>  I just have a doubt about the performance impact. Would there be any? Or
would the "constness" of these function pointers allow the compiler to
optimize and remove any indirection we'd get with mutable function pointers?

Both GCC and Clang can inline indirect function calls so long as they know that the function pointer is immutable for the lifetime of the program (i.e. const static). [https://gcc.godbolt.org/z/z78M57bq9]

Without IPO/LTO, however, the compiler cannot know the destination of the function pointer across compilation unit boundaries. [https://gcc.godbolt.org/z/WWKP6ncx6]

So with IPO/LTO I would expect to see little to no performance impact, but without it might be a bit more substantial. There aren’t many reasons not to use LTO outside of debugging, so this might be an acceptable performance cost. Size cost might be more problematic for debug images, but it’s difficult to estimate that.

Chris

From: Sandrine Bailleux via TF-A <tf-a@lists.trustedfirmware.org>
Date: Friday, 10 February 2023 at 12:16
To: Varun Wadekar <vwadekar@nvidia.com>, tf-a <tf-a@lists.trustedfirmware.org>
Subject: [TF-A] Re: Coding guidelines update: Discouraging usage of weak functions

Hi Varun,

On 2/9/23 16:20, Varun Wadekar wrote:
> Hi,
>
>>> Although we've not completely thought through all the details, we suspect we will have to isolate each strong implementation under plat/common/ in a separate file such that platforms can pick and choose just the files (and thus, just the functions) they need.
>
> [VW] It would be interesting to see if a function pointer based solution, similar to psci_ops, would be useful.

I see that Raghu raised the same idea in Gerrit, quoting his comment for
reference:

> Another alternative to providing interfaces is using const function pointers. People may barf at function pointers as a security issue, but conscious use of const function pointers that defines platform interfaces, that must be filled, is another way to get similar effect to weak functions without breaking security or potential for causing bugs.

I agree that *const* function pointers sound safe from a security point
of view.

I just have a doubt about the performance impact. Would there be any? Or
would the "constness" of these function pointers allow the compiler to
optimize and remove any indirection we'd get with mutable function pointers?

Best regards,
Sandrine
--
TF-A mailing list -- tf-a@lists.trustedfirmware.org
To unsubscribe send an email to tf-a-leave@lists.trustedfirmware.org