Step to UEFI (176)memset的实现方法

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之前的文章“哪里来的的 memset”【参考1】提到过因为编译器擅作主张使用memset优化引起了很诡异的问题。可以通过关闭编译优化来避免错误,这里从代码的角度分析 EDK2 是如何实现 memset 功能的。

  1. \MdePkg\Library\BaseMemoryLib\MemLibGeneric.c 提供了三个函数

InternalMemSetMem16

 InternalMemSetMem32

InternalMemSetMem64

以 InternalMemSetMem16  为例:

/**
  Fills a target buffer with a 16-bit value, and returns the target buffer.

  @param  Buffer  The pointer to the target buffer to fill.
  @param  Length  The count of 16-bit value to fill.
  @param  Value   The value with which to fill Length bytes of Buffer.

  @return Buffer

**/
VOID *
EFIAPI
InternalMemSetMem16 (
  OUT     VOID                      *Buffer,
  IN      UINTN                     Length,
  IN      UINT16                    Value
  )
{
  for (; Length != 0; Length--) {
    ((UINT16*)Buffer)[Length - 1] = Value;
  }
  return Buffer;
}

看起来for (; Length != 0; Length--) 这样的定义足够“迷惑”编译器避免优化。

2. \MdePkg\Library\BaseMemoryLib\SetMem.c 提供了InternalMemSetMem()

/**
  Set Buffer to Value for Size bytes.

  @param  Buffer   The memory to set.
  @param  Length   The number of bytes to set.
  @param  Value    The value of the set operation.

  @return Buffer

**/
VOID *
EFIAPI
InternalMemSetMem (
  OUT     VOID                      *Buffer,
  IN      UINTN                     Length,
  IN      UINT8                     Value
  )
{
  //
  // Declare the local variables that actually move the data elements as
  // volatile to prevent the optimizer from replacing this function with
  // the intrinsic memset()
  //
  volatile UINT8                    *Pointer8;
  volatile UINT32                   *Pointer32;
  volatile UINT64                   *Pointer64;
  UINT32                            Value32;
  UINT64                            Value64;

  if ((((UINTN)Buffer & 0x7) == 0) && (Length >= 8)) {
    // Generate the 64bit value
    Value32 = (Value << 24) | (Value << 16) | (Value << 8) | Value;
    Value64 = LShiftU64 (Value32, 32) | Value32;

    Pointer64 = (UINT64*)Buffer;
    while (Length >= 8) {
      *(Pointer64++) = Value64;
      Length -= 8;
    }

    // Finish with bytes if needed
    Pointer8 = (UINT8*)Pointer64;
  } else if ((((UINTN)Buffer &amp; 0x3) == 0) &amp;&amp; (Length >= 4)) {
    // Generate the 32bit value
    Value32 = (Value << 24) | (Value << 16) | (Value << 8) | Value;

    Pointer32 = (UINT32*)Buffer;
    while (Length >= 4) {
      *(Pointer32++) = Value32;
      Length -= 4;
    }

    // Finish with bytes if needed
    Pointer8 = (UINT8*)Pointer32;
  } else {
    Pointer8 = (UINT8*)Buffer;
  }
  while (Length-- > 0) {
    *(Pointer8++) = Value;
  }
  return Buffer;
}

避免被编译器优化的方法和上面的类似,此外还可以看出这个函数特地用 8 bytes填充提升效率。

3. \MdePkg\Library\UefiMemoryLib\MemLib.c 中的InternalMemSetMem 函数直接调用 gBS 提供的服务

/**
  Fills a target buffer with a byte value, and returns the target buffer.

  This function wraps the gBS->SetMem().

  @param  Buffer    Memory to set.
  @param  Size      The number of bytes to set.
  @param  Value     Value of the set operation.

  @return Buffer.

**/
VOID *
EFIAPI
InternalMemSetMem (
  OUT     VOID                      *Buffer,
  IN      UINTN                     Size,
  IN      UINT8                     Value
  )
{
  gBS->SetMem (Buffer, Size, Value);
  return Buffer;
}

4. 通过volatile 申明变量避免编译器的优化,简单粗暴,很前面2提到的没有本质差别。volatile是一个类型修饰符(type specifier).volatile的作用是作为指令关键字,确保本条指令不会因编译器的优化而省略,且要求每次直接读值。volatile的变量是说这变量可能会被意想不到地改变,这样,编译器就不会去假设这个变量的值了。【参考2】

 \EdkCompatibilityPkg\Foundation\Library\EdkIIGlueLib\Library\BaseMemoryLib\Ebc\SetMem.c

/**
  Set Buffer to Value for Size bytes.

  @param  Buffer Memory to set.
  @param  Size Number of bytes to set
  @param  Value Value of the set operation.

  @return Buffer

**/
VOID *
EFIAPI
InternalMemSetMem (
  IN      VOID                      *Buffer,
  IN      UINTN                     Size,
  IN      UINT8                     Value
  )
{
  //
  // Declare the local variables that actually move the data elements as
  // volatile to prevent the optimizer from replacing this function with
  // the intrinsic memset()
  //
  volatile UINT8                    *Pointer;

  Pointer = (UINT8*)Buffer;
  while (Size-- != 0) {
    *(Pointer++) = Value;
  }
  return Buffer;
}

5.汇编语言实现

\EdkCompatibilityPkg\Foundation\Library\CompilerStub\X64\memset.asm

\EdkCompatibilityPkg\Foundation\Library\CompilerStub\Ia32\memset.asm

IA32汇编的实现

    .686
    .model  flat,C
    .mmx
    .code

;------------------------------------------------------------------------------
;  VOID *
;  memset (
;    OUT VOID   *Buffer,
;    IN  UINT8  Value,
;    IN  UINTN  Count
;    )
;------------------------------------------------------------------------------
memset   PROC    USES    edi
    mov     al, [esp + 12]
    mov     ah, al
    shrd    edx, eax, 16
    shld    eax, edx, 16
    mov     ecx, [esp + 16]             ; ecx <- Count
    cmp     ecx, 0                      ; if Count == 0, do nothing
    je      @SetDone
    mov     edi, [esp + 8]              ; edi <- Buffer
    mov     edx, ecx
    and     edx, 7
    shr     ecx, 3                      ; # of Qwords to set
    jz      @SetBytes
    add     esp, -10h
    movq    [esp], mm0                  ; save mm0
    movq    [esp + 8], mm1              ; save mm1
    movd    mm0, eax
    movd    mm1, eax
    psllq   mm0, 32
    por     mm0, mm1                    ; fill mm0 with 8 Value's
@@:
    movq    [edi], mm0
    add     edi, 8
    loop    @B
    movq    mm0, [esp]                  ; restore mm0
    movq    mm1, [esp + 8]              ; restore mm1
    add     esp, 10h                    ; stack cleanup
@SetBytes:
    mov     ecx, edx
    rep     stosb
@SetDone:    
    mov     eax, [esp + 8]              ; eax <- Buffer as return value
    ret
memset   ENDP

    END

上面就是实现 SetMem 函数的基本方法,如果在 Porting 代码到 UEFI时遇到 MemSet 的错误,不妨试试直接将上面的代码搬迁到程序中。

参考:

  1. http://www.lab-z.com/stu136/  Step to UEFI (136)哪里来的的 memset 
  2. https://baike.baidu.com/item/volatile/10606957?fr=aladdin volatile

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