On Mar 12, 2012, at 12:45 PM, kalth...@googlemail.com wrote:
> Is there a way to determine whether a FreeBSD-system was compiled with gcc or
> clang?
> I thought of some libs or so that might significantly differ.
It's fairly easy to determine whether assembly code was compiled with gcc or
clang from idioms they use-- GCC emits .ascii for strings and then adds a
trailing null; clang uses .asciz, for example. From that you can also figure
out whether a particular executable or shared library was compiled with one or
the other-- gcc is doing a leaf frame caller optimization, where it leave / jmp
to puts() (using the stack frame allocated for main()), whereas clang is doing
normal stack frame handling of %rpb and explicit return.
Regards,
--
-Chuck
% cat h.c
#include <stdio.h>
int main() {
puts("Hello, world!\n");
}
% gcc -S -O2 -o h-gcc.s h.c
% clang -S -O2 -o h-clang.s h.c
% cat h-gcc.s
.cstring
LC0:
.ascii "Hello, world!\12\0"
.text
.align 4,0x90
.globl _main
_main:
LFB3:
pushq %rbp
LCFI0:
movq %rsp, %rbp
LCFI1:
leaq LC0(%rip), %rdi
leave
jmp _puts
LFE3:
.section
__TEXT,__eh_frame,coalesced,no_toc+strip_static_syms+live_support
EH_frame1:
.set L$set$0,LECIE1-LSCIE1
.long L$set$0
LSCIE1:
.long 0x0
.byte 0x1
.ascii "zR\0"
.byte 0x1
.byte 0x78
.byte 0x10
.byte 0x1
.byte 0x10
.byte 0xc
.byte 0x7
.byte 0x8
.byte 0x90
.byte 0x1
.align 3
LECIE1:
.globl _main.eh
_main.eh:
LSFDE1:
.set L$set$1,LEFDE1-LASFDE1
.long L$set$1
LASFDE1:
.long LASFDE1-EH_frame1
.quad LFB3-.
.set L$set$2,LFE3-LFB3
.quad L$set$2
.byte 0x0
.byte 0x4
.set L$set$3,LCFI0-LFB3
.long L$set$3
.byte 0xe
.byte 0x10
.byte 0x86
.byte 0x2
.byte 0x4
.set L$set$4,LCFI1-LCFI0
.long L$set$4
.byte 0xd
.byte 0x6
.align 3
LEFDE1:
.subsections_via_symbols
% cat h-clang.s
.section __TEXT,__text,regular,pure_instructions
.globl _main
.align 4, 0x90
_main: ## @main
Leh_func_begin0:
## BB#0:
pushq %rbp
Ltmp0:
movq %rsp, %rbp
Ltmp1:
leaq L_.str(%rip), %rdi
callq _puts
xorl %eax, %eax
popq %rbp
ret
Leh_func_end0:
.section __TEXT,__cstring,cstring_literals
L_.str: ## @.str
.asciz "Hello, world!\n"
.section
__TEXT,__eh_frame,coalesced,no_toc+strip_static_syms+live_support
EH_frame0:
Lsection_eh_frame0:
Leh_frame_common0:
Lset0 = Leh_frame_common_end0-Leh_frame_common_begin0 ## Length of Common
Information Entry
.long Lset0
Leh_frame_common_begin0:
.long 0 ## CIE Identifier Tag
.byte 1 ## DW_CIE_VERSION
.asciz "zR" ## CIE Augmentation
.byte 1 ## CIE Code Alignment Factor
.byte 120 ## CIE Data Alignment Factor
.byte 16 ## CIE Return Address Column
.byte 1 ## Augmentation Size
.byte 16 ## FDE Encoding = pcrel
.byte 12 ## DW_CFA_def_cfa
.byte 7 ## Register
.byte 8 ## Offset
.byte 144 ## DW_CFA_offset + Reg (16)
.byte 1 ## Offset
.align 3
Leh_frame_common_end0:
.globl _main.eh
_main.eh:
Lset1 = Leh_frame_end0-Leh_frame_begin0 ## Length of Frame Information Entry
.long Lset1
Leh_frame_begin0:
Lset2 = Leh_frame_begin0-Leh_frame_common0 ## FDE CIE offset
.long Lset2
Ltmp2: ## FDE initial location
.quad Leh_func_begin0-Ltmp2
Lset3 = Leh_func_end0-Leh_func_begin0 ## FDE address range
.quad Lset3
.byte 0 ## Augmentation size
.byte 4 ## DW_CFA_advance_loc4
Lset4 = Ltmp0-Leh_func_begin0
.long Lset4
.byte 14 ## DW_CFA_def_cfa_offset
.byte 16 ## Offset
.byte 134 ## DW_CFA_offset + Reg (6)
.byte 2 ## Offset
.byte 4 ## DW_CFA_advance_loc4
Lset5 = Ltmp1-Ltmp0
.long Lset5
.byte 13 ## DW_CFA_def_cfa_register
.byte 6 ## Register
.align 3
Leh_frame_end0:
.subsections_via_symbols
...and here's a disassembly of main() from gcc:
_main:
0000000100000f10 pushq %rbp
0000000100000f11 movq %rsp,%rbp
0000000100000f14 leaq 0x00000039(%rip),%rdi
0000000100000f1b leave
0000000100000f1c jmp 0x100000f28 ; symbol stub for: _puts
...and clang:
_main:
0000000100000f10 pushq %rbp
0000000100000f11 movq %rsp,%rbp
0000000100000f14 leaq 0x00000039(%rip),%rdi
0000000100000f1b callq 0x100000f2a ; symbol stub for: _puts
0000000100000f20 xorl %eax,%eax
0000000100000f22 popq %rbp
0000000100000f23 ret
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