ARM Cortex-A8:使用简单浮点乘法时，来自交叉编译器的不同程序集输出

Question

我正在试验一个带有linux ubuntu的I.MX53板。我正在主机系统上使用交叉编译器(arm-linux-gnueabihf)处理ssh。

对于在处理器(ARM Cortex-A8)上进行浮点运算的基准测试，我创建了以下两个不同的程序，它们都包含一个简单的循环。在第一个程序中，循环包含单个乘法，在第二个程序中包含相同的乘法和额外的加法。

我用以下编译器调用编译了这两个程序：

arm-linux-gnueabihf-g++-4.8 -O3 -ffast-math -Ofast -Wall -fmessage-length=0 -Wno-multichar -Wno-unknown-pragmas -std=c++11 -mcpu=cortex-a8 -mfpu=neon -mfloat-abi=hard -save-temps loopMul.cpp -o loopMul

现在我的问题是：为什么为循环部分输出编译器如此不同的汇编代码(参见装配代码)

我认识到，在第一个程序中，编译器生成用于乘法的NEON指令，而在第二个程序中只生成较慢的VFP指令。

第一个程序(loopMul.cpp)：

#include <iostream>
#include <ctime>
using namespace std;
int main(int argc, char **argv)
{
size_t length = 10E7;
float test = 1;
clock_t start = clock();    
for(size_t i=1; i<length; i++)
{
test *= i;
}
clock_t elapsed = clock() - start;
cout << test << endl;
float elapsed_seconds = float(elapsed) / float(CLOCKS_PER_SEC);
cout << "loop took " << elapsed_seconds <<  "seconds" << endl;
return 0;
}

第二个程序(loopMulAdd)：

#include <iostream>
#include <ctime>
using namespace std;
int main(int argc, char **argv)
{
size_t length = 10E7;
float test = 1;
clock_t start = clock();    
for(size_t i=1; i<length; i++)
{
test *= i;
test += 1;
}
clock_t elapsed = clock() - start;
cout << test << endl;
float elapsed_seconds = float(elapsed) / float(CLOCKS_PER_SEC);
cout << "loop took " << elapsed_seconds <<  "seconds" << endl;
return 0;
}

第一个程序的汇编输出(loopMul.s)：

.syntax unified
.cpu cortex-a8
.eabi_attribute 27, 3
.eabi_attribute 28, 1
.fpu neon
.eabi_attribute 23, 1
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.eabi_attribute 26, 2
.eabi_attribute 30, 2
.eabi_attribute 34, 1
.eabi_attribute 18, 4
.thumb
.file   "loopMul.cpp"
.section    .text.startup,"ax",%progbits
.align  2
.global main
.thumb
.thumb_func
.type   main, %function
main:
.fnstart
.LFB1265:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
push    {r4, lr}
.save {r4, lr}
fstmfdd sp!, {d8, d9, d10, d11}
.vsave {d8, d9, d10, d11}
bl  clock
vmov.i32    q11, #4  @ v4si
vmov.f32    q9, #1.0e+0  @ v4sf
movw    r2, #30783
movt    r2, 381
movs    r3, #0
vldr    d16, .L8
vldr    d17, .L8+8
mov r4, r0
.L3:
vcvt.f32.s32    q10, q8
adds    r3, r3, #1
cmp r3, r2
vadd.i32    q8, q8, q11
vmul.f32    q9, q9, q10
bne .L3
vmov.i32    q8, #0  @ v16qi
vext.8  q4, q9, q8, #8
vmul.f32    q4, q4, q9
vext.8  q5, q4, q8, #4
bl  clock
flds    s15, .L8+16
vmul.f32    q4, q5, q4
vmov.32 r3, d8[0]
fmsr    s0, r3
fmuls   s0, s0, s15
fcvtds  d0, s0
subs    r4, r0, r4
movw    r0, #:lower16:_ZSt4cout
movt    r0, #:upper16:_ZSt4cout
bl  _ZNSo9_M_insertIdEERSoT_
bl  _ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_
movw    r0, #:lower16:_ZSt4cout
movw    r1, #:lower16:.LC0
movt    r0, #:upper16:_ZSt4cout
movt    r1, #:upper16:.LC0
bl  _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
fmsr    s15, r4 @ int
fsitos  s0, s15
flds    s15, .L8+20
fmuls   s0, s0, s15
fcvtds  d0, s0
bl  _ZNSo9_M_insertIdEERSoT_
movw    r1, #:lower16:.LC1
movt    r1, #:upper16:.LC1
bl  _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
bl  _ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_
fldmfdd sp!, {d8-d11}
movs    r0, #0
pop {r4, pc}
.L9:
.align  3
.L8:
.word   1
.word   2
.word   3
.word   4
.word   1733542428
.word   897988541
.fnend
.size   main, .-main
.align  2
.thumb
.thumb_func
.type   _GLOBAL__sub_I_main, %function
_GLOBAL__sub_I_main:
.fnstart
.LFB1422:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
push    {r4, lr}
movw    r4, #:lower16:.LANCHOR0
movt    r4, #:upper16:.LANCHOR0
mov r0, r4
bl  _ZNSt8ios_base4InitC1Ev
mov r0, r4
movw    r1, #:lower16:_ZNSt8ios_base4InitD1Ev
movw    r2, #:lower16:__dso_handle
movt    r1, #:upper16:_ZNSt8ios_base4InitD1Ev
movt    r2, #:upper16:__dso_handle
pop {r4, lr}
b   __aeabi_atexit
.cantunwind
.fnend
.size   _GLOBAL__sub_I_main, .-_GLOBAL__sub_I_main
.section    .init_array,"aw",%init_array
.align  2
.word   _GLOBAL__sub_I_main(target1)
.section    .rodata.str1.4,"aMS",%progbits,1
.align  2
.LC0:
.ascii  "loop took 00"
.space  1
.LC1:
.ascii  "seconds00"
.bss
.align  2
.LANCHOR0 = . + 0
.type   _ZStL8__ioinit, %object
.size   _ZStL8__ioinit, 1
_ZStL8__ioinit:
.space  1
.hidden __dso_handle
.ident  "GCC: (Ubuntu/Linaro 4.8.1-10ubuntu7) 4.8.1"
.section    .note.GNU-stack,"",%progbits

