Messages in this thread | | | Date | Tue, 27 Aug 2019 09:51:02 -0500 | From | Josh Poimboeuf <> | Subject | Re: objtool warning "uses BP as a scratch register" with clang-9 |
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On Tue, Aug 27, 2019 at 02:30:06PM +0200, Arnd Bergmann wrote: > I upgraded to the latest clang-9 snapshot from http://apt.llvm.org/ today. > Many problems are fixed, but I still get tons of warnings like > > arch/x86/kernel/cpu/mtrr/generic.o: warning: objtool: > mtrr_type_lookup_variable uses BP as a scratch register > arch/x86/kernel/process.o: warning: objtool: get_tsc_mode()+0x21: call > without frame pointer save/setup > arch/x86/kernel/early_printk.o: warning: objtool: early_vga_write uses > BP as a scratch register > arch/x86/kernel/sysfb_simplefb.o: warning: objtool: parse_mode uses BP > as a scratch register > arch/x86/kernel/head64.o: warning: objtool: __startup_64 uses BP as a > scratch register > kernel/time/timeconv.o: warning: objtool: time64_to_tm uses BP as a > scratch register > kernel/trace/ring_buffer.o: warning: objtool: > ring_buffer_discard_commit uses BP as a scratch register > ... > > I created a reduced test case: > > $ cat crc32.i > typedef unsigned u32; > long a, c; > u32 b, f; > u32 *d, *e; > void fn1() { > u32 *g = &f, *h = e, *i = d; > for (; a < c; a++) > b = i[b >> 8 & 255] ^ h[b] ^ g[5]; > } > $ clang-9 -c crc32.i -O2 ; objtool check crc32.o > crc32.o: warning: objtool: fn1 uses BP as a scratch register
I assume your config has CONFIG_UNWINDER_FRAME_POINTER=y, since objtool is doing frame pointer checking. Though I'm not sure about that, since I don't see -fno-omit-frame-pointer on the reduced clang command line. Do you still see this warning with -fno-omit-frame-pointer (assuming clang has that option)?
-- Josh
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