On Tue, Aug 27, 2019 at 12:47 PM Arnd Bergmann <a...@arndb.de> wrote: > > On Tue, Aug 27, 2019 at 9:23 PM Josh Poimboeuf <jpoim...@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.ha...@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
