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 makes translate-c lower discards as `_ = @TypeOf(foo);` to avoid
tripping the "pointless discard" error.
Ideally, translate-c would avoid emitting pointless discards, in which
case this commit can be reverted, however, that is a separate
enhancement.
* Fix incorrect result when the first digit after the decimal point is not 0-9 - eg 0x0.ap0
* Fix compiler panic when the number starts with `0X` with a capital `X` - eg 0X0p0
* Fix compiler panic when the number has a decimal point immediately after `0x` - eg 0x.0p0
#8589 introduced correct handling of signed (possibly negative) array access
of pointers. Since unadorned integer literals in C are signed, this resulted
in inefficient generated code when indexing a pointer by a non-negative
integer literal.
Some macros (for example any macro that uses token pasting) cannot be
directly translated to Zig, but may nevertheless still admit a Zig
implementation. This provides a mechanism for matching macros against
templates and mapping them to functions implemented in c_translation.zig.
A macro matches a template if it contains the same sequence of tokens, except
that the name and parameters may be renamed. No attempt is made to
semantically analyze the macro. For example the following two macros are
considered equivalent:
```C
```
But the following two are not:
```C
```
Use `@` syntax to escape `_` when used as an identifier.
Remove the stage1 astgen prohibition against assigning from `_`
Note: there a few stage1 bugs preventing `_` from being used as an identifier
for a local variable or function parameter; these will be fixed by stage2.
They are unlikely to arise in real C code since identifiers starting with
underscore are reserved for the implementation.
Translate enum types as the underlying integer type. Translate enum constants
as top-level integer constants of the correct type (which does not necessarily
match the enum integer type).
If an enum constant's type cannot be translated for some reason, omit it.
See discussion https://github.com/ziglang/zig/issues/2115#issuecomment-827968279Fixes#9153
Don't move static local variables into the top-level scope since this
can cause name clashes if subsequently-defined variables or parameters
in different scopes share the name.
Instead, use a variable within a struct so that the variable's lexical
scope does not change. This solution was suggested by @LemonBoy
Note that a similar name-shadowing problem exists with `extern` variables
declared within block scope, but a different solution will be needed since
they do need to be moved to the top-level scope and we can't rename them.
