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@earthen94

C言語の型変換をARM32アセンブリで確認する（int, short, float）

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目的

前回x86でC言語のキャストが、実際にどのように型変換を行っているかを調べた。
今回は32ビットARMで同じ検証をする。

C言語の型変換をx86アセンブリで確認する（int, short, float）

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

int -> short

intは4バイト
shortは2バイト
strh命令でr3レジスタの下位16ビットをメモリへ格納している。
ここでintからshortへ変換している。

int a=100;
short b=(short)a;

movs    r3, #100
str     r3, [r7, #4]
ldr     r3, [r7, #4]
strh    r3, [r7, #2]    @ movhi
元のコード
test.c
int main(){
    int a=100;
    short b=(short)a;
    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
        .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
        @ link register save eliminated.
        push    {r7}
        sub     sp, sp, #12
        add     r7, sp, #0
        movs    r3, #100
        str     r3, [r7, #4]
        ldr     r3, [r7, #4]
        strh    r3, [r7, #2]    @ movhi
        movs    r3, #0
        mov     r0, r3
        adds    r7, r7, #12
        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

short -> int

ldrshは16ビットの符号を保ったままビットに広げる命令

short b=100;
int a=(int)b;

movs    r3, #100
strh    r3, [r7, #6]    @ movhi
ldrsh   r3, [r7, #6]
str     r3, [r7]
元のコード
test.c
int main(){
    short b=100;
    int a=(int)b;
    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
        .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
        @ link register save eliminated.
        push    {r7}
        sub     sp, sp, #12
        add     r7, sp, #0
        movs    r3, #100
        strh    r3, [r7, #6]    @ movhi
        ldrsh   r3, [r7, #6]
        str     r3, [r7]
        movs    r3, #0
        mov     r0, r3
        adds    r7, r7, #12
        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

int -> float

int a = 42;
float f;
f = (float)a;

movs    r3, #42
str     r3, [r7, #4]
ldr     r3, [r7, #4]
vmov    s15, r3 @ int42をFPレジスタs15に複製
vcvt.f32.s32    s15, s15 @ 32bit整数→32bit浮動小数点に変換
vstr.32 s15, [r7]
元のコード
test.c
int main() {
    int a = 42;
    float f;
    f = (float)a;
    return 0;
}
test.c
        .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 = 8
        @ frame_needed = 1, uses_anonymous_args = 0
        @ link register save eliminated.
        push    {r7}
        sub     sp, sp, #12
        add     r7, sp, #0
        movs    r3, #42
        str     r3, [r7, #4]
        ldr     r3, [r7, #4]
        vmov    s15, r3 @ int
        vcvt.f32.s32    s15, s15
        vstr.32 s15, [r7]
        movs    r3, #0
        mov     r0, r3
        adds    r7, r7, #12
        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

float -> int

float f = 12.3;
int a;
a = (int)f;

movw    r3, #52429
movt    r3, 16708
str     r3, [r7, #4]    @ float
vldr.32 s15, [r7, #4]
vcvt.s32.f32    s15, s15 @ float → int に変換
vmov    r3, s15 @ int
str     r3, [r7]
元のコード
test.c
int main() {
    float f = 12.3;
    int a;
    a = (int)f;
    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
        .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
        @ link register save eliminated.
        push    {r7}
        sub     sp, sp, #12
        add     r7, sp, #0
        movw    r3, #52429
        movt    r3, 16708
        str     r3, [r7, #4]    @ float
        vldr.32 s15, [r7, #4]
        vcvt.s32.f32    s15, s15
        vmov    r3, s15 @ int
        str     r3, [r7]
        movs    r3, #0
        mov     r0, r3
        adds    r7, r7, #12
        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

まとめ

int -> short　strh命令を使い、上位16ビットを切り捨てている。
short -> int　ldrsh命令を使って拡張している。
int -> float　vcvt.f32.s32命令を使って変換している。
float -> int　vcvt.s32.f32命令を使って変換している。

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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