[all-commits] [llvm/llvm-project] ebbc5d: [M68k] Correctly emit non-pic relocations (#89863)

[Peter Lafreniere via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: ebbc5de7db45b2fc81564a6c870a57f4b95d0477
      https://github.com/llvm/llvm-project/commit/ebbc5de7db45b2fc81564a6c870a57f4b95d0477
  Author: Peter Lafreniere <peter at n8pjl.ca>
  Date:   2024-05-03 (Fri, 03 May 2024)

  Changed paths:
    M llvm/lib/Target/M68k/M68kSubtarget.cpp
    M llvm/test/CodeGen/M68k/Arith/divide-by-constant.ll
    M llvm/test/CodeGen/M68k/Arith/imul.ll
    M llvm/test/CodeGen/M68k/Arith/mul64.ll
    M llvm/test/CodeGen/M68k/Arith/sdiv-exact.ll
    M llvm/test/CodeGen/M68k/Arith/smul-with-overflow.ll
    M llvm/test/CodeGen/M68k/Arith/sub-with-overflow.ll
    M llvm/test/CodeGen/M68k/Atomics/cmpxchg.ll
    M llvm/test/CodeGen/M68k/Atomics/load-store.ll
    M llvm/test/CodeGen/M68k/Atomics/rmw.ll
    M llvm/test/CodeGen/M68k/CodeModel/medium-static.ll
    M llvm/test/CodeGen/M68k/CodeModel/small-static.ll
    M llvm/test/CodeGen/M68k/TLS/tlsie.ll
    M llvm/test/CodeGen/M68k/TLS/tlsle.ll
    M llvm/test/CodeGen/M68k/gcc_except_table.ll

  Log Message:
  -----------
  [M68k] Correctly emit non-pic relocations (#89863)

The m68k backend will always emit external calls (including libcalls)
with
PC-relative PLT relocations, even when in non-pic mode or -fno-plt is
used.

This is unexpected, as other function calls are emitted with absolute
addressing, and a static code modes suggests that there is no PLT. It
also
leads to a miscompilation where the call instruction emitted expects an
immediate address, while the relocation emitted for that instruction is
PC-relative.

This miscompilation can even be seen in the default C function in
godbolt:
https://godbolt.org/z/zEoazovzo

Fix the issue by classifying external function references based upon the
pic
mode. This triggers a change in the static code model, making it more in
line
with the expected behaviour and allowing use of this backend in more
bare-metal
situations where a PLT does not exist.

The change avoids the issue where we emit a PLT32 relocation for an
absolute
call, and makes libcalls and other external calls use absolute
addressing modes
when a static code model is desired.

Further work should be done in instruction lowering and validation to
ensure
that miscompilations of the same type don't occur.



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