用於32位字旋轉的ARM內聯匯編

Question

我正在嘗試制作一些內聯匯編來測試ARM上的旋轉性能。 該代碼是C ++代碼庫的一部分，因此旋轉是模板專業化。 代碼在下面，但是它產生的消息對我來說沒有多大意義。

根據ARM匯編語言 ，說明大致如下：

# rotate - rotate instruction
# dst - output operand
# lhs - value to be rotated
# rhs - rotate amount (immediate or register)
<rotate> <dst>, <lhs>, <rhs>

它們沒有多大意義，因為例如（對我而言），我使用g來約束輸出寄存器，而這只是每個Simple Contraints的通用寄存器。 ARM應該有很多，並且特定於機器的約束似乎沒有改變約束的行為。

我不確定解決此問題的最佳方法，因此我將提出三個問題：

1. 使用常數或立即數時，如何編碼旋轉？
2. 使用通過寄存器傳遞的值時，如何編碼旋轉？
3. 拇指模式將如何改變內聯裝配

---

arm-linux-androideabi-g++ -DNDEBUG -g2 -Os -pipe -fPIC -mfloat-abi=softfp
-mfpu=vfpv3-d16 -mthumb --sysroot=/opt/android-ndk-r10e/platforms/android-21/arch-arm
-I/opt/android-ndk-r10e/sources/cxx-stl/stlport/stlport/ -c camellia.cpp
In file included from seckey.h:9:0,
             from camellia.h:9,
             from camellia.cpp:14:
misc.h: In function 'T CryptoPP::rotlFixed(T, unsigned int) [with T = unsigned int]':
misc.h:1121:71: error: matching constraint not valid in output operand
  __asm__ ("rol %2, %0, %1" : "=g2" (z) : "g0" (x), "M1" ((int)(y%32)));
                                                                       ^
misc.h:1121:71: error: matching constraint references invalid operand number
misc.h: In function 'T CryptoPP::rotrFixed(T, unsigned int) [with T = unsigned int]':
misc.h:1129:71: error: matching constraint not valid in output operand
  __asm__ ("ror %2, %0, %1" : "=g2" (z) : "g0" (x), "M1" ((int)(y%32)));
                                                                       ^
misc.h:1129:71: error: matching constraint references invalid operand number
misc.h: In function 'T CryptoPP::rotlVariable(T, unsigned int) [with T = unsigned int]':
misc.h:1137:72: error: matching constraint not valid in output operand
  __asm__ ("rol %2, %0, %1"  : "=g2" (z) : "g0" (x), "g1" ((int)(y%32)));
                                                                        ^
misc.h:1137:72: error: matching constraint references invalid operand number
misc.h: In function 'T CryptoPP::rotrVariable(T, unsigned int) [with T = unsigned int]':
misc.h:1145:72: error: matching constraint not valid in output operand
  __asm__ ("ror %2, %0, %1"  : "=g2" (z) : "g0" (x), "g1" ((int)(y%32)));
                                                                        ^
misc.h:1145:72: error: matching constraint references invalid operand number
misc.h: In function 'T CryptoPP::rotrFixed(T, unsigned int) [with T = unsigned int]':
misc.h:1129:71: error: matching constraint not valid in output operand
  __asm__ ("ror %2, %0, %1" : "=g2" (z) : "g0" (x), "M1" ((int)(y%32)));
                                                                       ^
misc.h:1129:71: error: invalid lvalue in asm output 0
misc.h:1129:71: error: matching constraint references invalid operand number
misc.h: In function 'T CryptoPP::rotlFixed(T, unsigned int) [with T = unsigned int]':
misc.h:1121:71: error: matching constraint not valid in output operand
  __asm__ ("rol %2, %0, %1" : "=g2" (z) : "g0" (x), "M1" ((int)(y%32)));
                                                                       ^
misc.h:1121:71: error: invalid lvalue in asm output 0
misc.h:1121:71: error: matching constraint references invalid operand number

---

// ROL #n Rotate left immediate
template<> inline word32 rotlFixed<word32>(word32 x, unsigned int y)
{
    int z;
    __asm__ ("rol %2, %0, %1" : "=g2" (z) : "g0" (x), "M1" ((int)(y%32)));
    return static_cast<word32>(z);
}

// ROR #n Rotate right immediate
template<> inline word32 rotrFixed<word32>(word32 x, unsigned int y)
{
    int z;
    __asm__ ("ror %2, %0, %1" : "=g2" (z) : "g0" (x), "M1" ((int)(y%32)));
    return static_cast<word32>(z);
}

// ROR rn Rotate left by a register
template<> inline word32 rotlVariable<word32>(word32 x, unsigned int y)
{
    int z;
    __asm__ ("rol %2, %0, %1"  : "=g2" (z) : "g0" (x), "g1" ((int)(y%32)));
    return static_cast<word32>(z);
}

// ROR rn Rotate right by a register
template<> inline word32 rotrVariable<word32>(word32 x, unsigned int y)
{
    int z;
    __asm__ ("ror %2, %0, %1"  : "=g2" (z) : "g0" (x), "g1" ((int)(y%32)));
    return static_cast<word32>(z);
}

template<> inline word32 rotlMod<word32>(word32 x, unsigned int y)
{
    return rotlVariable<word32>(x, y);
}

template<> inline word32 rotrMod<word32>(word32 x, unsigned int y)
{
    return rotrVariable<word32>(x, y);
}

Answer 1

首先，ARM沒有向左旋轉（ ROL ），您需要通過ROR進行模擬。

其次，由於某種原因， M約束接受0到32，但是ROL在處理立即數時僅接受0到31。

第三， g約束過於籠統，因為它還允許ROR不接受的內存操作數。 最好改用r 。

這是我想出的：

// Rotate right
inline word32 rotr(word32 x, unsigned int y)
{
    int z;
    if (__builtin_constant_p(y))
    {
        y &= 31;
        if (y != 0) // this should be optimized away by the compiler
        {
            __asm__ ("ror %0, %1, %2" : "=r" (z) : "r" (x), "M" (y));
        }
    } else {
        __asm__ ("ror %0, %1, %2" : "=r" (z) : "r" (x), "r" (y));
    }
    return static_cast<word32>(z);
}

// Rotate left
inline word32 rotl(word32 x, unsigned int y)
{
    int z;
    if (__builtin_constant_p(y))
    {
        y &= 31;
        if (y != 0) // this should be optimized away by the compiler
        {
            __asm__ ("ror %0, %1, %2" : "=r" (z) : "r" (x), "M" (32 - y));
        }
    } else {
        __asm__ ("ror %0, %1, %2" : "=r" (z) : "r" (x), "r" (32 - y));
    }
    return static_cast<word32>(z);
}