[cfe-dev] ifunc resolver parameters

Peter Smith via cfe-dev cfe-dev at lists.llvm.org
Mon Oct 1 08:32:01 PDT 2018

I've raised https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87482 to see
if I can get some clarification about the intent of the documentation.

On some targets like Arm and AArch64 the glibc dynamic loader will
pass a parameter HWCAPS to the ifunc resolver function that the
resolver can use choose the function, whereas I believe it does not on
x86_64 where it is common for the ifunc resolver to check the cpu id
directly to get the same information.

In the Arm and AArch64 ports of glibc ifunc resolvers with parameters
are used but __attribute__((ifunc("resolver"))) isn't used. A somewhat
lower level method using inline assembly is used instead: asm (".type
func1, %gnu_indirect_function");

My personal view is that the error would be useful in the context of
(X86_64, glibc) but not in the case of (AArch64, glibc) or (Arm,
glibc), it is also possible that FreeBSD have chosen to pass a
parameter even in X86_64. So I think that the error message, if it is
there at all, should be conditional on a Target.

On Mon, 1 Oct 2018 at 14:42, David Blaikie via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> (+echristo in case he has some thoughts here)
> On Sun., 30 Sep. 2018, 6:57 am Dmitry Polukhin via cfe-dev, <cfe-dev at lists.llvm.org> wrote:
>> When I implemented ifunc I followed GCC documentation and was not aware of ifunc resolvers parameter. But if it is so much platform dependent and undocumented in GCC, I'm doubt if it should be supported in clang.
>> On Sat, Sep 29, 2018 at 11:38 PM Ed Maste <emaste at freebsd.org> wrote:
>>> When ifunc support was added to Clang with r265917
>>> http://llvm.org/viewvc/llvm-project?rev=265917&view=rev it did not
>>> allow resolvers to take parameters. It appears GCC documents ifunc
>>> resolvers as taking no parameters, but GCC in fact allows it.
>>> In FreeBSD our rtld passes machine-dependent CPU ID information to
>>> ifunc resolvers, and we had to remove the prohibition:
>>> https://svnweb.freebsd.org/changeset/base/339019.
>>> In the glibc world, as far as I can tell it in fact passes dl_hwcap on
>>> several architectures (but not on x86 or x86_64).
>>> I believe GCC's documentation is simply incorrect. Is there any reason
>>> not to just remove this check and error case from Clang?
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