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SubjectRe: objtool warning "uses BP as a scratch register" with clang-9
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

The kernel configuration has frame pointers enabled:
$ make O=obj-x86 CC=clang-9 V=1 lib/crc32.o
...
clang-9 -Wp,-MD,lib/.crc32.o.d -nostdinc -isystem
/usr/lib/llvm-9/lib/clang/9.0.0/include -I../arch/x86/include
-I./arch/x86/include/generated -I../include -I./include
-I../arch/x86/include/uapi -I./arch/x86/include/generated/uapi
-I../include/uapi -I./include/generated/uapi -include
../include/linux/kconfig.h -include ../include/linux/compiler_types.h
-D__KERNEL__ -Qunused-arguments -Wall -Wundef
-Werror=strict-prototypes -Wno-trigraphs -fno-strict-aliasing
-fno-common -fshort-wchar -fno-PIE
-Werror=implicit-function-declaration -Werror=implicit-int
-Wno-format-security -std=gnu89 -no-integrated-as
-Werror=unknown-warning-option -mno-sse -mno-mmx -mno-sse2 -mno-3dnow
-mno-avx -m64 -mno-80387 -mstack-alignment=8 -mtune=generic
-mno-red-zone -mcmodel=kernel -DCONFIG_AS_CFI=1
-DCONFIG_AS_CFI_SIGNAL_FRAME=1 -DCONFIG_AS_CFI_SECTIONS=1
-DCONFIG_AS_SSSE3=1 -DCONFIG_AS_AVX=1 -DCONFIG_AS_AVX2=1
-DCONFIG_AS_AVX512=1 -DCONFIG_AS_SHA1_NI=1 -DCONFIG_AS_SHA256_NI=1
-Wno-sign-compare -fno-asynchronous-unwind-tables
-fno-delete-null-pointer-checks -Wno-address-of-packed-member -O2
-Wframe-larger-than=2048 -fstack-protector
-Wno-format-invalid-specifier -Wno-gnu -Wno-tautological-compare
-mno-global-merge -Wno-unused-const-variable -fno-omit-frame-pointer
-fno-optimize-sibling-calls -Wdeclaration-after-statement -Wvla
-Wno-pointer-sign -fno-strict-overflow -fno-merge-all-constants
-fno-stack-check -Werror=date-time -Werror=incompatible-pointer-types
-Wno-initializer-overrides -Wno-format -Wno-sign-compare
-Wno-format-zero-length -I ../lib -I ./lib
-DKBUILD_BASENAME='"crc32"' -DKBUILD_MODNAME='"crc32"' -c -o
lib/crc32.o ../lib/crc32.c
/bin/bash ../scripts/gen_ksymdeps.sh lib/crc32.o >> lib/.crc32.o.cmd
./tools/objtool/objtool check lib/crc32.o
lib/crc32.o: warning: objtool: crc32_le uses BP as a scratch register
lib/crc32.o: warning: objtool: __crc32c_le uses BP as a scratch register
lib/crc32.o: warning: objtool: crc32_be uses BP as a scratch register

I uploaded my .config file to https://pastebin.com/raw/dmQ1CsNs for reference.

I don't see anything unusual in the configuration, but it's interesting that
this configuration has more of those warnings that other configurations,
so there must be something to it. Here is a list of how many objtool warnings
I got in some recent randconfig builds:

$ find rand86/ -name \*success -mmin -300 | while read i ; do echo
`grep objtool $i | wc -l` $i ; done
0 rand86/0x694311F1-success
0 rand86/0x8D972A20-success
35 rand86/0x23203CF4-success
2 rand86/0xCBBF0EE0-success
2 rand86/0x4C14539-success
0 rand86/0xDAF4E782-success
0 rand86/0xC0A55734-success
0 rand86/0x66A9932-success
0 rand86/0x6EA0E018-success
3 rand86/0x39524E4C-success
0 rand86/0x1529A460-success
0 rand86/0xF8B3C01-success
0 rand86/0x3A38D796-success
1 rand86/0xBC23C91A-success
0 rand86/0xBB45D3A4-success
0 rand86/0xA907618F-success
0 rand86/0xE1FD6050-success
2 rand86/0x564C9F97-success
1 rand86/0x6B87B4EC-success
0 rand86/0x138BF05D-success
0 rand86/0xBEA383C4-success
4 rand86/0x13EDB348-success
0 rand86/0xDD0F59A6-success
1 rand86/0x1F629018-success
0 rand86/0x775CC509-success
8 rand86/0xC368A488-success
0 rand86/0x619AB1C8-success
0 rand86/0xF94B887A-success
197 rand86/0xBB7BE7F0-success
0 rand86/0x9C77525F-success
0 rand86/0x8FB025C4-success
50 rand86/0xA25C2C06-success
0 rand86/0xC96DF498-success
3 rand86/0xA9CE5E8F-success
0 rand86/0xC2F8A840-success
0 rand86/0xEAD8A021-success
0 rand86/0x5721D814-success
141 rand86/0x8259791A-success
0 rand86/0x9EB91B3D-success
0 rand86/0x5F1C100E-success
2 rand86/0xD132EF06-success
0 rand86/0x1C679BF4-success
0 rand86/0xE647B3E0-success
0 rand86/0xEA358480-success

Arnd

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