C++ 标准：：向量<；uint8_t>；C++；11/14启用_C++_C++11_Gcc_C++14_Stdvector

C++ 标准：：向量<；uint8_t>；C++；11/14启用

c++ c++11 gcc

C++ 标准：：向量<；uint8_t>；C++；11/14启用,c++,c++11,gcc,c++14,stdvector,C++,C++11,Gcc,C++14,Stdvector,使用gcc 4.9，使用Linaro工具链对ARM进行交叉编译，我发现添加-std=c++14时，vector.assign（）的编译结果发生了变化，从而造成了严重的性能问题我已经尝试了几种不同的方法来完成这个分配+复制，但是只要我使用std:：vector来完成，它们都有这个性能问题我可以用这个玩具例子重现这个问题： VectorTest.h #include <stdint.h> #include <stddef.h> #include <vector>

使用gcc 4.9，使用Linaro工具链对ARM进行交叉编译，我发现添加

-std=c++14

时，

vector.assign（）

的编译结果发生了变化，从而造成了严重的性能问题

我已经尝试了几种不同的方法来完成这个分配+复制，但是只要我使用

std:：vector

来完成，它们都有这个性能问题

我可以用这个玩具例子重现这个问题：

VectorTest.h

#include <stdint.h>
#include <stddef.h>
#include <vector>

struct VectorWrapper_t
{
    VectorWrapper_t(uint8_t const* pData, size_t length);
    std::vector<uint8_t> data;
};

gcc标志：

-std=c++14 \
-mthumb -march=armv7-a -mtune=cortex-a9 \
-mlittle-endian -mfloat-abi=hard -mfpu=neon -Wa,-mimplicit-it=thumb \
-O2 -g

查看程序集，我可以了解原因：原始版本（我假设是C++03？）调用

memmove

，而C++14版本添加了一个额外的循环，看起来像是手动复制数据。查看gcc添加的

.loc

标记和

-fverbose asm

，此循环中的指令来自

stl_construct.h

和

stl_uninitialized.h

更改为gcc 5.2.1（使用C++14），除了使用

memcpy

而不是

memmove

之外，它的编译与C++03示例几乎相同

我可以通过使用

std:：unique_ptr

而不是

vector

来解决这个问题。但是，我想弄清这个问题的根源，以了解其他使用

vector

s的地方是否存在性能问题，以及如何潜在地解决这些问题（更新到gcc 5.2是不切实际的）

所以我的问题是：为什么它在C++11/14下的编译方式不同？

作为参考，

gcc--version

报告：

arm linux gnueabihf gcc（Linaro gcc 4.9-2014.12）4.9.3 20141205（预发布）

以下是生成的程序集：

# C++03, gcc 4.9

    push    {r3, r4, r5, r6, r7, lr}    @
    movs    r3, #0  @ tmp118,
    mov r4, r0  @ this, this
    str r3, [r0]    @ tmp118, MEM[(struct _Vector_impl *)this_1(D)]._M_start
    mov r5, r2  @ length, length
    str r3, [r0, #4]    @ tmp118, MEM[(struct _Vector_impl *)this_1(D)]._M_finish
    str r3, [r0, #8]    @ tmp118, MEM[(struct _Vector_impl *)this_1(D)]._M_end_of_storage
    cbnz    r2, .L19    @ length,
    mov r0, r4  @, this
    pop {r3, r4, r5, r6, r7, pc}    @
.L19:
    mov r0, r2  @, length
    mov r6, r1  @ pData, pData
    bl  _Znwj   @
    mov r2, r5  @, length
    mov r1, r6  @, pData
    mov r7, r0  @ D.13516,
    bl  memmove @
    ldr r0, [r4]    @ D.13515, MEM[(struct vector *)this_1(D)].D.11902._M_impl._M_start
    cbz r0, .L3 @ D.13515,
    bl  _ZdlPv  @
.L3:
    add r5, r5, r7  @ D.13515, D.13516
    str r7, [r4]    @ D.13516, MEM[(struct vector *)this_1(D)].D.11902._M_impl._M_start
    str r5, [r4, #4]    @ D.13515, MEM[(struct vector *)this_1(D)].D.11902._M_impl._M_finish
    mov r0, r4  @, this
    str r5, [r4, #8]    @ D.13515, MEM[(struct vector *)this_1(D)].D.11902._M_impl._M_end_of_storage
    pop {r3, r4, r5, r6, r7, pc}    @
.L6:
    ldr r0, [r4]    @ D.13515, MEM[(struct _Vector_base *)this_1(D)]._M_impl._M_start
    cbz r0, .L5 @ D.13515,
    bl  _ZdlPv  @
.L5:
    bl  __cxa_end_cleanup   @

