Can a single core do multiple compare/add operations at once?

Question

I want to know if a CPU core can do multiple x86 comparison and add operations at once in parallel.

So if I wrote something like

Compare X y
Compare y z
Add X y
Add q p

Would the compares run at the same time? Would the adds run at the same time?

Answer 1

Yes, provided that the CPU has 2 free comparers/2 adders, the data are independent of each other and the instruction dispatcher can deliver to both units at the same time. That's how superscalar CPUs work. All x86 CPUs from P5 Pentium as well as all modern CPUs are superscalar.

That's one of the reasons newer microarchitectures run faster than older ones even though clock cycle and the number of cores are still the same, because they can have more execution units, bigger units, bigger cache, run more instructions at the same time along with numerous other improvements. For example

Current x86 processors can deliver 3 instructions per clock cycle. Conroe, however, has been architected to fetch, dispatch, execute and retire up to four full instructions simultaneously, offering a 33% boost over, say, a Pentium 4 CPU

https://hexus.net/tech/tech-explained/cpu/17976-intel-core-2-duo-conroe/

It's hard to define a clear value due to the use of micro-op and macro-fusion. However for some rough comparison, Sandy Bridge is able to achieve 6 micro-ops per cycle. e.g. it can sustain 1 iteration per clock throughput for a loop consisting of two add instructions with memory sources, a multiply, and a macro-fused dec-and-branch.

See also

• Out-of-order execution
• Is processor can do memory and arithmetic operation at the same time?
• Exactly how "fast" are modern CPUs?
• How could a CPU execute more than one instruction per clock cycle?
• Difference between superscalar and multi-core?
• superscalar and VLIW
• Instructions per cycle
• How many CPU cycles are needed for each assembly instruction?
• How can a CPU deliver more than one instruction per cycle?
• Superscalar Processor
• Modern Microprocessors A 90-Minute Guide! a nice article introducing pipelining, then superscalar, then out-of-order execution.
• David Kanter's Sandybridge microarchitecture deep dive, with block diagrams. (He's done writeups for earlier Intel uarches, and Haswell. And for Several AMD microarchitectures.)