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@earthen94

ARM32アセンブリで確認する符号有無 整数の違いと演算の挙動

Posted at

同じバイナリでも符号の有無で10進数は変わる

同じ0xFFFFFFFEでも符号ありと解釈するか無しと解釈するかで値が変わる。

test.c
#include <stdio.h>

int main() {
    signed int a = 0xFFFFFFFE;
    unsigned int b = 0xFFFFFFFE;

    printf("signed a = %d\n", a);
    printf("unsigned b = %u\n", b);

    return 0;
}

testuser@CasaOS:~/test$ gcc test.c -o test
testuser@CasaOS:~/test$ ./test
signed a = -2
unsigned b = 4294967294

アセンブリで二つの値に差があるかを確認

変数格納

逆アセンブルの命令は以下の通り

gcc -O0 -S test.c -o test.s

signed int a = 0xFFFFFFFE;
unsigned int b = 0xFFFFFFFE;

以下のようなコードへ変換された。
変数へ値を格納する段階ではsigned intとunsigned intの差異は見られない。

mvn     r3, #1
str     r3, [r7, #4]
mvn     r3, #1
str     r3, [r7]

指定した値をビット反転（NOT）してレジスタに入れる。
#1は32ビットで00000000 00000000 00000000 00000001
反転させると11111111 11111111 11111111 11111110となる。
16進数で表すと0xFFFFFFFE

元のコード
test.c
#include <stdio.h>

int main() {
    signed int a = 0xFFFFFFFE;
    unsigned int b = 0xFFFFFFFE;
    return 0;
}
test.s
        .arch armv7-a
        .fpu vfpv3-d16
        .eabi_attribute 28, 1
        .eabi_attribute 20, 1
        .eabi_attribute 21, 1
        .eabi_attribute 23, 3
        .eabi_attribute 24, 1
        .eabi_attribute 25, 1
        .eabi_attribute 26, 2
        .eabi_attribute 30, 6
        .eabi_attribute 34, 1
        .eabi_attribute 18, 4
        .file   "test.c"
        .text
        .section        .rodata
        .align  2
.LC0:
        .ascii  "signed a = %d\012\000"
        .align  2
.LC1:
        .ascii  "unsigned b = %u\012\000"
        .text
        .align  1
        .global main
        .syntax unified
        .thumb
        .thumb_func
        .type   main, %function
main:
        @ args = 0, pretend = 0, frame = 8
        @ frame_needed = 1, uses_anonymous_args = 0
        push    {r7, lr}
        sub     sp, sp, #8
        add     r7, sp, #0
        mvn     r3, #1
        str     r3, [r7, #4]
        mvn     r3, #1
        str     r3, [r7]
        ldr     r1, [r7, #4]
        ldr     r3, .L3
.LPIC0:
        add     r3, pc
        mov     r0, r3
        bl      printf(PLT)
        ldr     r1, [r7]
        ldr     r3, .L3+4
.LPIC1:
        add     r3, pc
        mov     r0, r3
        bl      printf(PLT)
        movs    r3, #0
        mov     r0, r3
        adds    r7, r7, #8
        mov     sp, r7
        @ sp needed
        pop     {r7, pc}
.L4:
        .align  2
.L3:
        .word   .LC0-(.LPIC0+4)
        .word   .LC1-(.LPIC1+4)
        .size   main, .-main
        .ident  "GCC: (Debian 12.2.0-14+deb12u1) 12.2.0"
        .section        .note.GNU-stack,"",%progbits

加算命令 符号有無による違い

どちらもadds命令を使っており違いはない

signed int a = 0xFFFFFFFE;   // 2
unsigned int b = 0xFFFFFFFE; // 4294967294
    
// ...省略...
    
a = a + 11;
b = b + 22;

mvn     r3, #1
str     r3, [r7, #12]
mvn     r3, #1
str     r3, [r7, #8]

@ ...省略...

