Xcode7でのembed-bitcodeオプション

Last updated at 2015-11-13Posted at 2015-09-05

こちらからの続きです。
LLVM bitcode基礎知識
http://qiita.com/gamako/items/f37dbb05de9d3832ce6b

Xcode7からデフォルトで有効になったembed-bitcodeオプションがどんな働きをしているか見てみましょう。

ふつうのoファイル

まず前回と同じHelloWorldのソースコードから、ふつうにoファイルを出力してみます。

clang -c sample.c -o sample.o

中身はバイナリなので、hexdumpしてみます。

% hexdump -C -v sample.o
00000000 cf fa ed fe 07 00 00 01 03 00 00 00 01 00 00 00 |................|
00000010 04 00 00 00 00 02 00 00 00 20 00 00 00 00 00 00 |......... ......|
00000020 19 00 00 00 88 01 00 00 00 00 00 00 00 00 00 00 |................|
00000030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000040 98 00 00 00 00 00 00 00 20 02 00 00 00 00 00 00 |........ .......|
00000050 98 00 00 00 00 00 00 00 07 00 00 00 07 00 00 00 |................|
00000060 04 00 00 00 00 00 00 00 5f 5f 74 65 78 74 00 00 |........__text..|

...

00000300 00 5f 6d 61 69 6e 00 5f 70 72 69 6e 74 66 00 4c |._main._printf.L|
00000310 5f 2e 73 74 72 00 00 00 |_.str...|
00000318

llvm-objdumpコマンドで、もう少し人間に優しい出力をしてくれます。当たり前ですが、この中にbitcodeの情報は含まれていません。

% /usr/local/Cellar/llvm/3.6.1/bin/llvm-objdump -private-headers -section-headers -disassemble -s sample.o

sample.o:	file format Mach-O 64-bit x86-64

Disassembly of section __TEXT,__text:
_main:
 0:	55 	pushq	%rbp
 1:	48 89 e5 	movq	%rsp, %rbp
 4:	48 83 ec 10 	subq	$16, %rsp
 8:	48 8d 3d 00 00 00 00 	leaq	(%rip), %rdi
 f:	c7 45 fc 00 00 00 00 	movl	$0, -4(%rbp)
 16:	b0 00 	movb	$0, %al
 18:	e8 00 00 00 00 	callq	0
 1d:	31 c9 	xorl	%ecx, %ecx
 1f:	89 45 f8 	movl	%eax, -8(%rbp)
 22:	89 c8 	movl	%ecx, %eax
 24:	48 83 c4 10 	addq	$16, %rsp
 28:	5d 	popq	%rbp
 29:	c3 	retq
Sections:
Idx Name Size Address Type
 0 __text 0000002a 0000000000000000 TEXT
 1 __cstring 0000000e 000000000000002a DATA
 2 __compact_unwind 00000020 0000000000000038 DATA
 3 __eh_frame 00000040 0000000000000058 DATA
Contents of section __text:
 0000 554889e5 4883ec10 488d3d00 000000c7 UH..H...H.=.....
 0010 45fc0000 0000b000 e8000000 0031c989 E............1..
 0020 45f889c8 4883c410 5dc3 E...H...].
Contents of section __cstring:
 002a 48656c6c 6f2c2057 6f726c64 0a00 Hello, World..
Contents of section __compact_unwind:
 0038 00000000 00000000 2a000000 00000001 ........*.......
 0048 00000000 00000000 00000000 00000000 ................
Contents of section __eh_frame:
 0058 14000000 00000000 037a5200 01781001 .........zR..x..
 0068 100c0708 90010000 24000000 1c000000 ........$.......
 0078 88ffffff ffffffff 2a000000 00000000 ........*.......
 0088 00410e10 8602430d 06000000 00000000 .A....C.........
Mach header
 magic cputype cpusubtype caps filetype ncmds sizeofcmds flags
MH_MAGIC_64 X86_64 ALL 0x00 OBJECT 4 512 SUBSECTIONS_VIA_SYMBOLS
Load command 0

...

osxのotoolコマンドでも中身をみることができます。

% otool -hfavltVdoj sample.o
sample.o:
Mach header
 magic cputype cpusubtype caps filetype ncmds sizeofcmds flags
MH_MAGIC_64 X86_64 ALL 0x00 OBJECT 4 512 SUBSECTIONS_VIA_SYMBOLS

...

embed-bitcodeでbitcodeを埋め込む

Xcode7 betaのclangで-fembed-bitcodeをつけてオブジェクトファイルを作ってみます

% /Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -c -fembed-bitcode sample.c -o sample1.o

出力されたオブジェクトファイルのサイズは、先ほどのものに比べてかなり大きいようです。

% ls -al
...
-rw-r--r-- 1 gamako staff 75 9 4 13:43 sample.c
-rw-r--r-- 1 gamako staff 792 9 4 14:38 sample.o
-rw-r--r-- 1 gamako staff 2676 9 4 15:25 sample1.o

中身をダンプしてみましょう

% hexdump -C -v sample1.o
00000000 cf fa ed fe 07 00 00 01 03 00 00 00 01 00 00 00 |................|
00000010 04 00 00 00 a0 02 00 00 00 20 00 00 00 00 00 00 |......... ......|
00000020 19 00 00 00 28 02 00 00 00 00 00 00 00 00 00 00 |....(...........|
00000030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|

...

