[llvm-dev] linkonce expected behavior?

[Andrew Kelley via llvm-dev]

On Tue, Oct 24, 2017 at 9:27 PM, Vedant Kumar <vsk at apple.com> wrote:

> Unreferenced linkonce globals are allowed to be discarded.
>
>
> Is __udivmodti4 referenced?
>


It's referenced by a different .o file, but nothing within the module.

My confusion comes from the missing direct object in the sentence.
Referenced by a function local to the module? Referenced by any object in
the linker job?


>
> vedant
>
> On Oct 24, 2017, at 6:09 PM, Andrew Kelley via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> http://llvm.org/docs/LangRef.html#linkage-types says:
>
> Globals with “linkonce” linkage are merged with other globals of the same
> name when linkage occurs. This can be used to implement some forms of
> inline functions, templates, or other code which must be generated in each
> translation unit that uses it, but where the body may be overridden with a
> more definitive definition later. Unreferenced linkonce globals are allowed
> to be discarded. Note that linkonce linkage does not actually allow the
> optimizer to inline the body of this function into callers because it
> doesn’t know if this definition of the function is the definitive
> definition within the program or whether it will be overridden by a
> stronger definition. To enable inlining and other optimizations, use
> “linkonce_odr” linkage.
>
> This makes me think that linkonce globals will not be optimized out of an
> .o file, but they can be deleted by the linker. Here's some IR I have:
>
> define linkonce i128 @__udivmodti4(i128, i128, i128*) local_unnamed_addr
> #0 !dbg !264 {
> ....
> }
>
> When I run:
>
> opt -O3 -S compiler_rt.ll
>
> It deletes the function. Is this expected behavior?
>
> Is there a linkage type that lets the linker merge functions of the same
> name but does not let the optimizer delete the function?
>
> (let me know if providing the entire .ll file would be helpful)
>
> Regards,
> Andrew Kelley
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>
>
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