Agner`s CPU blog

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Indexed registers
Author:  Date: 2016-10-04 10:57
Kurt Baumgardner wrote:
[...]
Well, that's a shame. I've been looking at some 80x86 disassembly of a C .exe recently, that contained a lot of function calls. Most of them do a bunch of PUSHes, read some stack vars, followed by the function's actual purpose, and end with the POPs, and finally the return. And, of course that's in the majority of function calls: this constant pattern of register save, fill, use, reload, followed by return (another stack pop). Seems like there should be a better way. Maybe instead of a set for every program/thread, you could have just enough space for a few levels deep of registers. This would correspond with the "call stack". Since most calls come with PUSHes, and most returns are proceeded by POPs, it seems like a prime candidate for optimization, for both speed and code size.

Maybe that's still too complex or slow for hardware, though.

Function calls and their associated push/pops are very common, but a large majority of them happen on cold paths:
- Anything that happens on a cold path rarely makes any noticeable speed difference... Even moderately warm paths tend to count a lot less than the hottest paths in overall speed.
- The hottest paths often are loops and tend to either not have any function calls, or have calls that are only occasionally called (error conditions etc), or have calls that can be inlined - which means the compiler can automatically allocate registers and even reorder operations within the parent function, as long as the function is small enough for this to be beneficial.
- Hot loops that do have function calls are often very large and complex loops that are likely to stall for all sorts of other reasons, such as not fitting in the instruction cache, having data cache misses, having pointer-following chains that stall due to data cache latency, having hard-to-predict branches, loading/storing lots of values from objects and so forth, so the function call overhead might not stand out in the wash, or if you're lucky, it could happen while the CPU is waiting after something else (which makes it free).

One option would be register windows like on SPARC, which acts as a hardware assisted stack. Here's an article about that:
http://ieng9.ucsd.edu/~cs30x/sparcstack.html

Another option would be load-dual and store-dual instructions, which let you combine two 64-bit register memory operations into a single 128-bit store (as long as your stack is 128 bit-aligned) and is probably as fast possible for saving/restoring a bunch of registers at the same time (especially if you can dual-issue it to a 2-port data cache).

 
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