CODEWITHSUNDEEP
c++performanceboolean
tupini07
tupini07

Reputation: 558

Setting or consulting boolean. Which has the best performance?

Just for curiosity, which process is fastest: setting a value to a boolean (ex: changing it from true to false) or simple checking its value (ex: if(boolean)...)

Upvotes: 0

Views: 65

Answers (2)

user395760
user395760

Reputation:

The problem I have with "which is fastest" is that they are too underspecified to actually be answered conclusively, and at the same time too broad to yield useful conclusions even if answered conclusively. The only productive avenue your curiosity can take is to build a mental model of the machine and run both cases through it.

foo = true stores the value true to the location allocated to foo. That raises the question: Where and how is foo stored? This is impossible to answer without actually running the complete source code through the compiler with the right settings. It could be anywhere in RAM, or it could be in a register, or it could use no storage at all, being completely eliminated by compiler optimizations. Depending on where foo resides, the cost of overwriting it can vary: Hundreds of CPU cycles (if in RAM and not in cache), a couple cycles (in RAM and cache), one cycle (register), zero cycles (not stored).

if (foo) generally means reading foo and then performing a conditional branch based on it. Regarding the aspects I'll discuss here (I have to omit many details and some major categories), reading is effectively like writing. The conditional branch that follows has is even less predictable, as its cost depends on the run-time behavior of the program. If the branch is always taken, branch prediction will make it virtually free (a few cycles). If it's unpredictable, it may take tens of cycles and blow the pipeline (further reducing throughput). However, it's also possible that the conditional code will be predicated, invaliding most of the above concerns and replacing it with reasoning about instruction latency, data dependencies, and the gory details of the pipeline.

As you can see from the sheer volume to be written about it (despite omitting many details and even some important secondary effects), it's virtually impossible to really answer this in any generality. You need to look at concrete, complete programs to make any sort of prediction, and you need to know your whole system from top to bottom. Note that I had to assume a very specific kind of machine to even get this far: For a GPGPU or a embedded system or an early 90's consumer CPU, I'd have to rewrite almost all of that.

Upvotes: 2

hivert
hivert

Reputation: 10667

I think that this is so dependent on the context :

  • CPU/Architecture
  • Is the boolean stored in memory, cache, or register
  • whether what's behind the if induce a jump or only a conditional move
  • whether the value of the boolean is predictable or not

that the question doesn't finally makes sense.

Upvotes: 2

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