ldr     r3, [r7, #12]
adds    r3, r3, #11
str     r3, [r7, #12]
ldr     r3, [r7, #8]
adds    r3, r3, #22
str     r3, [r7, #8]
元のコード
test.c
int main() {
    signed int a = 0xFFFFFFFE;   // 2
    unsigned int b = 0xFFFFFFFE; // 4294967294

    int signed_result, unsigned_result;

    a = a + 11;
    b = b + 22;
    
    return signed_result + unsigned_result;
}
test.s
        .arch armv7-a
        .fpu vfpv3-d16
        .eabi_attribute 28, 1
        .eabi_attribute 20, 1
        .eabi_attribute 21, 1
        .eabi_attribute 23, 3
        .eabi_attribute 24, 1
        .eabi_attribute 25, 1
        .eabi_attribute 26, 2
        .eabi_attribute 30, 6
        .eabi_attribute 34, 1
        .eabi_attribute 18, 4
        .file   "test.c"
        .text
        .align  1
        .global main
        .syntax unified
        .thumb
        .thumb_func
        .type   main, %function
main:
        @ args = 0, pretend = 0, frame = 16
        @ frame_needed = 1, uses_anonymous_args = 0
        @ link register save eliminated.
        push    {r7}
        sub     sp, sp, #20
        add     r7, sp, #0
        mvn     r3, #1
        str     r3, [r7, #12]
        mvn     r3, #1
        str     r3, [r7, #8]
        ldr     r3, [r7, #12]
        adds    r3, r3, #11
        str     r3, [r7, #12]
        ldr     r3, [r7, #8]
        adds    r3, r3, #22
        str     r3, [r7, #8]
        ldr     r2, [r7, #4]
        ldr     r3, [r7]
        add     r3, r3, r2
        mov     r0, r3
        adds    r7, r7, #20
        mov     sp, r7
        @ sp needed
        ldr     r7, [sp], #4
        bx      lr
        .size   main, .-main
        .ident  "GCC: (Debian 12.2.0-14+deb12u1) 12.2.0"
        .section        .note.GNU-stack,"",%progbits

減算命令 符号有無による違い

どちらもsubs命令を使っており違いはない

signed int a = 0xFFFFFFFE;   // 2
unsigned int b = 0xFFFFFFFE; // 4294967294
    
// ...省略...
    
a = a - 11;
b = b - 22;

mvn     r3, #1
str     r3, [r7, #12]
mvn     r3, #1
str     r3, [r7, #8]

@ ...省略...

ldr     r3, [r7, #12]
subs    r3, r3, #11
str     r3, [r7, #12]
ldr     r3, [r7, #8]
subs    r3, r3, #22
str     r3, [r7, #8]
元のコード
test.c
int main() {
    signed int a = 0xFFFFFFFE;   // 2
    unsigned int b = 0xFFFFFFFE; // 4294967294

    int signed_result, unsigned_result;

    a = a - 11;
    b = b - 22;

    return signed_result + unsigned_result;
}
test.s
        .arch armv7-a
        .fpu vfpv3-d16
        .eabi_attribute 28, 1
        .eabi_attribute 20, 1
        .eabi_attribute 21, 1
        .eabi_attribute 23, 3
        .eabi_attribute 24, 1
        .eabi_attribute 25, 1
        .eabi_attribute 26, 2
        .eabi_attribute 30, 6
        .eabi_attribute 34, 1
        .eabi_attribute 18, 4
        .file   "test.c"
        .text
        .align  1
        .global main
        .syntax unified
        .thumb
        .thumb_func
        .type   main, %function
main:
        @ args = 0, pretend = 0, frame = 16
        @ frame_needed = 1, uses_anonymous_args = 0
        @ link register save eliminated.
        push    {r7}
        sub     sp, sp, #20
        add     r7, sp, #0
        mvn     r3, #1
        str     r3, [r7, #12]
        mvn     r3, #1
        str     r3, [r7, #8]
        ldr     r3, [r7, #12]
        subs    r3, r3, #11
        str     r3, [r7, #12]
        ldr     r3, [r7, #8]
        subs    r3, r3, #22
        str     r3, [r7, #8]
        ldr     r2, [r7, #4]
        ldr     r3, [r7]
        add     r3, r3, r2
        mov     r0, r3
        adds    r7, r7, #20
        mov     sp, r7
        @ sp needed
        ldr     r7, [sp], #4
        bx      lr
        .size   main, .-main
        .ident  "GCC: (Debian 12.2.0-14+deb12u1) 12.2.0"
        .section        .note.GNU-stack,"",%progbits

