https://bugs.llvm.org/show_bug.cgi?id=46952
Bug ID: 46952
Summary: movl nearer, %ebx in .code16 block needs to have
16-bit displacement (required by grub)
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: i...@maskray.me
CC: craig.top...@gmail.com, llvm-bugs@lists.llvm.org,
llvm-...@redking.me.uk, spatel+l...@rotateright.com
grub cannot be assembled by integrated assembler because movl nearer, %ebx in a
.code16 block has 32-bit displacement instead of 16-bit. There is a
configure-time test catching MC's discrepancy
https://github.com/coreos/grub/commit/a0bf403f66dbaca4edd8e667bfc397dd91c8d71c
git clone https://github.com/coreos/grub
cd grub
% clang -c asm-tests/i386-pc.S
asm-tests/i386-pc.S:13:7: error: invalid .org offset '11' (at offset '14')
.org 11
^
asm-tests/i386-pc.S:13:7: error: invalid .org offset '11' (at offset '14')
.org 11
^
asm-tests/i386-pc.S:13:7: error: invalid .org offset '11' (at offset '14')
.org 11
^
The integrated assembler emits:
% llvm-mc -triple=i386 a.s --show-encoding --show-inst
.text
.code16
movl nearer, %ebx # encoding:
[0x67,0x66,0x8b,0x1d,A,A,A,A]
# fixup A - offset: 4, value: nearer,
kind: FK_Data_4
The expected GNU as behavior:
% as asm-tests/i386-pc.S -o a.o
% objdump -mi386 -Maddr16,data16 --start-address=6 -dr a.o L
a.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000006 <nearer-0x69>:
6: 66 8b 1e 00 00 mov 0x0,%ebx
9: R_X86_64_16 .text+0x6f
...
000000000000006f <nearer>:
...
0000000000000137 <far>:
...
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