When experimenting with compiling GRUB2 with clang using integrated as,
I found out that it generates a 16-bit code bigger than gas counterpart
and result gets too big for size constraints of bootsector. This was
traced mainly to 2 problems.
32-bit access to 16-bit addresses.
source:
movl LOCAL(kernel_sector), %ebx
movl %ebx, 8(%si)
clang:
7cbc: 67 66 8b 1d 5c 7c 00 addr32 mov 0x7c5c,%ebx
7cc3: 00
7cc4: 66 89 5c 08 mov %ebx,0x8(%si)
gas:
7cbc: 66 8b 1e 5c 7c mov 0x7c5c,%ebx
7cc1: 66 89 5c 08 mov %ebx,0x8(%si)
32-bit jump.
source:
jnb LOCAL(floppy_probe)
clang:
+ 7cb5: 66 0f 83 07 01 00 00 jae 7dc3 <L_floppy_probe>
gas:
- 7cb5: 0f 83 0a 01 jae 7dc3 <L_floppy_probe>
The last one is particularly problematic as it never makes sense to
issue 32-bit jump if %ip is only 16 bits and it eats 3 extra bytes per
jump. Is it possible to force clang to generate 16-bit jumps?
On bright side if I remove error strings the code is functional.
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