[llvm-bugs] [Bug 44138] New: please consider using a 128 bit fast build-id

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https://bugs.llvm.org/show_bug.cgi?id=44138

            Bug ID: 44138
           Summary: please consider using a 128 bit fast build-id
           Product: lld
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: ELF
          Assignee: unassignedbugs at nondot.org
          Reporter: rafael at espindo.la
                CC: llvm-bugs at lists.llvm.org, peter.smith at linaro.org

lld defaults to a fast build-id hash (which is currently xxhash). That produces
a 64 bit hash.

The other hash options are md5 and sha1, which are probably a bit too slow to
be the default. Linking a release version of scylla with split dwarf I get:

none 2.022252056 seconds
fast 2.061659922 seconds
md5  2.740843349 seconds
sha1 2.318114449 seconds

One issue with fast producing a 64 bit hash is that rpm considers it too small.
I initially reported it as a problem in rpm
(https://github.com/rpm-software-management/rpm/issues/950), but there is an
argument why given the birthday problem and the desire to have a small
probability of a collision on an entire distro 64 bits might not be enough.

I wonder if there is 128 bit hash that would be fast enough to be the default. 
Farmhash comes to mind.

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