[PATCH v3 1/2] Makefile: Fix -mno-save-restore compile warning with CLANG

[Jessica Clarke]

On 2 Dec 2021, at 14:28, Xiang W <wxjstz at 126.com> wrote:
> 
> 在 2021-12-02星期四的 17:59 +0530，Anup Patel写道：
>> The riscv target of CLANG does not support -m(no-)save-restore option
>> so we get compile warnings. This patch fixes compile warning by using
>> -m(no-)save-restore option only for GCC.
>> 
>> Signed-off-by: Anup Patel <anup.patel at wdc.com>
> 
> ➜  /tmp clang -target riscv64 -mno-save-restore -O2 -S test.c
> ➜  /tmp clang -target riscv64 -msave-restore -O2 -S test.c 
> ➜  /tmp clang --version                                      
> Debian clang version 11.0.1-2
> Target: x86_64-pc-linux-gnu
> Thread model: posix
> InstalledDir: /usr/bin
> ➜  /tmp lsb_release -a
> No LSB modules are available.
> Distributor ID:	Debian
> Description:	Debian GNU/Linux 11 (bullseye)
> Release:	11
> Codename:	bullseye
> ➜  /tmp
> 
> clang supports -m(no-)save-restore option on my computer

Yes, Clang 11 and above support -m(no-)save-restore. Clang 9 and 10
support the driver option but were buggy and would cause the backend to
give several warnings even if you just gave -mno-save-restore
(https://reviews.llvm.org/D64008, which did make it into Clang 9, was
not a complete fix for that, see my comment there). Clang 8 and below
didn’t support the option in the driver, but the RISCV backend only
graduated from experimental status in Clang 9 so, whilst it was in
relatively decent shape before then, such versions should probably be
discouraged.

In theory not passing the option is fine, there isn’t currently a way
to configure Clang to default to -msave-restore other than patching it,
though only skipping it for Clang 10 and below would be preferable.
Either way the commit message needs updating.

Or you could just say it’s not worth caring and leave the warnings for
Clang 9 and 10; with time they’ll stop being relevant, and they’re
stderr output, not real diagnostics, so -Werror doesn’t make them fatal.
That’s probably the approach I’d take, but I don’t have to work with
old versions of Clang on RISC-V.

Jess