KDOC 449: 32bitマシンでint64型を使うとどうなるのか?

この文書のステータス

  • 作成
    • 2025-10-12 貴島
  • レビュー
    • 2025-10-17 貴島

概要

32bitマシンだとレジスタに入り切らないため1度で処理できず、スタックを使う想定だが、正しいだろうか。レジスタよりサイズが大きい場合にどう扱うか、ともいえる。

32bit/64bitでコンパイルして、アセンブリのdiffを見る。 

tmpfile=$(mktemp /tmp/tmp.XXXXXX.c)

cat > $tmpfile <<EOF
  #include <stdio.h>

  int main() {
      long long a = 1234567890123;
      int b = 42;
      printf("%lld %d\n", a, b);
      return 0;
  }
EOF

docker run --rm -v /tmp:/tmp -w /tmp i386/ubuntu bash -c "apt update -y && apt install -y build-essential && gcc -m32 -S $tmpfile -o 32.s && cat 32.s"

rm $tmpfile

tmpfile=$(mktemp /tmp/tmp.XXXXXX.c)

cat > $tmpfile <<EOF
  #include <stdio.h>

  int main() {
      long long a = 1234567890123;
      int b = 42;
      printf("%lld %d\n", a, b);
      return 0;
  }
EOF
gcc -S $tmpfile -o /tmp/64.s