# C++14, gcc 4.9

    push    {r3, r4, r5, r6, r7, lr}    @
    movs    r3, #0  @ tmp157,
    mov r6, r0  @ this, this
    str r3, [r0]    @ tmp157, MEM[(struct _Vector_impl *)this_1(D)]._M_start
    mov r5, r2  @ length, length
    str r3, [r0, #4]    @ tmp157, MEM[(struct _Vector_impl *)this_1(D)]._M_finish
    str r3, [r0, #8]    @ tmp157, MEM[(struct _Vector_impl *)this_1(D)]._M_end_of_storage
    cbnz    r2, .L25    @ length,
    mov r0, r6  @, this
    pop {r3, r4, r5, r6, r7, pc}    @
.L25:
    mov r0, r2  @, length
    mov r4, r1  @ pData, pData
    bl  _Znwj   @
    adds    r3, r4, r5  @ D.20345, pData, length
    mov r7, r0  @ __result,
    cmp r4, r3  @ pData, D.20345
    ittt    ne
    addne   r1, r4, #-1 @ ivtmp.76, pData,
    movne   r3, r0  @ __result, __result
    addne   r4, r0, r5  @ D.20346, __result, length
    beq .L26    @,
.L7:
    ldrb    r2, [r1, #1]!   @ zero_extendqisi2  @ D.20348, MEM[base: _48, offset: 0]
    cbz r3, .L6 @ __result,
    strb    r2, [r3]    @ D.20348, MEM[base: __result_23, offset: 0B]
.L6:
    adds    r3, r3, #1  @ __result, __result,
    cmp r3, r4  @ __result, D.20346
    bne .L7 @,
.L8:
    ldr r0, [r6]    @ D.20346, MEM[(struct vector *)this_1(D)].D.18218._M_impl._M_start
    cbz r0, .L5 @ D.20346,
    bl  _ZdlPv  @
.L5:
    str r7, [r6]    @ __result, MEM[(struct vector *)this_1(D)].D.18218._M_impl._M_start
    mov r0, r6  @, this
    str r4, [r6, #4]    @ D.20346, MEM[(struct vector *)this_1(D)].D.18218._M_impl._M_finish
    str r4, [r6, #8]    @ D.20346, MEM[(struct vector *)this_1(D)].D.18218._M_impl._M_end_of_storage
    pop {r3, r4, r5, r6, r7, pc}    @
.L26:
    adds    r4, r0, r5  @ D.20346, __result, length
    b   .L8 @
.L11:
    ldr r0, [r6]    @ D.20346, MEM[(struct _Vector_base *)this_1(D)]._M_impl._M_start
    cbz r0, .L10    @ D.20346,
    bl  _ZdlPv  @
.L10:
    bl  __cxa_end_cleanup   @

# C++14, gcc 5.2

    push    {r3, r4, r5, r6, r7, lr}    @
    movs    r3, #0  @ tmp118,
    mov r4, r0  @ this, this
    str r3, [r0]    @ tmp118, MEM[(struct _Vector_impl *)this_1(D)]._M_start
    str r3, [r0, #4]    @ tmp118, MEM[(struct _Vector_impl *)this_1(D)]._M_finish
    str r3, [r0, #8]    @ tmp118, MEM[(struct _Vector_impl *)this_1(D)]._M_end_of_storage
    cbnz    r2, .L19    @ length,
    mov r0, r4  @, this
    pop {r3, r4, r5, r6, r7, pc}    @
.L19:
    mov r0, r2  @, length
    mov r6, r1  @ pData, pData
    mov r5, r2  @ length, length
    bl  _Znwj   @
    mov r2, r5  @, length
    mov r1, r6  @, pData
    mov r7, r0  @ D.20824,
    bl  memcpy  @
    ldr r0, [r4]    @ D.20823, MEM[(struct vector *)this_1(D)].D.18751._M_impl._M_start
    cbz r0, .L3 @ D.20823,
    bl  _ZdlPv  @
.L3:
    add r5, r5, r7  @ D.20823, D.20824
    str r7, [r4]    @ D.20824, MEM[(struct vector *)this_1(D)].D.18751._M_impl._M_start
    str r5, [r4, #4]    @ D.20823, MEM[(struct vector *)this_1(D)].D.18751._M_impl._M_finish
    mov r0, r4  @, this
    str r5, [r4, #8]    @ D.20823, MEM[(struct vector *)this_1(D)].D.18751._M_impl._M_end_of_storage
    pop {r3, r4, r5, r6, r7, pc}    @
.L6:
    ldr r0, [r4]    @ D.20823, MEM[(struct _Vector_base *)this_1(D)]._M_impl._M_start
    cbz r0, .L5 @ D.20823,
    bl  _ZdlPv  @
.L5:
    bl  __cxa_end_cleanup   @