加算命令 符号有無による違い

どちらもadds命令を使っており違いはない

signed int a = 0xFFFFFFFE;   // 2
unsigned int b = 0xFFFFFFFE; // 4294967294
    
// ...省略...
    
a = a + 11;
b = b + 22;

mvn     r3, #1
str     r3, [r7, #12]
mvn     r3, #1
str     r3, [r7, #8]

@ ...省略...

ldr     r3, [r7, #12]
adds    r3, r3, #11
str     r3, [r7, #12]
ldr     r3, [r7, #8]
adds    r3, r3, #22
str     r3, [r7, #8]
元のコード
test.c
int main() {
    signed int a = 0xFFFFFFFE;   // 2
    unsigned int b = 0xFFFFFFFE; // 4294967294

    int signed_result, unsigned_result;

    a = a + 11;
    b = b + 22;
    
    return signed_result + unsigned_result;
}
test.s
        .arch armv7-a
        .fpu vfpv3-d16
        .eabi_attribute 28, 1
        .eabi_attribute 20, 1
        .eabi_attribute 21, 1
        .eabi_attribute 23, 3
        .eabi_attribute 24, 1
        .eabi_attribute 25, 1
        .eabi_attribute 26, 2
        .eabi_attribute 30, 6
        .eabi_attribute 34, 1
        .eabi_attribute 18, 4
        .file   "test.c"
        .text
        .align  1
        .global main
        .syntax unified
        .thumb
        .thumb_func
        .type   main, %function
main:
        @ args = 0, pretend = 0, frame = 16
        @ frame_needed = 1, uses_anonymous_args = 0
        @ link register save eliminated.
        push    {r7}
        sub     sp, sp, #20
        add     r7, sp, #0
        mvn     r3, #1
        str     r3, [r7, #12]
        mvn     r3, #1
        str     r3, [r7, #8]
        ldr     r3, [r7, #12]
        adds    r3, r3, #11
        str     r3, [r7, #12]
        ldr     r3, [r7, #8]
        adds    r3, r3, #22
        str     r3, [r7, #8]
        ldr     r2, [r7, #4]
        ldr     r3, [r7]
        add     r3, r3, r2
        mov     r0, r3
        adds    r7, r7, #20
        mov     sp, r7
        @ sp needed
        ldr     r7, [sp], #4
        bx      lr
        .size   main, .-main
        .ident  "GCC: (Debian 12.2.0-14+deb12u1) 12.2.0"
        .section        .note.GNU-stack,"",%progbits

乗算命令 符号有無による違い

どちらもmul命令を使っており違いはない

signed int a = 0xFFFFFFFE;   // 2
unsigned int b = 0xFFFFFFFE; // 4294967294
    
// ...省略...
    
a = a * 11;
b = b * 22;

mvn     r3, #1
str     r3, [r7, #12]
mvn     r3, #1
str     r3, [r7, #8]

@ ...省略...

ldr     r3, [r7, #12]
movs    r2, #11
mul     r3, r2, r3
str     r3, [r7, #12]
ldr     r3, [r7, #8]
movs    r2, #22
mul     r3, r2, r3
str     r3, [r7, #8]
元のコード
test.c
int main() {
    signed int a = 0xFFFFFFFE;   // 2
    unsigned int b = 0xFFFFFFFE; // 4294967294

    int signed_result, unsigned_result;

    a = a * 11;
    b = b * 22;

