
pub const struct_bar = extern struct {
    a: u8 align(1),
    b: c_int align(1),
    c: c_short align(1),
    d: c_long align(8),
};
```

Also note for reference: Since the last stable release (0.9.1),
there was a change in the language spec
related to the alignment of packed structures.

The docs for Zig 0.9.1 read:
> Packed structs have 1-byte alignment.

So the old behavior of translate-c (not specifying any alignment) was possibly correct back then.

However the current docs read:

> Packed structs have the same alignment as their backing integer

Suggsestive both to the change to an integer-backed representation
which is incompatible with C's notation.
This commit is contained in:
Techcable 2022-09-04 17:16:26 -07:00
parent 34835bbbcf
commit 5b689d389f
No known key found for this signature in database
GPG Key ID: D7B98ADFF827CD17
6 changed files with 106 additions and 50 deletions
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6
src/clang.zig
View File

@ -470,6 +470,9 @@ pub const FieldDecl = opaque {
pub const getAlignedAttribute = ZigClangFieldDecl_getAlignedAttribute;
extern fn ZigClangFieldDecl_getAlignedAttribute(*const FieldDecl, *const ASTContext) c_uint;
pub const getPackedAttribute = ZigClangFieldDecl_getPackedAttribute;
extern fn ZigClangFieldDecl_getPackedAttribute(*const FieldDecl) bool;
pub const isAnonymousStructOrUnion = ZigClangFieldDecl_isAnonymousStructOrUnion;
extern fn ZigClangFieldDecl_isAnonymousStructOrUnion(*const FieldDecl) bool;
@ -1015,6 +1018,9 @@ pub const VarDecl = opaque {
pub const getAlignedAttribute = ZigClangVarDecl_getAlignedAttribute;
extern fn ZigClangVarDecl_getAlignedAttribute(*const VarDecl, *const ASTContext) c_uint;
pub const getPackedAttribute = ZigClangVarDecl_getPackedAttribute;
extern fn ZigClangVarDecl_getPackedAttribute(*const VarDecl) bool;
pub const getCleanupAttribute = ZigClangVarDecl_getCleanupAttribute;
extern fn ZigClangVarDecl_getCleanupAttribute(*const VarDecl) ?*const FunctionDecl;

77
src/translate_c.zig
View File

@ -878,7 +878,7 @@ fn visitVarDecl(c: *Context, var_decl: *const clang.VarDecl, mangled_name: ?[]co
.is_export = is_export,
.is_threadlocal = is_threadlocal,
.linksection_string = linksection_string,
.alignment = zigAlignment(var_decl.getAlignedAttribute(c.clang_context)),
.alignment = ClangAlignment.forVar(c, var_decl).zigAlignment(),
.name = var_name,
.type = type_node,
.init = init_node,
@ -1096,7 +1096,6 @@ fn transRecordDecl(c: *Context, scope: *Scope, record_decl: *const clang.RecordD
break :blk Tag.opaque_literal.init();
};
const is_packed = record_decl.getPackedAttribute();
var fields = std.ArrayList(ast.Payload.Record.Field).init(c.gpa);
defer fields.deinit();
@ -1153,7 +1152,7 @@ fn transRecordDecl(c: *Context, scope: *Scope, record_decl: *const clang.RecordD
const alignment = if (has_flexible_array and field_decl.getFieldIndex() == 0)
@intCast(c_uint, record_alignment)
else
zigAlignment(field_decl.getAlignedAttribute(c.clang_context));
ClangAlignment.forField(c, field_decl, record_def).zigAlignment();
if (is_anon) {
try c.decl_table.putNoClobber(c.gpa, @ptrToInt(field_decl.getCanonicalDecl()), field_name);
@ -1166,15 +1165,11 @@ fn transRecordDecl(c: *Context, scope: *Scope, record_decl: *const clang.RecordD
});
}
if (!c.zig_is_stage1 and is_packed) {
return failDecl(c, record_loc, name, "cannot translate packed record union", .{});
}
const record_payload = try c.arena.create(ast.Payload.Record);
record_payload.* = .{
.base = .{ .tag = ([2]Tag{ .@"struct", .@"union" })[@boolToInt(is_union)] },
.data = .{
.layout = if (is_packed) .@"packed" else .@"extern",
.layout = .@"extern",
.fields = try c.arena.dupe(ast.Payload.Record.Field, fields.items),
.functions = try c.arena.dupe(Node, functions.items),
.variables = &.{},
@ -1851,12 +1846,62 @@ fn transCStyleCastExprClass(
return maybeSuppressResult(c, scope, result_used, cast_node);
}
/// Clang reports the alignment in bits, we use bytes
/// Clang uses 0 for "no alignment specified", we use null
fn zigAlignment(bit_alignment: c_uint) ?c_uint {
if (bit_alignment == 0) return null;
return bit_alignment / 8;
}
/// The alignment of a variable or field
const ClangAlignment = struct {
/// Clang reports the alignment in bits, we use bytes
/// Clang uses 0 for "no alignment specified", we use null
bit_alignment: c_uint,
/// If the field or variable is marked as 'packed'
///
/// According to the GCC variable attribute docs, this impacts alignment
/// https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html
///
/// > The packed attribute specifies that a structure member
/// > should have the smallest possible alignment
///
/// Note also that specifying the 'packed' attribute on a structure
/// implicitly packs all its fields (making their alignment 1).
