• Lynn Boger's avatar
    cmd/link: implement trampolines for ppc64le with ext linking · 738739f5
    Lynn Boger authored
    When using golang on ppc64le there have been issues
    when building executables that generate extremely large text
    sections.  This is due to the call instruction and the limitation
    on the offset field, which is smaller than most platforms.  If the
    size of the call target offset is too big for the offset field in
    the call instruction, then link errors can occur.
    
    The original solution to this problem in golang was to split the
    text section when it became too large, allowing the external (GNU)
    linker to insert the necessary stub to handle the long call.  That
    worked fine until the another size limit for the program size was hit,
    where a plt_branch was created instead of a long branch.  In that case
    the plt_branch code sequence expects r2 to contain the address of the
    TOC, but when golang creates dynamic executables by default
    (-buildmode=exe) r2 does not always contain the address of the TOC
    and as a result when building programs that reach this extremely
    large size, a runtime SEGV or SIGILL can occur due to branching to a bad
    address.
    
    When using internal linking, trampolines are generated to handle the
    long calls but the text sections are not split.  With this change,
    text sections will still be split approrpriately with external linking
    but if the buildmode being used does not maintain r2 as the TOC
    addresses, then trampolines will be created for those calls.
    
    Fixes #20497
    
    Change-Id: If5400b0f86c2c08e106b332be6db0b259b07d93d
    Reviewed-on: https://go-review.googlesource.com/45130
    Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: 's avatarCherry Zhang <cherryyz@google.com>
    738739f5
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