    return signed_result + unsigned_result;
}
test.s
        .arch armv7-a
        .fpu vfpv3-d16
        .eabi_attribute 28, 1
        .eabi_attribute 20, 1
        .eabi_attribute 21, 1
        .eabi_attribute 23, 3
        .eabi_attribute 24, 1
        .eabi_attribute 25, 1
        .eabi_attribute 26, 2
        .eabi_attribute 30, 6
        .eabi_attribute 34, 1
        .eabi_attribute 18, 4
        .file   "test.c"
        .text
        .align  1
        .global main
        .syntax unified
        .thumb
        .thumb_func
        .type   main, %function
main:
        @ args = 0, pretend = 0, frame = 16
        @ frame_needed = 1, uses_anonymous_args = 0
        @ link register save eliminated.
        push    {r7}
        sub     sp, sp, #20
        add     r7, sp, #0
        mvn     r3, #1
        str     r3, [r7, #12]
        mvn     r3, #1
        str     r3, [r7, #8]
        ldr     r3, [r7, #12]
        movs    r2, #11
        mul     r3, r2, r3
        str     r3, [r7, #12]
        ldr     r3, [r7, #8]
        movs    r2, #22
        mul     r3, r2, r3
        str     r3, [r7, #8]
        ldr     r2, [r7, #4]
        ldr     r3, [r7]
        add     r3, r3, r2
        mov     r0, r3
        adds    r7, r7, #20
        mov     sp, r7
        @ sp needed
        ldr     r7, [sp], #4
        bx      lr
        .size   main, .-main
        .ident  "GCC: (Debian 12.2.0-14+deb12u1) 12.2.0"
        .section        .note.GNU-stack,"",%progbits

除算命令 符号有無による違い

smullとumullを使い分けている。

signed int a = 0xFFFFFFFE;   // 2
unsigned int b = 0xFFFFFFFE; // 4294967294
    
// ...省略...
    
a = a / 11;
b = b / 22;

mvn     r3, #1
str     r3, [r7, #12]
mvn     r3, #1
str     r3, [r7, #8]

@ ...省略...

ldr     r2, [r7, #12]
movw    r3, #41705
movt    r3, 11915
smull   r1, r3, r3, r2
asrs    r1, r3, #1
asrs    r3, r2, #31
subs    r3, r1, r3
str     r3, [r7, #12]
ldr     r2, [r7, #8]
movw    r3, #35747
movt    r3, 47662
umull   r2, r3, r3, r2
lsrs    r3, r3, #4
str     r3, [r7, #8]
元のコード
test.c
int main() {
    signed int a = 0xFFFFFFFE;   // 2
    unsigned int b = 0xFFFFFFFE; // 4294967294

    int signed_result, unsigned_result;

    a = a / 11;
    b = b / 22;

    return signed_result + unsigned_result;
}
test.s
        .arch armv7-a
        .fpu vfpv3-d16
        .eabi_attribute 28, 1
        .eabi_attribute 20, 1
        .eabi_attribute 21, 1
        .eabi_attribute 23, 3
        .eabi_attribute 24, 1
        .eabi_attribute 25, 1
        .eabi_attribute 26, 2
        .eabi_attribute 30, 6
        .eabi_attribute 34, 1
        .eabi_attribute 18, 4
        .file   "test.c"
        .text
        .align  1
        .global main
        .syntax unified
        .thumb
        .thumb_func
        .type   main, %function
main:
        @ args = 0, pretend = 0, frame = 16
        @ frame_needed = 1, uses_anonymous_args = 0
        @ link register save eliminated.
        push    {r7}
        sub     sp, sp, #20
        add     r7, sp, #0
        mvn     r3, #1
        str     r3, [r7, #12]
        mvn     r3, #1
        str     r3, [r7, #8]
        ldr     r2, [r7, #12]
        movw    r3, #41705
        movt    r3, 11915
        smull   r1, r3, r3, r2
        asrs    r1, r3, #1
        asrs    r3, r2, #31
        subs    r3, r1, r3
        str     r3, [r7, #12]
        ldr     r2, [r7, #8]
        movw    r3, #35747
        movt    r3, 47662
        umull   r2, r3, r3, r2
        lsrs    r3, r3, #4
        str     r3, [r7, #8]
        ldr     r2, [r7, #4]
        ldr     r3, [r7]
        add     r3, r3, r2
        mov     r0, r3
        adds    r7, r7, #20
        mov     sp, r7
        @ sp needed
        ldr     r7, [sp], #4
        bx      lr
        .size   main, .-main
        .ident  "GCC: (Debian 12.2.0-14+deb12u1) 12.2.0"
        .section        .note.GNU-stack,"",%progbits