rm $tmpfile

cat /tmp/32.s
cat /tmp/64.s
diff /tmp/32.s /tmp/64.s

	.file	"tmp.Q8B7li.c"
	.text
	.section	.rodata
.LC0:
	.string	"%lld %d\n"
	.text
	.globl	main
	.type	main, @function
main:
.LFB0:
	.cfi_startproc
	leal	4(%esp), %ecx
	.cfi_def_cfa 1, 0
	andl	$-16, %esp
	pushl	-4(%ecx)
	pushl	%ebp
	.cfi_escape 0x10,0x5,0x2,0x75,0
	movl	%esp, %ebp
	pushl	%ebx
	pushl	%ecx
	.cfi_escape 0xf,0x3,0x75,0x78,0x6
	.cfi_escape 0x10,0x3,0x2,0x75,0x7c
	subl	$16, %esp
	call	__x86.get_pc_thunk.ax
	addl	$_GLOBAL_OFFSET_TABLE_, %eax
	movl	$1912276171, -16(%ebp)
	movl	$287, -12(%ebp)
	movl	$42, -20(%ebp)
	pushl	-20(%ebp)
	pushl	-12(%ebp)
	pushl	-16(%ebp)
	leal	.LC0@GOTOFF(%eax), %edx
	pushl	%edx
	movl	%eax, %ebx
	call	printf@PLT
	addl	$16, %esp
	movl	$0, %eax
	leal	-8(%ebp), %esp
	popl	%ecx
	.cfi_restore 1
	.cfi_def_cfa 1, 0
	popl	%ebx
	.cfi_restore 3
	popl	%ebp
	.cfi_restore 5
	leal	-4(%ecx), %esp
	.cfi_def_cfa 4, 4
	ret
	.cfi_endproc
.LFE0:
	.size	main, .-main
	.section	.text.__x86.get_pc_thunk.ax,"axG",@progbits,__x86.get_pc_thunk.ax,comdat
	.globl	__x86.get_pc_thunk.ax
	.hidden	__x86.get_pc_thunk.ax
	.type	__x86.get_pc_thunk.ax, @function
__x86.get_pc_thunk.ax:
.LFB1:
	.cfi_startproc
	movl	(%esp), %eax
	ret
	.cfi_endproc
.LFE1:
	.ident	"GCC: (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0"
	.section	.note.GNU-stack,"",@progbits
	.file	"tmp.wWxeed.c"
	.text
	.section	.rodata
.LC0:
	.string	"%lld %d\n"
	.text
	.globl	main
	.type	main, @function
main:
.LFB0:
	.cfi_startproc
	endbr64
	pushq	%rbp
	.cfi_def_cfa_offset 16
	.cfi_offset 6, -16
	movq	%rsp, %rbp
	.cfi_def_cfa_register 6
	subq	$16, %rsp
	movabsq	$1234567890123, %rax
	movq	%rax, -8(%rbp)
	movl	$42, -12(%rbp)
	movl	-12(%rbp), %edx
	movq	-8(%rbp), %rax
	movq	%rax, %rsi
	leaq	.LC0(%rip), %rax
	movq	%rax, %rdi
	movl	$0, %eax
	call	printf@PLT
	movl	$0, %eax
	leave
	.cfi_def_cfa 7, 8
	ret
	.cfi_endproc
.LFE0:
	.size	main, .-main
	.ident	"GCC: (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0"
	.section	.note.GNU-stack,"",@progbits
	.section	.note.gnu.property,"a"
	.align 8
	.long	1f - 0f
	.long	4f - 1f
	.long	5
0:
	.string	"GNU"
1:
	.align 8
	.long	0xc0000002
	.long	3f - 2f
2:
	.long	0x3
3:
	.align 8
4:
1c1
< 	.file	"tmp.Q8B7li.c"
---
> 	.file	"tmp.wWxeed.c"
12,34c12,27
< 	leal	4(%esp), %ecx
< 	.cfi_def_cfa 1, 0
< 	andl	$-16, %esp
< 	pushl	-4(%ecx)
< 	pushl	%ebp
< 	.cfi_escape 0x10,0x5,0x2,0x75,0
< 	movl	%esp, %ebp
< 	pushl	%ebx
< 	pushl	%ecx
< 	.cfi_escape 0xf,0x3,0x75,0x78,0x6
< 	.cfi_escape 0x10,0x3,0x2,0x75,0x7c
< 	subl	$16, %esp
< 	call	__x86.get_pc_thunk.ax
< 	addl	$_GLOBAL_OFFSET_TABLE_, %eax
< 	movl	$1912276171, -16(%ebp)
< 	movl	$287, -12(%ebp)
< 	movl	$42, -20(%ebp)
< 	pushl	-20(%ebp)
< 	pushl	-12(%ebp)
< 	pushl	-16(%ebp)
< 	leal	.LC0@GOTOFF(%eax), %edx
< 	pushl	%edx
< 	movl	%eax, %ebx
---
> 	endbr64
> 	pushq	%rbp
> 	.cfi_def_cfa_offset 16
> 	.cfi_offset 6, -16
> 	movq	%rsp, %rbp
> 	.cfi_def_cfa_register 6
> 	subq	$16, %rsp
> 	movabsq	$1234567890123, %rax
> 	movq	%rax, -8(%rbp)
> 	movl	$42, -12(%rbp)
> 	movl	-12(%rbp), %edx
> 	movq	-8(%rbp), %rax
> 	movq	%rax, %rsi
> 	leaq	.LC0(%rip), %rax
> 	movq	%rax, %rdi
> 	movl	$0, %eax
36d28
< 	addl	$16, %esp
38,47c30,31
< 	leal	-8(%ebp), %esp
< 	popl	%ecx
< 	.cfi_restore 1
< 	.cfi_def_cfa 1, 0
< 	popl	%ebx
< 	.cfi_restore 3
< 	popl	%ebp
< 	.cfi_restore 5
< 	leal	-4(%ecx), %esp
< 	.cfi_def_cfa 4, 4
---
> 	leave
> 	.cfi_def_cfa 7, 8
52,63c36
< 	.section	.text.__x86.get_pc_thunk.ax,"axG",@progbits,__x86.get_pc_thunk.ax,comdat
< 	.globl	__x86.get_pc_thunk.ax
< 	.hidden	__x86.get_pc_thunk.ax
< 	.type	__x86.get_pc_thunk.ax, @function
< __x86.get_pc_thunk.ax:
< .LFB1:
< 	.cfi_startproc
< 	movl	(%esp), %eax
< 	ret
< 	.cfi_endproc
< .LFE1:
< 	.ident	"GCC: (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0"
---
> 	.ident	"GCC: (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0"
64a38,53
> 	.section	.note.gnu.property,"a"
> 	.align 8
> 	.long	1f - 0f
> 	.long	4f - 1f
> 	.long	5
> 0:
> 	.string	"GNU"
> 1:
> 	.align 8
> 	.long	0xc0000002
> 	.long	3f - 2f
> 2:
> 	.long	0x3
> 3:
> 	.align 8
> 4:

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