[LLVMdev] How to use clang -intergrated-as to compile cross-(os/target) assembly file.

Tue Oct 22 07:08:47 PDT 2013

For example, I execute the following command sequences:

H:\CI\bld\compilers\musl\src\math\i386>type sqrt.s
#.globl _sqrt
.global sqrt
#.type sqrt, at function
#_sqrt:
sqrt:   fldl 4(%esp)
        fsqrt
        fstsw %ax
        sub $12,%esp
        fld %st(0)
        fstpt (%esp)
        mov (%esp),%ecx
        and $0x7ff,%ecx
        cmp $0x400,%ecx
        jnz 1f
        and $0x200,%eax
        sub $0x100,%eax
        sub %eax,(%esp)
        fstp %st(0)
        fldt (%esp)
1:      add $12,%esp
        fstpl 4(%esp)
        fldl 4(%esp)
        ret

H:\CI\bld\compilers\musl\src\math\i386>type sqrt.c
float __cdecl sqrt(float z)
{
    return 0.0;
}
H:\CI\bld\compilers\musl\src\math\i386>clang  -integrated-as -c -v
--target=i686
-pc-win32 sqrt.s
clang version 3.4 (trunk)
Target: i686-pc-win32
Thread model: posix
 "C:\Program Files (x86)\LLVM 3.4.svn\bin\clang.exe" -cc1as -triple
i686-pc-win3
2 -filetype obj -main-file-name sqrt.s -target-cpu pentium4
-fdebug-compilation-
dir "H:\\CI\\bld\\compilers\\musl\\src\\math\\i386" -dwarf-debug-producer
clang
version 3.4 (trunk) -o sqrt.o sqrt.s

H:\CI\bld\compilers\musl\src\math\i386>llvm-objdump -s -r -t sqrt.o

sqrt.o: file format COFF-i386

Contents of section .text:
 0000 dd442404 d9fa9bdf e083ec0c d9c0db3c  .D$............<
 0010 248b0c24 81e1ff07 000081f9 00040000  $..$............
 0020 75122500 0200002d 00010000 290424dd  u.%....-....).$.
 0030 d8db2c24 83c40cdd 5c2404dd 442404c3  ..,$....\$..D$..
SYMBOL TABLE:
[ 0](sec  1)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .text
AUX scnlen 0x40 nreloc 0 nlnno 0 checksum 0x0 assoc 1 comdat 0
[ 2](sec  1)(fl 0x00)(ty   0)(scl   2) (nx 0) 0x00000000 sqrt

H:\CI\bld\compilers\musl\src\math\i386>clang --target=i686-pc-win32 -c
 sqrt.c
sqrt.c:1:15: warning: incompatible redeclaration of library function 'sqrt'
      [-Wincompatible-library-redeclaration]
float __cdecl sqrt(float z)
              ^
sqrt.c:1:15: note: 'sqrt' is a builtin with type 'double (double)'
1 warning generated.

H:\CI\bld\compilers\musl\src\math\i386>llvm-objdump -s -r -t sqrt.o

sqrt.o: file format COFF-i386

Contents of section .text:
 0000 5589e583 ec08f30f 1045080f 57c9f30f  U........E..W...
 0010 1145fcd9 eef30f11 4df883c4 085dc3    .E......M....].
SYMBOL TABLE:
[ 0](sec  1)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .text
AUX scnlen 0x1f nreloc 0 nlnno 0 checksum 0x0 assoc 1 comdat 0
[ 2](sec -1)(fl 0x00)(ty   0)(scl   3) (nx 0) 0x00000001 @feat.00
[ 3](sec  1)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 _sqrt

H:\CI\bld\compilers\musl\src\math\i386>
H:\CI\bld\compilers\musl\src\math\i386>clang  -integrated-as -c -v
--target=amd6
4-pc-win32 sqrt.s
clang version 3.4 (trunk)
Target: amd64-pc-win32
Thread model: posix
 "C:\Program Files (x86)\LLVM 3.4.svn\bin\clang.exe" -cc1as -triple
amd64-pc-win
32 -filetype obj -main-file-name sqrt.s -target-cpu x86-64
-fdebug-compilation-d
ir "H:\\CI\\bld\\compilers\\musl\\src\\math\\i386" -dwarf-debug-producer
clang v
ersion 3.4 (trunk) -o sqrt.o sqrt.s

H:\CI\bld\compilers\musl\src\math\i386>llvm-objdump -s -r -t sqrt.o

sqrt.o: file format COFF-x86-64

Contents of section .text:
 0000 67dd4424 04d9fa9b dfe083ec 0cd9c067  g.D$...........g
 0010 db3c2467 8b0c2481 e1ff0700 0081f900  .<$g..$.........
 0020 04000075 14250002 00002d00 01000067  ...u.%....-....g
 0030 290424dd d867db2c 2483c40c 67dd5c24  ).$..g.,$...g.\$
 0040 0467dd44 2404c3                      .g.D$..
SYMBOL TABLE:
[ 0](sec  1)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .text
AUX scnlen 0x47 nreloc 0 nlnno 0 checksum 0x0 assoc 1 comdat 0
[ 2](sec  1)(fl 0x00)(ty   0)(scl   2) (nx 0) 0x00000000 sqrt

H:\CI\bld\compilers\musl\src\math\i386>clang --target=amd64-pc-win32 -c
 sqrt.c
sqrt.c:1:15: warning: incompatible redeclaration of library function 'sqrt'
      [-Wincompatible-library-redeclaration]
float __cdecl sqrt(float z)
              ^
sqrt.c:1:15: note: 'sqrt' is a builtin with type 'double (double)'
1 warning generated.

H:\CI\bld\compilers\musl\src\math\i386>llvm-objdump -s -r -t sqrt.o

sqrt.o: file format COFF-x86-64

Contents of section .text:
 0000 554889e5 50f30f11 45fc0f57 c04883c4  UH..P...E..W.H..
 0010 085dc3                               .].
SYMBOL TABLE:
[ 0](sec  1)(fl 0x00)(ty   0)(scl   3) (nx 1) 0x00000000 .text
AUX scnlen 0x13 nreloc 0 nlnno 0 checksum 0x0 assoc 1 comdat 0
[ 2](sec  1)(fl 0x00)(ty  20)(scl   2) (nx 0) 0x00000000 sqrt

H:\CI\bld\compilers\musl\src\math\i386>

*
*
*My problem is:*

Because C __cdecl calling convention vary on x86/amd64 architecture under
win32.
for example, the sqrt function in is expressed as
float __cdecl sqrt(float)

but in the compiled object file, the final COFF symbol table:
on x86/win32(i686-pc-win32), it's named with "*_sqrt*"
on amd64/win32(amd64-pc-win32), it's named with "*sqrt*"

on linux(*-pc-linux), in ELF symbol table, it's named with "*sqrt*",

So I want to control the function name in assembly file, by the means of
Assembly macros,
but I don't know LLVM-MC support which kinds of macros to control the
assembly directives so that I can unified the assembly function name can
match the c function name  in object symbol table under different
CPU(x86/x64) and different OS(Win32/Linux/MacOS).
The simplest way is to duplicate the function name is to duplicate the
function name in two form:
.globl _sqrt
.global sqrt
#.type sqrt, at function
_sqrt:
sqrt:   fldl 4(%esp)
But that will leading redundant object symbol name.
besides, the* .type *didn't support under COFF symbol table, so how did I
use assembly macro to comment it out when we are targeting COFF object but
reserved when we are targeting ELF.
didn't know if Mach-O support for this directive.

-- 
         此致
礼
罗勇刚
Yours
    sincerely,
Yonggang Luo
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