这是4.9.2版本中的一个GCC错误，请参阅。默认的

-std=gnu++03

模式和

-std=c++14

之间的区别在于，对于c++11和更高版本，可能会有不可分配的琐碎类型（因为它们可能有一个已删除的赋值运算符），这会导致

std:：uninitialized_copy

的实现采用不同（较慢）的代码路径。对可分配性的检查是错误的，这意味着我们在不需要的时候走了一条慢路

我在两年前为GCC4.9.3修复了它，但是您的编译器基于4.9.2和4.9.3版本之间的快照，并且已经过了几周没有修复了

您可以要求Linaro将其GCC 4.9编译器更新为4.9.4，或者至少应用修补程序来修复此错误。

这可能不会有太大区别，但为什么要分配而不是初始化？后者可以避免一次分配

VectorWrapper_t（uint8_t const*pData，size_t length）：data（pData，pData+length）{}

这个问题似乎更适合Linaro bug报告。2种可能性：要么代码开始相同，但gcc-5+检测到循环等效于memcpy，并用对memcpy的调用替换它，或者libstdc++获得了一些新的特殊路径，称为memcpy。尝试传递

-fdump tree all

，查看一些早期/晚期转储以确定发生了什么。我手头没有任何标准，但分配的规范在它们之间是否发生了变化？如果不是，我会称之为回归bug@AndreiR.

memmove

中的

move

与C++11移动语义无关。这两个函数都不修改源数组。

# C++03, gcc 4.9

    push    {r3, r4, r5, r6, r7, lr}    @
    movs    r3, #0  @ tmp118,
    mov r4, r0  @ this, this
    str r3, [r0]    @ tmp118, MEM[(struct _Vector_impl *)this_1(D)]._M_start
    mov r5, r2  @ length, length
    str r3, [r0, #4]    @ tmp118, MEM[(struct _Vector_impl *)this_1(D)]._M_finish
    str r3, [r0, #8]    @ tmp118, MEM[(struct _Vector_impl *)this_1(D)]._M_end_of_storage
    cbnz    r2, .L19    @ length,
    mov r0, r4  @, this
    pop {r3, r4, r5, r6, r7, pc}    @
.L19:
    mov r0, r2  @, length
    mov r6, r1  @ pData, pData
    bl  _Znwj   @
    mov r2, r5  @, length
    mov r1, r6  @, pData
    mov r7, r0  @ D.13516,
    bl  memmove @
    ldr r0, [r4]    @ D.13515, MEM[(struct vector *)this_1(D)].D.11902._M_impl._M_start
    cbz r0, .L3 @ D.13515,
    bl  _ZdlPv  @
.L3:
    add r5, r5, r7  @ D.13515, D.13516
    str r7, [r4]    @ D.13516, MEM[(struct vector *)this_1(D)].D.11902._M_impl._M_start
    str r5, [r4, #4]    @ D.13515, MEM[(struct vector *)this_1(D)].D.11902._M_impl._M_finish
    mov r0, r4  @, this
    str r5, [r4, #8]    @ D.13515, MEM[(struct vector *)this_1(D)].D.11902._M_impl._M_end_of_storage
    pop {r3, r4, r5, r6, r7, pc}    @
.L6:
    ldr r0, [r4]    @ D.13515, MEM[(struct _Vector_base *)this_1(D)]._M_impl._M_start
    cbz r0, .L5 @ D.13515,
    bl  _ZdlPv  @
.L5:
    bl  __cxa_end_cleanup   @

