Re: [PATCH] i386: Update -mfunction-return= for return with pop

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Mon, Feb 26, 2018 at 2:58 AM, Jan Hubicka <hubicka@ucw.cz> wrote:
>> On Mon, Feb 26, 2018 at 2:05 AM, Martin Liška <mliska@suse.cz> wrote:
>> > On 02/25/2018 02:37 AM, H.J. Lu wrote:
>> >>       PR target/r84530
>> >
>> > Just a nit I've noticed:
>> >
>> > s/r84530/84530
>>
>> Fixed.
>>
>> --
>> H.J.
>
>> From f08b8721ed038cefcee5a0bb1329b90a2d322269 Mon Sep 17 00:00:00 2001
>> From: "H.J. Lu" <hjl.tools@gmail.com>
>> Date: Sat, 24 Feb 2018 05:05:49 -0800
>> Subject: [PATCH] i386: Update -mfunction-return= for return with pop
>>
>> When -mfunction-return= is used, simple_return_pop_internal should pop
>> return address into ECX register, adjust stack by bytes to pop from stack
>> and jump to the return thunk via ECX register.
>>
>> Tested on i686 and x86-64.
>>
>>       PR target/84530
>>       * config/i386/i386-protos.h (ix86_output_indirect_jmp): Remove
>>       the bool argument.
>>       (ix86_output_indirect_function_return): New prototype.
>>       (ix86_split_simple_return_pop_internal): Likewise.
>>       * config/i386/i386.c (indirect_return_via_cx): New.
>>       (indirect_return_via_cx_bnd): Likewise.
>>       (indirect_thunk_name): Handle return va CX_REG.
>>       (output_indirect_thunk_function): Create alias for
>>       __x86_return_thunk_[re]cx and __x86_return_thunk_[re]cx_bnd.
>>       (ix86_output_indirect_jmp): Remove the bool argument.
>>       (ix86_output_indirect_function_return): New function.
>>       (ix86_split_simple_return_pop_internal): Likewise.
>>       * config/i386/i386.md (*indirect_jump): Don't pass false
>>       to ix86_output_indirect_jmp.
>>       (*tablejump_1): Likewise.
>>       (simple_return_pop_internal): Change it to define_insn_and_split.
>>       Call ix86_split_simple_return_pop_internal to split it for
>>       -mfunction-return=.
>>       (simple_return_indirect_internal): Call
>>       ix86_output_indirect_function_return instead of
>>       ix86_output_indirect_jmp.
>
> It seems to make sense. Since LLVM has picked up our thunk names now,
> I wonder if they have same bug or someohow already assigned a name
> to this thunk. In the second case we probably ought to match it. Do
> you know what LLVM uses?
>

I don't think LLVM has implemented -mfunction-return=.   I disclosed
my -mindirect-branch= approach to LLVM last year.  At the time, they
wanted freedom in their implementation and didn't want to be compatible
with mine.   Later they added GCC compatibility so that LLVM may be
used to compile Linux kernel.  I believe it will be the same case if this
option is used to compile Linux kernel.

BTW, is my patch:

https://gcc.gnu.org/ml/gcc-patches/2018-01/msg02233.html

OK for trunk?  The review died at

https://gcc.gnu.org/ml/gcc-patches/2018-02/msg01355.html

Thanks.

-- 
H.J.