比較命令

符号有りはbge、符号無しはbcs命令に変換された。

volatile signed int a = 0xFFFFFFFF;
volatile unsigned int b = 0xFFFFFFFF;
volatile signed int x = 0;
volatile unsigned int y = 0;
    
// ...省略...
    
if (a < x)
    
// ...省略...
    
if (b < x)

cmp     r2, r3
bge     .L2

@ ...省略...

cmp     r3, r2
bcs     .L4
元のコード
test.c
int main() {
    volatile signed int a = 0xFFFFFFFF;
    volatile unsigned int b = 0xFFFFFFFF;
    volatile signed int x = 0;
    volatile unsigned int y = 0;

    // 符号あり比較
    if (a < x) {
        a = 1;
    } else {
        a = 2;
    }

    // 符号なし比較
    if (b < x) {
        b = 1;
    } else {
        b = 2;
    }

    return a + b;
}
test.s
        .arch armv7-a
        .fpu vfpv3-d16
        .eabi_attribute 28, 1
        .eabi_attribute 20, 1
        .eabi_attribute 21, 1
        .eabi_attribute 23, 3
        .eabi_attribute 24, 1
        .eabi_attribute 25, 1
        .eabi_attribute 26, 2
        .eabi_attribute 30, 6
        .eabi_attribute 34, 1
        .eabi_attribute 18, 4
        .file   "test.c"
        .text
        .align  1
        .global main
        .syntax unified
        .thumb
        .thumb_func
        .type   main, %function
main:
        @ args = 0, pretend = 0, frame = 16
        @ frame_needed = 1, uses_anonymous_args = 0
        @ link register save eliminated.
        push    {r7}
        sub     sp, sp, #20
        add     r7, sp, #0
        mov     r3, #-1
        str     r3, [r7, #12]
        mov     r3, #-1
        str     r3, [r7, #8]
        movs    r3, #0
        str     r3, [r7, #4]
        movs    r3, #0
        str     r3, [r7]
        ldr     r2, [r7, #12]
        ldr     r3, [r7, #4]
        cmp     r2, r3
        bge     .L2
        movs    r3, #1
        str     r3, [r7, #12]
        b       .L3
.L2:
        movs    r3, #2
        str     r3, [r7, #12]
.L3:
        ldr     r3, [r7, #8]
        ldr     r2, [r7, #4]
        cmp     r3, r2
        bcs     .L4
        movs    r3, #1
        str     r3, [r7, #8]
        b       .L5
.L4:
        movs    r3, #2
        str     r3, [r7, #8]
.L5:
        ldr     r3, [r7, #12]
        mov     r2, r3
        ldr     r3, [r7, #8]
        add     r3, r3, r2
        mov     r0, r3
        adds    r7, r7, #20
        mov     sp, r7
        @ sp needed
        ldr     r7, [sp], #4
        bx      lr
        .size   main, .-main
        .ident  "GCC: (Debian 12.2.0-14+deb12u1) 12.2.0"
        .section        .note.GNU-stack,"",%progbits

大きい型へのキャスト(拡張)

このように拡張される。
ldrsh : 0xFFFF → 0xFFFFFFFF
ldrh : 65535 → 0x0000FFFF

short s = 0xFFFF;
unsigned short us = 0xFFFF;

int a = (int)s;           //  （符号拡張）
int b = (int)us;          // 65535 → 0x0000FFFF （ゼロ拡張）

movw    r3, #65535
strh    r3, [r7, #14]   @ movhi
movw    r3, #65535
strh    r3, [r7, #12]   @ movhi
ldrsh   r3, [r7, #14]
str     r3, [r7, #8]
ldrh    r3, [r7, #12]
str     r3, [r7, #4]
元のコード
test.c
int main() {
    short s = 0xFFFF;
    unsigned short us = 0xFFFF;