00000a40 00 5f 6d 61 69 6e 00 5f 70 72 69 6e 74 66 00 5f |._main._printf._|
00000a50 6c 6c 76 6d 2e 63 6d 64 6c 69 6e 65 00 5f 6c 6c |llvm.cmdline._ll|
00000a60 76 6d 2e 65 6d 62 65 64 64 65 64 2e 6d 6f 64 75 |vm.embedded.modu|
00000a70 6c 65 00 00 |le..|
00000a74

llvm-objdumpで中身を見てみると、__bitcodeと__cmdlineのセクションが増えているのがわかります。

% /usr/local/Cellar/llvm/3.6.1/bin/llvm-objdump -private-headers -section-headers -disassemble -s sample1.o

sample1.o:	file format Mach-O 64-bit x86-64

Disassembly of section __TEXT,__text:
_main:
 0:	55 
 
...
 
Sections:
Idx Name Size Address Type
 0 __text 0000002a 0000000000000000 TEXT
 1 __cstring 0000000e 000000000000002a DATA
 2 __bitcode 00000640 0000000000000040 DATA
 3 __cmdline 00000042 0000000000000680 DATA
 4 __compact_unwind 00000020 00000000000006c8 DATA
 5 __eh_frame 00000040 00000000000006e8 DATA
Contents of section __text:
 0000 554889e5 4883ec10 488d3d1b 000000c7 UH..H...H.=.....
 0010 45fc0000 0000b000 e8000000 0031c989 E............1..
 0020 45f889c8 4883c410 5dc3 E...H...].
Contents of section __cstring:
 002a 48656c6c 6f2c2057 6f726c64 0a00 Hello, World..
Contents of section __bitcode:
 0040 dec0170b 00000000 14000000 28060000 ............(...
 0050 07000001 4243c0de 210c0000 87010000 ....BC..!.......
 0060 0b822000 02000000 12000000 07812391 .. ...........#.
 0070 41c80449 06103239 9201840c 25050819 A..I..29....%...
 0080 1e048b62 80104502 42920b42 84103214 ...b..E.B..B..2.

...

 0640 14010000 61200000 0b000000 1304412c ....a ........A,
 0650 10000000 03000000 34230064 43190230 ........4#.dC..0
 0660 18830100 3311ca40 0c8311c1 00002306 ....3..@......#.
 0670 04001c42 12000000 00000000 00000000 ...B............
Contents of section __cmdline:
 0680 2d747269 706c6500 7838365f 36342d61 -triple.x86_64-a
 0690 70706c65 2d6d6163 6f737831 302e3130 pple-macosx10.10
 06a0 2e30002d 656d6974 2d6f626a 002d6469 .0.-emit-obj.-di
 06b0 7361626c 652d6c6c 766d2d6f 70747a6e sable-llvm-optzn
 06c0 7300 s.
Contents of section __compact_unwind:
 06c8 00000000 00000000 2a000000 00000001 ........*.......
 06d8 00000000 00000000 00000000 00000000 ................
Contents of section __eh_frame:
 06e8 14000000 00000000 037a5200 01781001 .........zR..x..
 06f8 100c0708 90010000 24000000 1c000000 ........$.......
 0708 f8f8ffff ffffffff 2a000000 00000000 ........*.......
 0718 00410e10 8602430d 06000000 00000000 .A....C.........
Mach header
 magic cputype cpusubtype caps filetype ncmds sizeofcmds flags
MH_MAGIC_64 X86_64 ALL 0x00 OBJECT 4 672 SUBSECTIONS_VIA_SYMBOLS
Load command 0

...

Section
 sectname __bitcode
 segname __LLVM
 addr 0x0000000000000040
 size 0x0000000000000640
 offset 768
 align 2^4 (16)
 reloff 0
 nreloc 0
 type S_REGULAR
attributes (none)
 reserved1 0
 reserved2 0
Section
 sectname __cmdline
 segname __LLVM
 addr 0x0000000000000680
 size 0x0000000000000042
 offset 2368
 align 2^4 (16)
 reloff 0
 nreloc 0
 type S_REGULAR
attributes (none)
 reserved1 0
 reserved2 0
Section
 sectname __compact_unwind
...

bitcodeの抜き出し

bitcodeのセクションはoffsetが768でsizeが0x640とあるので、この部分をファイルに抜き出してみましょう。ddコマンドでできます。

% dd if=sample1.o bs=1 skip=768 count=0x640 of=sample1.bc
1600+0 records in
1600+0 records out
1600 bytes transferred in 0.002806 secs (570217 bytes/sec)

% hexdump -C -v sample1.bc
00000000 de c0 17 0b 00 00 00 00 14 00 00 00 28 06 00 00 |............(...|
00000010 07 00 00 01 42 43 c0 de 21 0c 00 00 87 01 00 00 |....BC..!.......|
00000020 0b 82 20 00 02 00 00 00 12 00 00 00 07 81 23 91 |.. ...........#.|

...

