In another thread, I was told that a switch
may be better than a lookup table in terms of speed and compactness.
So I'd like to understand the differences between this:
static void func1(){} static void func2(){} typedef enum { FUNC1, FUNC2, FUNC_COUNT } state_e; typedef void (*func_t)(void); const func_t lookUpTable[FUNC_COUNT] = { [FUNC1] = &func1, [FUNC2] = &func2 }; void fsm(state_e state) { if (state < FUNC_COUNT) lookUpTable[state](); else ;// Error handling }
and this:
static void func1(){} static void func2(){} void fsm(int state) { switch(state) { case FUNC1: func1(); break; case FUNC2: func2(); break; default: ;// Error handling } }
I thought that a lookup table was faster since compilers try to transform switch statements into jump tables when possible. Since this may be wrong, I'd like to know why!
Thanks for your help!
As I was the original author of the comment, I have to add a very important issue you did not mention in your question. That is, the original was about an embedded system. Presuming this is a typical bare-metal system with integrated Flash, there are very important differences from a PC on which I will concentrate.
Such embedded systems typically have the following constraints.
For e.g. the STM32F4xx a read takes 6 clocks at 150MHz/3.3V for 128 bits (4 words). So if a data-access is required, chances are good it adds more than 12 clocks delay for all data to be fetched (there are additional cycles involved).
Presuming compact state-codes, for the actual problem, this has the following effects on this architecture (Cortex-M4):
Also note that the switch
does not need functions, thus the compiler can fully optimise the code. This is not possible for a lookup table. At least code for function entry/exit is not required.
Due to the aforementioned and other factors, an estimate is hard to tell. It heavily depends on your platform and the code structure. But assuming the system given above, the switch is very likely faster (and clearer, btw.).
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