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Subjectobjtool warning "uses BP as a scratch register" with clang-9
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
$ objdump -d crc32.o
0000000000000000 <fn1>:
0: 55 push %rbp
1: 53 push %rbx
2: 4c 8b 05 00 00 00 00 mov 0x0(%rip),%r8 # 9 <fn1+0x9>
9: 48 8b 05 00 00 00 00 mov 0x0(%rip),%rax # 10 <fn1+0x10>
10: 4c 39 c0 cmp %r8,%rax
13: 7e 7f jle 94 <fn1+0x94>
15: 48 8b 0d 00 00 00 00 mov 0x0(%rip),%rcx # 1c <fn1+0x1c>
1c: 48 8b 15 00 00 00 00 mov 0x0(%rip),%rdx # 23 <fn1+0x23>
23: 8b 1d 00 00 00 00 mov 0x0(%rip),%ebx # 29 <fn1+0x29>
29: 8b 35 00 00 00 00 mov 0x0(%rip),%esi # 2f <fn1+0x2f>
2f: 89 c7 mov %eax,%edi
31: 44 29 c7 sub %r8d,%edi
34: 40 f6 c7 01 test $0x1,%dil
38: 75 05 jne 3f <fn1+0x3f>
3a: 4c 89 c7 mov %r8,%rdi
3d: eb 15 jmp 54 <fn1+0x54>
3f: 0f b6 ff movzbl %bh,%edi
42: 8b 1c 99 mov (%rcx,%rbx,4),%ebx
45: 33 1c ba xor (%rdx,%rdi,4),%ebx
48: 31 f3 xor %esi,%ebx
4a: 89 1d 00 00 00 00 mov %ebx,0x0(%rip) # 50 <fn1+0x50>
50: 49 8d 78 01 lea 0x1(%r8),%rdi
54: 49 83 c0 01 add $0x1,%r8
58: 4c 39 c0 cmp %r8,%rax
5b: 74 30 je 8d <fn1+0x8d>
5d: 0f 1f 00 nopl (%rax)
60: 0f b6 ef movzbl %bh,%ebp
63: 89 db mov %ebx,%ebx
65: 8b 1c 99 mov (%rcx,%rbx,4),%ebx
68: 33 1c aa xor (%rdx,%rbp,4),%ebx
6b: 31 f3 xor %esi,%ebx
6d: 89 1d 00 00 00 00 mov %ebx,0x0(%rip) # 73 <fn1+0x73>
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
7c: 31 f3 xor %esi,%ebx
7e: 89 1d 00 00 00 00 mov %ebx,0x0(%rip) # 84 <fn1+0x84>
84: 48 83 c7 02 add $0x2,%rdi
88: 48 39 c7 cmp %rax,%rdi
8b: 7c d3 jl 60 <fn1+0x60>
8d: 48 89 3d 00 00 00 00 mov %rdi,0x0(%rip) # 94 <fn1+0x94>
94: 5b pop %rbx
95: 5d pop %rbp
96: c3 retq

This happens with clang-9 and clang-10 at the moment, but not clang-8.

Arnd

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