00000620 18 83 01 00 33 11 ca 40 0c 83 11 c1 00 00 23 06 |....3..@......#.|
00000630 04 00 1c 42 12 00 00 00 00 00 00 00 00 00 00 00 |...B............|
00000640

抜き出した部分はllvm-disコマンドでLLVM IRのテキストに変換できます。

% /usr/local/Cellar/llvm/3.6.1/bin/llvm-dis sample1.bc

% cat sample1.ll
; ModuleID = 'sample1.bc'
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.10.0"

@.str = private unnamed_addr constant [14 x i8] c"Hello, World\0A\00", align 1

; Function Attrs: nounwind ssp uwtable
define i32 @main() #0 {
 %1 = alloca i32, align 4
 store i32 0, i32* %1
 %2 = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([14 x i8]* @.str, i32 0, i32 0))
 ret i32 0
}

declare i32 @printf(i8*, ...) #1

attributes #0 = { nounwind ssp uwtable "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="core2" "target-features"="+ssse3,+cx16,+sse,+sse2,+sse3" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #1 = { "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="core2" "target-features"="+ssse3,+cx16,+sse,+sse2,+sse3" "unsafe-fp-math"="false" "use-soft-float"="false" }

!llvm.module.flags = !{!0}
!llvm.ident = !{!1}

!0 = !{i32 1, !"PIC Level", i32 2}
!1 = !{!"Apple LLVM version 7.0.0 (clang-700.0.72)"}

ソースからコンパイルしたbitcodeとの比較

これをxcode7 betaのclangで出力したbitcodeと比較してみましょう。

% /Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -c -emit-llvm sample.c

% hexdump -C -v sample.bc
00000000 de c0 17 0b 00 00 00 00 14 00 00 00 38 06 00 00 |............8...|
00000010 07 00 00 01 42 43 c0 de 21 0c 00 00 8b 01 00 00 |....BC..!.......|
00000020 0b 82 20 00 02 00 00 00 12 00 00 00 07 81 23 91 |.. ...........#.|

...

00000630 04 00 1c 42 12 21 31 00 02 00 00 00 0b 0a 60 08 |...B.!1.......`.|
00000640 04 03 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000650

% /usr/local/Cellar/llvm/3.6.1/bin/llvm-dis sample.bc

% cat sample.ll
; ModuleID = 'sample.bc'
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.10.0"

@.str = private unnamed_addr constant [14 x i8] c"Hello, World\0A\00", align 1

; Function Attrs: nounwind ssp uwtable
define i32 @main() #0 {
 %1 = alloca i32, align 4
 store i32 0, i32* %1
 %2 = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([14 x i8]* @.str, i32 0, i32 0))
 ret i32 0
}

declare i32 @printf(i8*, ...) #1

attributes #0 = { nounwind ssp uwtable "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="core2" "target-features"="+ssse3,+cx16,+sse,+sse2,+sse3" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #1 = { "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="core2" "target-features"="+ssse3,+cx16,+sse,+sse2,+sse3" "unsafe-fp-math"="false" "use-soft-float"="false" }

!llvm.module.flags = !{!0}
!llvm.ident = !{!1}

!0 = !{i32 1, !"PIC Level", i32 2}
!1 = !{!"Apple LLVM version 7.0.0 (clang-700.0.72)"}

bitcodeの中身とサイズは若干違うようですが、、、

% ls -al
...
-rw-r--r-- 1 gamako staff 1616 9 4 15:38 sample.bc
-rw-r--r-- 1 gamako staff 1268 9 4 15:43 sample.ll
-rw-r--r-- 1 gamako staff 1600 9 4 15:47 sample1.bc
-rw-r--r-- 1 gamako staff 1269 9 4 15:55 sample1.ll

llファイルのdiffをとってみるとわかるように、中の情報はほぼ同一であることがわかります。

% diff sample.ll sample1.ll
1c1
< ; ModuleID = 'sample.bc'
---
> ; ModuleID = 'sample1.bc'

__cmdlineセクション

一方__cmdlineセクションは以下のようになっていました。ビルド時のオプションが格納されているようにみえます。これをヒントにストア側での再最適化が行われるのでしょう。

Contents of section __cmdline:
 0680 2d747269 706c6500 7838365f 36342d61 -triple.x86_64-a
 0690 70706c65 2d6d6163 6f737831 302e3130 pple-macosx10.10
 06a0 2e30002d 656d6974 2d6f626a 002d6469 .0.-emit-obj.-di
 06b0 7361626c 652d6c6c 766d2d6f 70747a6e sable-llvm-optzn
 06c0 7300 s.

最後に

Xcodeでのビルドについては、こちらも参考にどうぞ

XcodeでArchive以外のビルド結果にはbitcodeが埋め込まれない
http://qiita.com/gamako/items/66d3a8164678e525a26e

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URL: https://qiita.com/gamako/items/4ebfd048c5aed4f68595