///
/// This will be null if the AST node doesn't support packing (functions)
is_packed: ?bool,
/// Get the alignment for a field, optionally taking into account the parent record
pub fn forField(c: *const Context, field: *const clang.FieldDecl, parent: ?*const clang.RecordDecl) ClangAlignment {
const parent_packed = if (parent) |record| record.getPackedAttribute() else false;
// NOTE: According to GCC docs, parent attribute packed implies child attribute packed
return ClangAlignment{
.bit_alignment = field.getAlignedAttribute(c.clang_context),
.is_packed = field.getPackedAttribute() or parent_packed,
};
}
pub fn forVar(c: *const Context, var_decl: *const clang.VarDecl) ClangAlignment {
return ClangAlignment{
.bit_alignment = var_decl.getAlignedAttribute(c.clang_context),
.is_packed = var_decl.getPackedAttribute(),
};
}
pub fn forFunc(c: *const Context, fun: *const clang.FunctionDecl) ClangAlignment {
return ClangAlignment{
.bit_alignment = fun.getAlignedAttribute(c.clang_context),
.is_packed = null, // not supported by GCC/clang (or meaningful),
};
}
/// Translate the clang alignment info into a zig alignment
///
/// Returns null if there is no special alignment info
pub fn zigAlignment(self: ClangAlignment) ?c_uint {
if (self.bit_alignment != 0) {
return self.bit_alignment / 8;
} else if (self.is_packed orelse false) {
return 1;
} else {
return null;
}
}
};
fn transDeclStmtOne(
c: *Context,
@ -1910,7 +1955,7 @@ fn transDeclStmtOne(
.is_export = false,
.is_threadlocal = var_decl.getTLSKind() != .None,
.linksection_string = null,
.alignment = zigAlignment(var_decl.getAlignedAttribute(c.clang_context)),
.alignment = ClangAlignment.forVar(c, var_decl).zigAlignment(),
.name = var_name,
.type = type_node,
.init = init_node,
@ -5054,7 +5099,7 @@ fn finishTransFnProto(
break :blk null;
};
const alignment = if (fn_decl) |decl| zigAlignment(decl.getAlignedAttribute(c.clang_context)) else null;
const alignment = if (fn_decl) |decl| ClangAlignment.forFunc(c, decl).zigAlignment() else null;
const explicit_callconv = if ((is_inline or is_export or is_extern) and cc == .C) null else cc;

15
src/zig_clang.cpp
View File

@ -1941,10 +1941,7 @@ const char* ZigClangVarDecl_getSectionAttribute(const struct ZigClangVarDecl *se
bool ZigClangRecordDecl_getPackedAttribute(const ZigClangRecordDecl *zig_record_decl) {
const clang::RecordDecl *record_decl = reinterpret_cast<const clang::RecordDecl *>(zig_record_decl);
if (record_decl->getAttr<clang::PackedAttr>()) {
return true;
}
return false;
return record_decl->hasAttr<clang::PackedAttr>();
}
unsigned ZigClangVarDecl_getAlignedAttribute(const struct ZigClangVarDecl *self, const ZigClangASTContext* ctx) {
@ -1985,6 +1982,16 @@ unsigned ZigClangFunctionDecl_getAlignedAttribute(const struct ZigClangFunctionD
return 0;
}
bool ZigClangVarDecl_getPackedAttribute(const struct ZigClangVarDecl *self) {
auto casted_self = reinterpret_cast<const clang::VarDecl *>(self);
return casted_self->hasAttr<clang::PackedAttr>();
}
bool ZigClangFieldDecl_getPackedAttribute(const struct ZigClangFieldDecl *self) {
