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    SubjectRe: objtool warning "uses BP as a scratch register" with clang-9
    On Tue, Aug 27, 2019 at 12:47 PM Arnd Bergmann <arnd@arndb.de> wrote:
    >
    > On Tue, Aug 27, 2019 at 9:23 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
    > > On Tue, Aug 27, 2019 at 09:00:52PM +0200, Arnd Bergmann wrote:
    > > > On Tue, Aug 27, 2019 at 5:00 PM Ilie Halip <ilie.halip@gmail.com> wrote:
    > > > >
    > > > > > > $ clang-9 -c crc32.i -O2 ; objtool check crc32.o
    > > > > > > crc32.o: warning: objtool: fn1 uses BP as a scratch register
    > > > >
    > > > > Yes, I see it too. https://godbolt.org/z/N56HW1
    > > > >
    > > > > > Do you still see this warning with -fno-omit-frame-pointer (assuming
    > > > > > clang has that option)?
    > > > >
    > > > > Using this makes the warning go away. Running objtool with --no-fp
    > > > > also gets rid of it.
    > > >
    > > > I still see the warning after adding back the -fno-omit-frame-pointer
    > > > in my reduced test case:
    > > >
    > > > $ clang-9 -c crc32.i -Werror -Wno-address-of-packed-member -Wall
    > > > -Wno-pointer-sign -Wno-unused-value -Wno-constant-logical-operand -O2
    > > > -Wno-unused -fno-omit-frame-pointer
    > > > $ objtool check crc32.o
    > > > crc32.o: warning: objtool: fn1 uses BP as a scratch register
    > >
    > > This warning most likely means that clang is clobbering RBP in leaf
    > > functions. With -fno-omit-frame-pointer, leaf functions don't need to
    > > set up the frame pointer, but they at least need to leave RBP untouched,
    > > so that an interrupts/exceptions can unwind through the function.
    >
    > Yes, that clearly matches what I see in the output where it does
    >
    > 0: 55 push %rbp
    > ...
    > 73: 0f b6 ef movzbl %bh,%ebp
    > 76: 8b 1c 99 mov (%rcx,%rbx,4),%ebx
    > 79: 33 1c aa xor (%rdx,%rbp,4),%ebx
    > ...
    > 95: 5d pop %rbp
    > 96: c3 retq
    >
    > I just did another simple test: an x86-64 defconfig build with
    > UNWINDER_FRAME_POINTER shows the exact symptom as
    > my randconfig, so it sounds like any configuration with frame
    > pointers would, and there is nothing else to it (this also makes
    > sense given that it happens with a relatively simple test case
    > outside of the kernel).
    >
    > Arnd

    Thanks for the description of the issue and the reduced test case. It
    almost reminds me of
    https://github.com/ClangBuiltLinux/linux/issues/612.

    I've filed https://bugs.llvm.org/show_bug.cgi?id=43128, anything I
    should add to the bug report?

    --
    Thanks,
    ~Nick Desaulniers

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