Am 23.01.25 um 14:58 schrieb Richard Biener:
On Thu, Jan 23, 2025 at 2:23 PM Georg-Johann Lay <a...@gjlay.de> wrote:
Hi, this is Ping #2 for a patch from 2024.
It adds a new target hook that allows to output
assembly code for a VAR_DECL in a custom way.
The default action is an obvious no-op,
i.e. assemble_variable() behaves like before.
I tried to understand the AVR part of the series - there I fail to see
what exactly special handling "io" and friends requires and how
that was made work with the TLS noswitch section.
I do not think the sentence the middle-end doesn't allow custom
NOSWITCH sections is correct (it would be a matter of exporting
get_noswitch_section), but I also don't exactly see how
"io" and friends require a NOSWITCH.
A noswitch section is one way to go.
However, there is no way to attach a noswitch section to a VAR_DECL.
noswitch sections are selected by flags like DECL_COMMON or
DECL_INITIAL and command line options like -f[no-]common and
-f[no-]data-sections.
In the case of tls_comm_section, the section isn't attached to
a decl, but by means of flags like:
set_decl_section_name (decl, (const char *) nullptr);
set_decl_tls_model (decl, (tls_model) 2);
Anyway, all the noswitch stuff was only a hack since it was
the only way to do in in the backend alone.
A proper implementation of the feature required some changes in
varasm, and a hook like proposed seems a clear and sound approach.
The implementation of the new hook is basically a complete override
of assemble_variable which IMO is bad style at least. The missing
So what's a better approach? Hookizing assemble_variable altogether,
and putting the current assemble_variable code into the hook default?
Johann
requirements for "io" and friends should instead be made available
by other means (for example more targeted hooks).
That said, I can see how the patch works and doesn't affect anything
besides AVR.
Richard.
This hook is needed in the avr backend to properly implement
some variable attributes. The avr part has already been approved.
The patch is bootstrapped, and it tests without new regressions.
Ok for trunk?
Johann
--
Original submission
https://gcc.gnu.org/pipermail/gcc-patches/2025-January/673459.html
Ping #1
https://gcc.gnu.org/pipermail/gcc-patches/2025-January/673459.html
avr part and approval
https://gcc.gnu.org/pipermail/gcc-patches/2024-December/672051.html
https://gcc.gnu.org/pipermail/gcc-patches/2024-December/672084.html
--
Add new target hook TARGET_ASM_VARIABLE.
This patch adds a new target hook that allows the backend to asm output
a variable definition in its own way. This hook is needed because
varasm.cc imposes a very restrictive layout for all variable definitions
which will be basically ELF style (on ELF targets as least). To date,
there is no way for a backend to output a variable definition in a
different way.
This hook is required by the avr backend when it outputs definitions
for variables defined with the "io", "io_low" or "address" attribute that
don't follow ELF style. These attributes are basically symbol definitions
of the form
.global var_io
var_io = 32
with some additional assertions.
gcc/
* target.def (TARGET_ASM_OUT) <variable>: Add new DEFHOOK.
* targhooks.cc (default_asm_out_variable): New function.
* targhooks.h (default_asm_out_variable): New prototype.
* doc/tm.texi.in (TARGET_ASM_VARIABLE): Place hook documentation.
* doc/tm.texi: Rebuild.
* varasm.cc (assemble_variable): Call targetm.asm_out.variable
in order to allow the backend to output a variable definition
in its own style.
