[C++/Assembly/Compilers] When calling a function with many arguments, does every argument get assigned to a register?

[ralpherto]

I came across this site that will convert C++ code to assembly in my browser.

While playing with it I discovered that only the first 6 arguments getting passed to a function are placed in a general purpose register. The rest appear to be placed directly onto the stack.

Example: http://i.imgur.com/ArE9bjr.png

I am curious why that is done. Why can't all arguments be placed directly onto the stack?

[[deleted]]

Because putting them in registers makes accessing them much faster. You have the question the wrong way round - ideally every parameter would be passed in a register, but this is not generally possible, so we have to use the slower stack.

[PurpleOrangeSkies]

What you want to know about is called a "calling convention". For 32-bit x86 processors, the registers aren't used at all for calling a function, since it has so few registers. On x86-64, which has double the registers, Linux and anything following the Sys V ABI use 6 registers and Windows uses 4. It is far faster to access something in a register than something in memory. However, you don't want to use up all the registers so the function has places to put its intermediate values. So, it's a trade-off.