[llvm-bugs] [Bug 42717] New: Make -thinlto-index-only work well with thin static archives

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42717

            Bug ID: 42717
           Summary: Make -thinlto-index-only work well with thin static
                    archives
           Product: lld
           Version: unspecified
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: All Bugs
          Assignee: akhuang at google.com
          Reporter: rnk at google.com
                CC: llvm-bugs at lists.llvm.org, llvm at inglorion.net,
                    peter.smith at linaro.org

Many build systems (such as LLVM's) are organized as collections of static
archives. Currently, the recommended way to use -thinlto-index-only to
distribute LTO backend actions is to use --start-lib / --end-lib on the command
line:
http://lists.llvm.org/pipermail/llvm-dev/2019-June/133145.html

To ease adoption of thinlto in legacy build systems, LLD should at least handle
thin archives, if not true static archives. There is a discussion of some of
these issues and how to solve them here:
https://reviews.llvm.org/D64461#1586526

It boils down to coming up with a good path for the .thinlto.bc file calculated
here:
http://llvm-cs.pcc.me.uk/lib/LTO/LTO.cpp#1206

With a very Chromium / LLVM perspective on things, I think the ideal index file
name would be something like:

${pathtoobj}/${object_noext}.o   # stage1 output
${pathtoobj}/${object_noext}.${binary_noext}.thinlto.bc # index file
${pathtoobj}/${object_noext}.${binary_noext}.imports  # import list
${pathtoobj}/${object_noext}.${binary_noext}.o        # native output

So, for chromium on Windows, we'd have:

obj/v8/v8_libbase/bits.obj
obj/v8/v8_libbase/bits.d8.thinlto.bc
obj/v8/v8_libbase/bits.d8.o
obj/v8/v8_libbase/bits.chrome_child.thinlto.bc
obj/v8/v8_libbase/bits.chrome_child.o
... one per thinlto binary

I (think?) today the current behavior is that every link will attempt to write
obj/v8/v8_libbase/bits.thinlto.bc, which won't work if the same object is
compiled into two binaries.

Amy expressed interest in working on this.

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