On Tue, Aug 27, 2019 at 09:00:52PM +0200, Arnd Bergmann wrote: > On Tue, Aug 27, 2019 at 5:00 PM Ilie Halip <[email protected]> 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. -- Josh
