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问题描述
我在读手臂手册和来此建议，但没有被提及的原因。
为什么无符号类型更快？
解决方案
解决方案
我在读手臂手册和来此建议，但没有被提及的原因。
为什么无符号类型更快？
ARMv4之前，ARM曾装载半字和有符号字节没有原生支持
LDRB 一个符号字节再签收延长值（ LSL 它那么 ASR 它背下来）。这是痛苦所以字符是无符号默认在ARM版本的指令被添加到处理半字和符号值。这些新指令必须挤进可用指令空间。可在空间限制意味着它们不能被制成柔性的原始指令，加载值时，其能够执行各种地址计算
所以，你可能会发现， LDRSB ，例如，是无法结合的内存地址计算，而 LDRB 可能。这可以花费的周期。有时候，我们可以返工短 - 重型code上对 int的操作来避免这个问题。
有在这里我的网站的详细信息：http://www.davespace.co.uk/arm/efficient-c-for-arm/memaccess.html
I'm reading an arm manual and come to this suggestion, but the reason is not mentioned.
Why unsigned types are faster?
Prior to ARMv4, ARM had no native support for loading halfwords and signed bytes. To load a signed byte you had to LDRB then sign extend the value (LSL it up then ASR it back down). This is painful so char is unsigned by default.
In ARMv4 instructions were added to handle halfwords and signed values. These new instructions had to be squeezed into the available instruction space. Limits on the space available meant that they could not be made as flexible as the original instructions, which are able to do various address computations when loading the value.
So you may find that LDRSB, for example, is unable to combine a fetch from memory with an address computation whereas LDRB could. This can cost cycles. Sometimes we can rework short-heavy code to operate on pairs of ints to avoid this.
There's more info on my site here: http://www.davespace.co.uk/arm/efficient-c-for-arm/memaccess.html
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