# C++14, gcc 4.9

    push    {r3, r4, r5, r6, r7, lr}    @
    movs    r3, #0  @ tmp157,
    mov r6, r0  @ this, this
    str r3, [r0]    @ tmp157, MEM[(struct _Vector_impl *)this_1(D)]._M_start
    mov r5, r2  @ length, length
    str r3, [r0, #4]    @ tmp157, MEM[(struct _Vector_impl *)this_1(D)]._M_finish
    str r3, [r0, #8]    @ tmp157, MEM[(struct _Vector_impl *)this_1(D)]._M_end_of_storage
    cbnz    r2, .L25    @ length,
    mov r0, r6  @, this
    pop {r3, r4, r5, r6, r7, pc}    @
.L25:
    mov r0, r2  @, length
    mov r4, r1  @ pData, pData
    bl  _Znwj   @
    adds    r3, r4, r5  @ D.20345, pData, length
    mov r7, r0  @ __result,
    cmp r4, r3  @ pData, D.20345
    ittt    ne
    addne   r1, r4, #-1 @ ivtmp.76, pData,
    movne   r3, r0  @ __result, __result
    addne   r4, r0, r5  @ D.20346, __result, length
    beq .L26    @,
.L7:
    ldrb    r2, [r1, #1]!   @ zero_extendqisi2  @ D.20348, MEM[base: _48, offset: 0]
    cbz r3, .L6 @ __result,
    strb    r2, [r3]    @ D.20348, MEM[base: __result_23, offset: 0B]
.L6:
    adds    r3, r3, #1  @ __result, __result,
    cmp r3, r4  @ __result, D.20346
    bne .L7 @,
.L8:
    ldr r0, [r6]    @ D.20346, MEM[(struct vector *)this_1(D)].D.18218._M_impl._M_start
    cbz r0, .L5 @ D.20346,
    bl  _ZdlPv  @
.L5:
    str r7, [r6]    @ __result, MEM[(struct vector *)this_1(D)].D.18218._M_impl._M_start
    mov r0, r6  @, this
    str r4, [r6, #4]    @ D.20346, MEM[(struct vector *)this_1(D)].D.18218._M_impl._M_finish
    str r4, [r6, #8]    @ D.20346, MEM[(struct vector *)this_1(D)].D.18218._M_impl._M_end_of_storage
    pop {r3, r4, r5, r6, r7, pc}    @
.L26:
    adds    r4, r0, r5  @ D.20346, __result, length
    b   .L8 @
.L11:
    ldr r0, [r6]    @ D.20346, MEM[(struct _Vector_base *)this_1(D)]._M_impl._M_start
    cbz r0, .L10    @ D.20346,
    bl  _ZdlPv  @
.L10:
    bl  __cxa_end_cleanup   @

# C++14, gcc 5.2

    push    {r3, r4, r5, r6, r7, lr}    @
    movs    r3, #0  @ tmp118,
    mov r4, r0  @ this, this
    str r3, [r0]    @ tmp118, MEM[(struct _Vector_impl *)this_1(D)]._M_start
    str r3, [r0, #4]    @ tmp118, MEM[(struct _Vector_impl *)this_1(D)]._M_finish
    str r3, [r0, #8]    @ tmp118, MEM[(struct _Vector_impl *)this_1(D)]._M_end_of_storage
    cbnz    r2, .L19    @ length,
    mov r0, r4  @, this
    pop {r3, r4, r5, r6, r7, pc}    @
.L19:
    mov r0, r2  @, length
    mov r6, r1  @ pData, pData
    mov r5, r2  @ length, length
    bl  _Znwj   @
    mov r2, r5  @, length
    mov r1, r6  @, pData
    mov r7, r0  @ D.20824,
    bl  memcpy  @
    ldr r0, [r4]    @ D.20823, MEM[(struct vector *)this_1(D)].D.18751._M_impl._M_start
    cbz r0, .L3 @ D.20823,
    bl  _ZdlPv  @
.L3:
    add r5, r5, r7  @ D.20823, D.20824
    str r7, [r4]    @ D.20824, MEM[(struct vector *)this_1(D)].D.18751._M_impl._M_start
    str r5, [r4, #4]    @ D.20823, MEM[(struct vector *)this_1(D)].D.18751._M_impl._M_finish
    mov r0, r4  @, this
    str r5, [r4, #8]    @ D.20823, MEM[(struct vector *)this_1(D)].D.18751._M_impl._M_end_of_storage
    pop {r3, r4, r5, r6, r7, pc}    @
.L6:
    ldr r0, [r4]    @ D.20823, MEM[(struct _Vector_base *)this_1(D)]._M_impl._M_start
    cbz r0, .L5 @ D.20823,
    bl  _ZdlPv  @
.L5:
    bl  __cxa_end_cleanup   @