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Re: [PATCH] xen: Use asm inline when available for alternatives
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 22 Apr 2025 14:52:35 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 22 Apr 2025 12:52:47 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 22.04.2025 13:39, Andrew Cooper wrote:
> Compilers estimate the size of an asm() block for inlining purposes.
>
> Constructs such as ALTERNATIVE appear large due to the metadata, depsite often
> only being a handful of instructions. asm inline() overrides the estimation
> to identify the block as being small.
>
> This has a substantial impact on inlining decisions, expected to be for the
> better given that the compiler has a more accurate picture to work with.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
So this is effectively a generalized version of
https://lists.xen.org/archives/html/xen-devel/2022-08/msg00712.html
with ...
> --- a/xen/Kconfig
> +++ b/xen/Kconfig
> @@ -29,6 +29,10 @@ config LD_IS_GNU
> config LD_IS_LLVM
> def_bool $(success,$(LD) --version | head -n 1 | grep -q "^LLD")
>
> +config CC_HAS_ASM_INLINE
> + # GCC >= 9, Clang >= 11
> + def_bool $(success,echo 'void foo(void) { asm inline (""); }' | $(CC)
> -x c - -c -o /dev/null)
... detection once again done in Kconfig (only). I don't think I had any
feedback there; I'm hence having a hard time seeing why I should now like
this any better than mine was liked.
In any event it's clearly increasingly necessary that we finally get the
detection aspect settled.
Jan
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