machine-instruction

I am working on a very low level part of the application in which performance is critical. While investigating the generated assembly, I noticed the following instruction: lea eax,[edx*8+8] I am used to seeing additions when using memory references (e.g. [edx+4]), but this is the first time I see a multiplication. Does this mean that the x86 processor can perform simple multiplications in the lea instruction? Does this multiplication have an impact on the number of cycles needed to execute the instruction? Is the multiplication limited to powers of 2 (I would assume this is the case)? Thanks

I am trying to understand more deeply the instruction selection process in llvm and for that I am debuging step-by-step the CodeGenAndEmitDAG function. I have printed a small function (see below) just before the combine step - the first step in the above function. In the graph I see blue lines and it seems that they are always pointing at "ch" , which I think means "other" machine value type. What I don't understand is the meaning of the blue lines... what is this dependency ? And, am I right about the meaning of "ch" ? is it "other" ? Dashed blue arrows represent non-dataflow dependencies

On today's modern processors, is there any performance difference between greater than and greater than or equal comparison for a branch condition? If I have a condition that could just as easily be either, is there any slight advantage to choosing > over >= or vice-versa? (This would be for a compiled language on Intel or AMD hardware) There shouldn't be any noticeable difference between comparing different predicates, because of the way they're computed (beware I haven't read the x86 manuals in detail so it may work different): Most instructions produce several flags as a byproduct, usually

In Processor, how are Data, Address and instructions are differentiated? When a program is executed, how is processor differentiate instructions when everything is in 0 s and 1 s and register`s load both data, addresses? The processor only does what you tell it to do. As you noted, the processor can't tell the difference between "data" and "code" in memory: it's all just a sequence of bytes. It's what you tell it to do with those bytes that defines how it's treated. When a program is compiled, the generated executable file has information in it that says which parts are code and which parts

I always wondered what's the purpose of the rotate instructions some CPUs have (ROL, RCL on x86, for example). What kind of software makes use of these instructions? I first thought they may be used for encryption/computing hash codes, but these libraries are written usually in C, which doesn't have operators that map to these instructions. Has anybody found an use for them? Why where they added to the instructions set? Rotates are required for bit shifts across multiple words. When you SHL the lower word, the high-order bit spills out into the carry. To complete the operation, you need to

I thought that there was zero. But, I see here, Instructions with two memory operands are extremely rare I can't find anything that explains what instructions, though rare, exist. What are the exceptions? Peter Cordes I can't find anything that explains the rarity. An x86 instruction can have at most one ModR/M + SIB + disp0/8/32. So there are zero instructions with two explicit memory operands. The x86 memory-memory instructions all have at least one implicit memory operand whose location is baked in to the opcode, like push which accesses the stack, or the string instructions movs and cmps .

I heard about the chicken and the egg and bootstrapping. I have a few questions. What wrote the first compiler that converted something into binary instructions? Is assembly compiled or translated into binary instructions? ...I'd find it hard to believe they wrote a compiler in binary. Assembly instructions are (generally) a direct mapping to opcodes, which are (multi-)byte values of machine code that can be directly interpreted by the processor. It is quite possible to write a program in opcodes directly by looking them up from a table (such as this one for the 6039 microprocessor , for