Re: [PATCH 2/2] x86, boot: Use regparm=0 for memcpy and memset when using clang

Ingo Molnar
2017-03-16T07:46:08Z

* Michael Davidson <md@google.com> wrote:

> Use the standard regparm=0 calling convention for memcpy and
> memset when building with clang.
>
> This is a work around for a long standing clang bug
> (see https://llvm.org/bugs/show_bug.cgi?id=3997) where
> clang always uses the standard regparm=0 calling convention
> for any implcit calls to memcpy and memset that it generates
> (eg for structure assignments and initialization) even if an
> alternate calling convention such as regparm=3 has been specified.
>
> Signed-off-by: Michael Davidson <md@google.com>
> ---
> arch/x86/boot/copy.S | 15 +++++++++++++--
> arch/x86/boot/string.h | 13 +++++++++++++
> 2 files changed, 26 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/boot/copy.S b/arch/x86/boot/copy.S
> index 1eb7d298b47d..57142d1ad0d2 100644
> --- a/arch/x86/boot/copy.S
> +++ b/arch/x86/boot/copy.S
> @@ -18,6 +18,12 @@
> .text
>
> GLOBAL(memcpy)
> +#ifdef __clang__ /* Use normal ABI calling conventions */
> + movw 4(%esp), %ax
> + movw 8(%esp), %dx
> + movw 12(%esp), %cx
> +#endif
> +_memcpy:
> pushw %si
> pushw %di
> movw %ax, %di
> @@ -34,6 +40,11 @@ GLOBAL(memcpy)
> ENDPROC(memcpy)
>
> GLOBAL(memset)
> +#ifdef __clang__ /* Use normal ABI calling conventions */
> + movw 4(%esp), %ax
> + movw 8(%esp), %dx
> + movw 12(%esp), %cx
> +#endif

I don't think we want to add such ugly workarounds for clang bugs in a piecemail
wise manner. If this was the _only_ workaround required to have a working kernel
built with clang then maybe, but there's no way to tell from this submission.

Thanks,

Ingo