第二个程序(loopMulAdd.s)的汇编输出：

.syntax unified
.cpu cortex-a8
.eabi_attribute 27, 3
.eabi_attribute 28, 1
.fpu neon
.eabi_attribute 23, 1
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.eabi_attribute 26, 2
.eabi_attribute 30, 2
.eabi_attribute 34, 1
.eabi_attribute 18, 4
.thumb
.file   "loopMulAdd.cpp"
.section    .text.startup,"ax",%progbits
.align  2
.global main
.thumb
.thumb_func
.type   main, %function
main:
.fnstart
.LFB1265:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
push    {r4, lr}
.save {r4, lr}
fstmfdd sp!, {d8}
.vsave {d8}
bl  clock
fconsts s16, #112
mov r2, #57600
movt    r2, 1525
movs    r3, #1
fcpys   s14, s16
mov r4, r0
.L3:
fmsr    s13, r3 @ int
adds    r3, r3, #1
cmp r3, r2
fsitos  s15, s13
fcpys   s13, s14
fmacs   s13, s16, s15
fcpys   s16, s13
bne .L3
bl  clock
fcvtds  d0, s16
subs    r4, r0, r4
movw    r0, #:lower16:_ZSt4cout
movt    r0, #:upper16:_ZSt4cout
bl  _ZNSo9_M_insertIdEERSoT_
bl  _ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_
movw    r0, #:lower16:_ZSt4cout
movw    r1, #:lower16:.LC0
movt    r0, #:upper16:_ZSt4cout
movt    r1, #:upper16:.LC0
bl  _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
fmsr    s15, r4 @ int
fsitos  s0, s15
flds    s15, .L7
fmuls   s0, s0, s15
fcvtds  d0, s0
bl  _ZNSo9_M_insertIdEERSoT_
movw    r1, #:lower16:.LC1
movt    r1, #:upper16:.LC1
bl  _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
bl  _ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_
fldmfdd sp!, {d8}
movs    r0, #0
pop {r4, pc}
.L8:
.align  2
.L7:
.word   897988541
.fnend
.size   main, .-main
.align  2
.thumb
.thumb_func
.type   _GLOBAL__sub_I_main, %function
_GLOBAL__sub_I_main:
.fnstart
.LFB1422:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
push    {r4, lr}
movw    r4, #:lower16:.LANCHOR0
movt    r4, #:upper16:.LANCHOR0
mov r0, r4
bl  _ZNSt8ios_base4InitC1Ev
mov r0, r4
movw    r1, #:lower16:_ZNSt8ios_base4InitD1Ev
movw    r2, #:lower16:__dso_handle
movt    r1, #:upper16:_ZNSt8ios_base4InitD1Ev
movt    r2, #:upper16:__dso_handle
pop {r4, lr}
b   __aeabi_atexit
.cantunwind
.fnend
.size   _GLOBAL__sub_I_main, .-_GLOBAL__sub_I_main
.section    .init_array,"aw",%init_array
.align  2
.word   _GLOBAL__sub_I_main(target1)
.section    .rodata.str1.4,"aMS",%progbits,1
.align  2
.LC0:
.ascii  "loop took 00"
.space  1
.LC1:
.ascii  "seconds00"
.bss
.align  2
.LANCHOR0 = . + 0
.type   _ZStL8__ioinit, %object
.size   _ZStL8__ioinit, 1
_ZStL8__ioinit:
.space  1
.hidden __dso_handle
.ident  "GCC: (Ubuntu/Linaro 4.8.1-10ubuntu7) 4.8.1"
.section    .note.GNU-stack,"",%progbits

Answer 1

Q：为什么为循环输出编译器如此不同的汇编代码部分

原因很简单。

在第一种情况(乘法循环)中，编译器能够自动向量化循环，并使用NEON单元的SIMD功能。

在第二种情况下，编译器就不那么聪明了。它没有找到对循环进行矢量化的方法，因此没有使用NEON，而是使用vfp指令集。

据我记忆所及，您可以使用选项强制编译器使用NEON而不是FPU。不过，这是否会提高性能在很大程度上取决于浮点代码的性质。