auto casted_self = reinterpret_cast<const clang::FieldDecl *>(self);
return casted_self->hasAttr<clang::PackedAttr>();
}
ZigClangQualType ZigClangParmVarDecl_getOriginalType(const struct ZigClangParmVarDecl *self) {
return bitcast(reinterpret_cast<const clang::ParmVarDecl *>(self)->getOriginalType());
}

2
src/zig_clang.h
View File

@ -1101,6 +1101,8 @@ ZIG_EXTERN_C const struct ZigClangFunctionDecl *ZigClangVarDecl_getCleanupAttrib
ZIG_EXTERN_C unsigned ZigClangVarDecl_getAlignedAttribute(const struct ZigClangVarDecl *self, const ZigClangASTContext* ctx);
ZIG_EXTERN_C unsigned ZigClangFunctionDecl_getAlignedAttribute(const struct ZigClangFunctionDecl *self, const ZigClangASTContext* ctx);
ZIG_EXTERN_C unsigned ZigClangFieldDecl_getAlignedAttribute(const struct ZigClangFieldDecl *self, const ZigClangASTContext* ctx);
ZIG_EXTERN_C bool ZigClangVarDecl_getPackedAttribute(const struct ZigClangVarDecl *self);
ZIG_EXTERN_C bool ZigClangFieldDecl_getPackedAttribute(const struct ZigClangFieldDecl *self);
ZIG_EXTERN_C const struct ZigClangStringLiteral *ZigClangFileScopeAsmDecl_getAsmString(const struct ZigClangFileScopeAsmDecl *self);

26
test/run_translated_c.zig
View File

@ -250,20 +250,18 @@ pub fn addCases(cases: *tests.RunTranslatedCContext) void {
\\}
, "");
if (@import("builtin").zig_backend == .stage1) {
cases.add("struct initializer - packed",
\\#define _NO_CRT_STDIO_INLINE 1
\\#include <stdint.h>
\\#include <stdlib.h>
\\struct s {uint8_t x,y;
\\ uint32_t z;} __attribute__((packed)) s0 = {1, 2};
\\int main() {
\\ /* sizeof nor offsetof currently supported */
\\ if (((intptr_t)&s0.z - (intptr_t)&s0.x) != 2) abort();
\\ return 0;
\\}
, "");
}
cases.add("struct initializer - packed",
\\#define _NO_CRT_STDIO_INLINE 1
\\#include <stdint.h>
\\#include <stdlib.h>
\\struct s {uint8_t x,y;
\\ uint32_t z;} __attribute__((packed)) s0 = {1, 2};
\\int main() {
\\ /* sizeof nor offsetof currently supported */
\\ if (((intptr_t)&s0.z - (intptr_t)&s0.x) != 2) abort();
\\ return 0;
\\}
, "");
cases.add("cast signed array index to unsigned",
\\#include <stdlib.h>

30
test/translate_c.zig
View File

@ -728,22 +728,20 @@ pub fn addCases(cases: *tests.TranslateCContext) void {
\\}
});
if (builtin.zig_backend == .stage1) {
cases.add("struct initializer - packed",
\\struct {int x,y,z;} __attribute__((packed)) s0 = {1, 2};
, &[_][]const u8{
\\const struct_unnamed_1 = packed struct {
\\ x: c_int,
\\ y: c_int,
\\ z: c_int,
\\};
\\pub export var s0: struct_unnamed_1 = struct_unnamed_1{
\\ .x = @as(c_int, 1),
\\ .y = @as(c_int, 2),
\\ .z = 0,
\\};
});
}
cases.add("struct initializer - packed",
\\struct {int x,y,z;} __attribute__((packed)) s0 = {1, 2};
, &[_][]const u8{
\\const struct_unnamed_1 = extern struct {
\\ x: c_int align(1),
\\ y: c_int align(1),
\\ z: c_int align(1),
\\};
\\pub export var s0: struct_unnamed_1 = struct_unnamed_1{
\\ .x = @as(c_int, 1),
\\ .y = @as(c_int, 2),
\\ .z = 0,
\\};
});
// Test case temporarily disabled:
// https://github.com/ziglang/zig/issues/12055