为什么汇编指令可以在"lea"中包含乘法?操作说明? [英] Why can assembly instructions contain multiplications in the "lea" instruction?
问题描述
我正在研究性能非常关键的应用程序的非常低级的部分.
I am working on a very low level part of the application in which performance is critical.
在调查生成的程序集时，我注意到以下说明:
While investigating the generated assembly, I noticed the following instruction:
lea eax,[edx*8+8]
使用内存引用(例如[edx + 4])时，我习惯于看到加法，但这是我第一次看到乘法.
I am used to seeing additions when using memory references (e.g. [edx+4]), but this is the first time I see a multiplication.
- 这是否意味着x86处理器可以在lea指令中执行简单的乘法运算?
- 这种乘法对执行指令所需的周期数有影响吗?
- 乘法是否仅限于2的幂(我会假设是这种情况)?
谢谢.
推荐答案
扩大我的评论并回答其余问题...
To expand on my comment and to answer the rest of the question...
是的，仅限于2的幂. (特别是2、4和8)，因此不需要乘数，因为它只是一个移位.这样做的目的是从索引变量和指针快速生成地址-数据类型是简单的2、4或8字节字. (尽管它也经常被滥用作其他用途.)
Yes, it's limited to powers of two. (2, 4, and 8 specifically) So no multiplier is needed since it's just a shift. The point of it is to quickly generate an address from an index variable and a pointer - where the datatype is a simple 2, 4, or 8 byte word. (Though it's often abused for other uses as well.)
关于所需的循环数:根据 Agner Fog的表，它看起来像
As for the number of cycles that are needed: According to Agner Fog's tables it looks like the lea instruction is constant on some machines and variable on others.
在桑迪桥(Sandy Bridge)上，如果是复杂或相对亲戚"，将处以2个周期的罚款.但这并没有说明复杂"的含义...因此，除非您进行基准测试，否则我们只能猜测.
On Sandy Bridge there's a 2-cycle penalty if it's "complex or rip relative". But it doesn't say what "complex" means... So we can only guess unless you do a benchmark.
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