    int a = (int)s;
    int b = (int)us;

    return a + b;
}
test.s
        .arch armv7-a
        .fpu vfpv3-d16
        .eabi_attribute 28, 1
        .eabi_attribute 20, 1
        .eabi_attribute 21, 1
        .eabi_attribute 23, 3
        .eabi_attribute 24, 1
        .eabi_attribute 25, 1
        .eabi_attribute 26, 2
        .eabi_attribute 30, 6
        .eabi_attribute 34, 1
        .eabi_attribute 18, 4
        .file   "test.c"
        .text
        .align  1
        .global main
        .syntax unified
        .thumb
        .thumb_func
        .type   main, %function
main:
        @ args = 0, pretend = 0, frame = 16
        @ frame_needed = 1, uses_anonymous_args = 0
        @ link register save eliminated.
        push    {r7}
        sub     sp, sp, #20
        add     r7, sp, #0
        movw    r3, #65535
        strh    r3, [r7, #14]   @ movhi
        movw    r3, #65535
        strh    r3, [r7, #12]   @ movhi
        ldrsh   r3, [r7, #14]
        str     r3, [r7, #8]
        ldrh    r3, [r7, #12]
        str     r3, [r7, #4]
        ldr     r2, [r7, #8]
        ldr     r3, [r7, #4]
        add     r3, r3, r2
        mov     r0, r3
        adds    r7, r7, #20
        mov     sp, r7
        @ sp needed
        ldr     r7, [sp], #4
        bx      lr
        .size   main, .-main
        .ident  "GCC: (Debian 12.2.0-14+deb12u1) 12.2.0"
        .section        .note.GNU-stack,"",%progbits

加減乗除 符号有無で処理が変わらない理由

CPUは演算中の32ビット値が符号有りか符号無しかは知らない。
符号の有無は演算に関係無く、計算結果をどちらとして解釈するかによって変わる。

補数表現

例として-2を２進数で表す。
0x00000002 = 00000000 00000000 00000000 00000010

全ビット反転
00000000 00000000 00000000 00000010 → 11111111 11111111 11111111 11111101
0x00000002 → 0xFFFFFFFD

1を足す
0xFFFFFFFD + 1 = 0xFFFFFFFE

-2を２進数で表すと0xFFFFFFFEとなる

加算

0x00000002 + 0x00000003 = 0x00000005
0xFFFFFFFE + 0x00000003 = 0x00000001

計算式 ビット列 signed 解釈 unsigned 解釈
0x00000002 + 0x00000003 0x00000005 5 5
0xFFFFFFFE + 0x00000003 0x00000001 1 1

減算

0x00000008 - 0x00000003 = 0x00000005
0xFFFFFFFE - 0x00000003 = 0xFFFFFFFB

計算式 ビット列 signed 解釈 unsigned 解釈
0x00000008 - 0x00000003 0x00000005 5 5
0xFFFFFFFE - 0x00000003 0xFFFFFFFB -5 4294967291

乗算

0x00000004 × 0x00000003 = 0x0000000C
0xFFFFFFFE × 0x00000003 = 0xFFFFFFFA

計算式 ビット列 signed 解釈 unsigned 解釈
0x00000004 × 0x00000003 0x0000000C 12 12
0xFFFFFFFE × 0x00000003 0xFFFFFFFA -6 4294967290

符号有無の差異違い

操作 ARM命令 / 実装 符号依存
加算 adds なし
減算 subs なし
乗算 mul なし
除算 sdiv / udiv あり
比較 cmp + 条件分岐 あり
型変換 / キャスト movsx（符号拡張） / movzx（ゼロ拡張） あり

動作環境（Linux / ARM 版）

OS: Armbian 23.08.0-trunk (Debian 12 Bookworm)
Kernel: Linux 6.1.38-meson (armv7l)
CPU: ARM Cortex-A5, 4 cores, Little Endian
GCC: 12.2.0
Architecture: armv7l
備考: Windows10からSSH接続して使用

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