On 2017-Jan-7, at 12:51 AM, Roman Divacky <rdivacky at vlakno.cz> wrote:
> That's a great progress. Can you produce minimal self contained test case that
> exhibits this bug? And submit it to llvm bugzilla?
>
> Also, clang3.9 defaults to using it's own internal asm, what happens if you
> add -no-integrated-as to CFLAGS and recompile the kernel? That should remove
> this llvm assembly problem. Does it boot?
>
> Thanks Mark, really great progress.
>
> Roman
In attempting this I found how to control the behavior based on
the assembler notation @toc being missing vs. being present.
If llvm should change is strongly tied to llvm's criteria for
gcc compatibility relative to filling-in/defaulting omitted
@toc's in the assembler notation.
FreeBSD has the option of always being explicit with @toc in order
to avoid differences in handling of omitted notation.
So I've no clue if FreebSD wants to claim that a llvm change
is a requirement for using clang as the powerpc64 system compiler.
[The issue of the distinction is submittable to llvm either way.]
Details. . .
For:
.section ".toc","aw"
tmpstk.L: .tc tmpstk[TC],tmpstk
. . .
/* Set up the stack pointer */
ld %r1,tmpstk.L(%r2)
using devel/powerpc64-gcc gets:
# /usr/local/bin/powerpc64-unknown-freebsd12.0-gcc \ -c \ -x assembler-with-cpp \ -pipe \ locore64_simplified.S
locore64_simplified.S: Assembler messages:
locore64_simplified.S:80: Warning: assuming @toc on symbol
and produces (with R_PPC64_TOC16_DS for .toc):
# /usr/local/powerpc64-freebsd/bin/objdump -r locore64_simplified.o
locore64_simplified.o: file format elf64-powerpc-freebsd
RELOCATION RECORDS FOR [.text]:
OFFSET TYPE VALUE
0000000000000028 R_PPC64_REL64 __tocbase+0x0000000000008000
0000000000000046 R_PPC64_TOC16_DS .toc
RELOCATION RECORDS FOR [.toc]:
OFFSET TYPE VALUE
0000000000000000 R_PPC64_ADDR64 tmpstk
RELOCATION RECORDS FOR [.opd]:
OFFSET TYPE VALUE
0000000000000000 R_PPC64_ADDR64 .__start
0000000000000008 R_PPC64_TOC *ABS*
By contrast clang is silent (cross compiler used):
# /usr/obj/powerpc64vtsc_clang_kernel/powerpc.powerpc64/usr/src/tmp/usr/bin/cc \ -target powerpc64-unknown-freebsd12.0 \ --sysroot=/usr/obj/powerpc64vtsc_clang_kernel/powerpc.powerpc64/usr/src/tmp \ -B/usr/obj/powerpc64vtsc_clang_kernel/powerpc.powerpc64/usr/src/tmp/usr/bin \ -c \ -x assembler-with-cpp \ -pipe \ locore64_simplified.S
and produces code with R_PPC64_ADDR16_DS for the .toc instead:
# /usr/local/powerpc64-freebsd/bin/objdump -r locore64_simplified.o | more
locore64_simplified.o: file format elf64-powerpc-freebsd
RELOCATION RECORDS FOR [.text]:
OFFSET TYPE VALUE
0000000000000028 R_PPC64_REL64 __tocbase+0x0000000000008000
0000000000000046 R_PPC64_ADDR16_DS .toc
RELOCATION RECORDS FOR [.toc]:
OFFSET TYPE VALUE
0000000000000000 R_PPC64_ADDR64 tmpstk
RELOCATION RECORDS FOR [.opd]:
OFFSET TYPE VALUE
0000000000000000 R_PPC64_ADDR64 .__start
0000000000000008 R_PPC64_TOC *ABS*
But for:
.section ".toc","aw"
tmpstk.L: .tc tmpstk[TC],tmpstk
. . .
/* Set up the stack pointer */
ld %r1,tmpstk.L at toc(%r2)
(note the @toc notation) both compilers agree and use
R_PPC64_TOC16_DS for the .toc:
# /usr/local/bin/powerpc64-unknown-freebsd12.0-gcc \ -c \ -x assembler-with-cpp \ -pipe \ locore64_simplified.S
# /usr/local/powerpc64-freebsd/bin/objdump -r locore64_simplified.o | more
locore64_simplified.o: file format elf64-powerpc-freebsd
RELOCATION RECORDS FOR [.text]:
OFFSET TYPE VALUE
0000000000000028 R_PPC64_REL64 __tocbase+0x0000000000008000
0000000000000046 R_PPC64_TOC16_DS .toc
RELOCATION RECORDS FOR [.toc]:
OFFSET TYPE VALUE
0000000000000000 R_PPC64_ADDR64 tmpstk
RELOCATION RECORDS FOR [.opd]:
OFFSET TYPE VALUE
0000000000000000 R_PPC64_ADDR64 .__start
0000000000000008 R_PPC64_TOC *ABS*
# /usr/obj/powerpc64vtsc_clang_kernel/powerpc.powerpc64/usr/src/tmp/usr/bin/cc \ -target powerpc64-unknown-freebsd12.0 \ --sysroot=/usr/obj/powerpc64vtsc_clang_kernel/powerpc.powerpc64/usr/src/tmp \ -B/usr/obj/powerpc64vtsc_clang_kernel/powerpc.powerpc64/usr/src/tmp/usr/bin \ -c \ -x assembler-with-cpp \ -pipe \ locore64_simplified.S
# /usr/local/powerpc64-freebsd/bin/objdump -r locore64_simplified.o | more
locore64_simplified.o: file format elf64-powerpc-freebsd
RELOCATION RECORDS FOR [.text]:
OFFSET TYPE VALUE
0000000000000028 R_PPC64_REL64 __tocbase+0x0000000000008000
0000000000000046 R_PPC64_TOC16_DS .toc
RELOCATION RECORDS FOR [.toc]:
OFFSET TYPE VALUE
0000000000000000 R_PPC64_ADDR64 tmpstk
RELOCATION RECORDS FOR [.opd]:
OFFSET TYPE VALUE
0000000000000000 R_PPC64_ADDR64 .__start
0000000000000008 R_PPC64_TOC *ABS*
I omitted "-f -gdwarf-2" to simplify things but with such
clang complains about:
locore64_simplified.S:36:2: warning: DWARF2 only supports one section per compilation unit
.section ".toc","aw"
^
locore64_simplified.S:47:2: warning: DWARF2 only supports one section per compilation unit
.section ".opd","aw"
^
(buildkernel gets such messages.)
I expect I can simplify the .S code more than I have so far but
I figured I'd report the discovery of the choice FreeBSD needs
to make for powerpc64 for if llvm changes are to be required
vs. not.
===
Mark Millard
markmi at dsl-only.net