[LLVMdev] Linking tools

[Reid Kleckner]

On Mon, Jul 27, 2015 at 9:59 AM, Russell Wallace <russell.wallace at gmail.com>
wrote:

> On Mon, Jul 27, 2015 at 5:52 PM, Reid Kleckner <rnk at google.com> wrote:
>
>> I think your original description of the situation is accurate. llvm-link
>> will take multiple bitcode files and spit out a big ball o' bitcode, but
>> that's usually not sufficient for LTO, which is the main use case that we
>> want to support.
>>
>
> To be clear I understand you: the reason it's usually not sufficient is
> because most programs use build systems that don't really provide an
> opportunity for such a step; they assume the compiler only needs to be told
> about one source file at a time right up until machine code linking time?
>

In practice, it's not sufficient because there are usually pre-compiled
objects passed into the link step, and symbols from the bitcode are
referenced from those object files. Getting the precise list of symbols
that are actually referenced externally is a big part of the value of LTO.
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