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clang .code16 with -Os producing larger code that it needs to
From: |
Vladimir 'φ-coder/phcoder' Serbinenko |
Subject: |
clang .code16 with -Os producing larger code that it needs to |
Date: |
Fri, 20 Feb 2015 15:58:37 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Icedove/31.4.0 |
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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- clang .code16 with -Os producing larger code that it needs to,
Vladimir 'φ-coder/phcoder' Serbinenko <=
- Re: clang .code16 with -Os producing larger code that it needs to, Vladimir 'φ-coder/phcoder' Serbinenko, 2015/02/20
- Re: clang .code16 with -Os producing larger code that it needs to, David Woodhouse, 2015/02/20
- Re: clang .code16 with -Os producing larger code that it needs to, Vladimir 'φ-coder/phcoder' Serbinenko, 2015/02/20
- Re: clang .code16 with -Os producing larger code that it needs to, David Woodhouse, 2015/02/20
- Re: clang .code16 with -Os producing larger code that it needs to, David Woodhouse, 2015/02/20
- Re: [LLVMdev] clang .code16 with -Os producing larger code that it needs to, Rafael Espíndola, 2015/02/20
- Re: [LLVMdev] clang .code16 with -Os producing larger code that it needs to, David Woodhouse, 2015/02/23
- Re: [LLVMdev] clang .code16 with -Os producing larger code that it needs to, Craig Topper, 2015/02/24
- Re: [LLVMdev] clang .code16 with -Os producing larger code that it needs to, David Woodhouse, 2015/02/24