ARM memcpy和校准

问题描述

我使用的是执行内存复制与Windows嵌入在Cortex-A8处理器Compact 7的ARM汇编。

I am using the NEON memory copy with preload implementation from the ARM website with the Windows Embedded Compact 7 ARM assembler on a Cortex-A8 processor.

我注意到，我得到的数据类型不对例外，当我提供非字对齐值的功能

I notice that I get datatype misalignment exceptions when I provide that function with non word aligned values

例如：

; NEON memory copy with preload
ALIGN
LEAF_ENTRY NEONCopyPLD
    PLD [r1, #0xC0]
    VLDM r1!,{d0-d7} ;datatype misalignment
    VSTM r0!,{d0-d7}
    SUBS r2,r2,#0x40
    MOV R0, #0
    MOV PC, LR
ENTRY_END

size_t size = /* arbitrary */;
size_t offset = 1;
char* src = new char[ size + offset ];
char* dst = new char[ size ];

NEONCopyPLD( dst, src + offset, size );

memcpy( dst, src + offset, size ); /* works perfectly */

这是预期的VLDM命令？文章没有提到这个实现仅限于字对齐值。它是可以解决的？如果是这样，怎么样？

Is this expected for the VLDM command? The article doesn't mention that this implementation is limited to word-aligned values. Is it fixable? If so, how?

推荐答案

即使你没有指定你仍然需要对齐数据元素边界上（即双字边界在这种情况下）明确的对齐要求。也有一些例外情况，但它可能最好不要依赖它们，除非你有一个很好的理由这样做。

Even if you don't specify an explicit alignment requirement you still need to align the data on an element boundary (i.e. on a doubleword boundary in this case). There are some exceptions to this rule, but it's probably best not to rely on them unless you have a really good reason to do so.

有关更多信息，请参见Cortex-A8的技术参考手册（ARM DDI 0344J）。

See the Cortex-A8 technical reference manual (ARM DDI 0344J) for more information.

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