On Wed, 22 Jan 2020 06:36:17 +0900
rgc <[email protected]> wrote:
> --- gcc4-obj/locore.s Wed Jan 22 04:29:10 2020
> +++ clang-obj/locore.s Wed Jan 22 04:29:05 2020
> ...
You might have found a problem with mftb (move from time base) in
clang's assembler. Some of the other differences in your disassembly
might not cause problems.
This is a sample of the differences:
$ cat sample.s
lwz %r31,battable+4@l(%r31)
mftb %r28
bla s_dsitrap
bc 4,17,s_trap
addi %r30,%r30,idledone@l
nop
s_dsitrap:
nop
cpu_switchto_asm:
nop
idledone:
nop
$ gcc -c sample.s
$ clang -c -o sample-clang.o sample.s
$ objdump -d sample.o > sample.ds
$ objdump -d sample-clang.o > sample-clang.ds
fishport$ diff -u sample.ds sample-clang.ds
...
00000000 <s_dsitrap-0x18>:
- 0: 83 ff 00 04 lwz r31,4(r31)
- 4: 7f 8c 42 e6 mftb r28
- 8: 48 00 00 1b bla 18 <s_dsitrap>
+ 0: 83 ff 00 00 lwz r31,0(r31)
+ 4: 7f 8c 42 a6 mfspr r28,268
+ 8: 48 00 00 03 bla 0 <s_dsitrap-0x18>
c: 40 91 00 10 ble- cr4,1c <s_trap>
- 10: 3b de 00 20 addi r30,r30,32
+ 10: 3b de 00 00 addi r30,r30,0
14: 60 00 00 00 nop
...
The change from 4 to 0 in 'lwz r31,4(r31)' is not significant if the
relocation for 'battable+4@l' overwrites the 4 or 0 with a different
value. I see that gcc and clang use the same relocations:
=begin
$ readelf -r sample.o sample-clang.o
File: sample.o
Relocation section '.rela.text' at offset 0x270 contains 3 entries:
Offset Info Type Sym.Value Sym. Name + Addend
00000002 00000704 R_PPC_ADDR16_LO 00000000 battable + 4
00000008 00000102 R_PPC_ADDR24 00000000 .text + 18
00000012 00000104 R_PPC_ADDR16_LO 00000000 .text + 20
File: sample-clang.o
Relocation section '.rela.text' at offset 0xc8 contains 3 entries:
Offset Info Type Sym.Value Sym. Name + Addend
00000002 00000604 R_PPC_ADDR16_LO 00000000 battable + 4
00000008 00000502 R_PPC_ADDR24 00000000 .text + 18
00000012 00000504 R_PPC_ADDR16_LO 00000000 .text + 20
=end
Both gcc and clang use 'battable + 4' as the relocation for the
'battable+4@l' in the source code, so the 4 is not lost. Each
relocation has a different symbol index (Info >> 8, see ELF32_R_SYM in
<sys/exec_elf.h>) because gcc and clang wrote the symbols in a
different order (readelf -s *.o), but the symbols 'battable' and
'.text' are the same.
Branches with predictions like bge- beq+ bne- might differ, because
newer versions of the Power ISA changed how to set the branch
prediction bits (but tried to be backward-compatible).
I recall that mftb (move from time base) is different between 32-bit
and 64-bit PowerPC, and suspect that clang might be using the wrong
mftb for our 32-bit target, but I won't know until I check the ISA
manual.
--George
