Re: How are switches optimized

[Johan Engelen via Digitalmars-d-learn]

On Friday, 1 June 2018 at 21:18:25 UTC, IntegratedDimensions 
wrote:
What is the best optimizations that a compiler does to switches 
in general and in the D compilers?


The best possible depends a lot on the specific case at hand. 
Best possible is to fully elide the switch, which does happen.


You can use d.godbolt.org to investigate what happens for 
different pieces of code.

Sometimes a jumptable is used, sometimes an if-chain.
LLVM's (LDC) and GCC's (GDC) optimizers are strong and the 
optimized code will often do extra calculations before indexing 
in the table or before doing the comparisons in the if-chain. 
Different compilers will make different optimizations.


https://godbolt.org/g/pHptff
https://godbolt.org/g/AwZ69o

A switch can be seen as if statements, or safer, nested if 
elses.


but surely the cost per case does not grow with depth in the 
switch? If one has a switch of N case then the last cost surely 
does not cost N times the cost of the first, approximately?


Depends on the code, but it's not O(N).


This is the cost when implementing a switch as nested ifs.


Not true. Nested if's are optimized as well. Sometimes switch is 
faster, sometimes if-chain, sometimes it's the same.


Tables can be used to give O(1) cost, are these used in D's 
compilers?


Yes (LDC and GDC).

How are they generally implemented? Hash tables? If the switch 
is on an enum of small values is it optimized for a simple 
calculating offset table?


Table stored in the instruction stream. Simple offset table with 
calculation on the index value (I guess you could say it is a 
simplified hash table).


Note that with performance, the rest of the program and the 
execution flow also matters...


cheers,
  Johan

Re: How are switches optimized

[Dennis via Digitalmars-d-learn]

On Friday, 1 June 2018 at 21:18:25 UTC, IntegratedDimensions 
wrote:
If one has a switch of N case then the last cost surely does 
not cost N times the cost of the first, approximately?


It depends on the compiler and optimization level. In general, no 
optimization or just a handful of cases means it's as if you 
wrote a bunch of if-then-else statements. When there are many 
cases, a jump table is the go-to way to optimize it. But the 
compiler may do more clever transformations:


```
void smallCase();
void largeCase();
void defaultCase();

int switchfunc(int i)
{
switch(i) {
case 8: .. case 64:
smallCase(); return 1;
case 256: .. case 512:
largeCase(); return 2;
default:
defaultCase(); return 3;
}
}
```
Even though the front end expands the ranged cases to individual 
case statements, LDC with -O1 or higher will actually output code 
akin to this:


```
int switchfunc(int i) {
  if (cast(uint) (i - 256) >= 257) {
if (cast(uint) (i - 8) > 56) {
  smallCase(); return 1;
}
defaultCase(); return 3;
  }
  largeCase(); return 2;
}
```
Notice how even though I put the 256-512 range second, LDC chose 
to check it first because it's a larger range than 8-64. Also 
notice the use of unsigned integer underflow to check whether an 
integer is in a certain range.


Something I like to do for text reading functions is this:

```
bool isOneOf(string str)(char chr) {
  switch (chr) {
static foreach(c; str) {
  case c: return true;
}
default: return false;
  }
}

alias isHexadecimalDigit = isOneOf!"abcdefABCDEF0123456789";
```
This will efficiently check if a character is in a compile-time 
known character set using compile-time optimizations. While DMD 
makes a jumptable for this, LDC actually transforms it into this:

```
bool isHexadecimalDigit(char chr) {
  ubyte x = cast(ubyte) (chr - 48);
  if (x > 54) return false;
  return 
(0b11001100011 >> x) 
& 1;

}

```
The cases can be encoded in a binary number that is shifted by 
the value of the character, so that the correct boolean return 
value (1 or 0) ends up as the least significant bit.

DMD uses the same trick if you write it in this way:
```
return (chr == 'a' || chr == 'b' || chr == 'c' || ...);
```

So the most common optimization is indexing in (jump) tables, but 
smart transformations are done too sometimes. By the way, a chain 
of if-then-else statements may also be optimized as if it was 
written in a switch statement.


If you want to see how the compiler optimizes your switch 
statement, check out run.dlang.io (DMD and LDC, press ASM button) 
or d.godbolt.org (DMD, LDC and GDC).