F# Breakable Array Iteration With Bounds Checking Elided? - arrays

Question

I am interested in trying F# in a high-performance application. I do not want to have a large array's bounds checked during iteration and the lack of break/return statements is concerning.
This is a contrived example that will break upon finding a value, but can someone tell me if bounds checking is also elided?
let innerExists (item: Char) (items: Char array): bool =
let mutable state = false
let mutable i = 0
while not state && i < items.Length do
state <- item = items.[i]
i <- i + 1
state
let exists (input: Char array)(illegalChars: Char array): bool =
let mutable state = false
let mutable i = 0
while not state && i < input.Length do
state <- innerExists input.[i] illegalChars
i <- i + 1
state
exists [|'A'..'z'|] [|'.';',';';'|]
Here is the relevant disassembly:
while not state && i < input.Length do
000007FE6EB4237A cmp dword ptr [rbp-14h],0
000007FE6EB4237E jne 000007FE6EB42383
000007FE6EB42380 nop
000007FE6EB42381 jmp 000007FE6EB42386
000007FE6EB42383 nop
000007FE6EB42384 jmp 000007FE6EB423A9
000007FE6EB42386 nop
000007FE6EB42387 mov r8d,dword ptr [rbp-18h]
000007FE6EB4238B mov rdx,qword ptr [rbp+18h]
000007FE6EB4238F cmp r8d,dword ptr [rdx+8]
000007FE6EB42393 setl r8b
000007FE6EB42397 movzx r8d,r8b
000007FE6EB4239B mov dword ptr [rbp-24h],r8d
000007FE6EB4239F mov r8d,dword ptr [rbp-24h]
000007FE6EB423A3 mov dword ptr [rbp-1Ch],r8d
000007FE6EB423A7 jmp 000007FE6EB423B1
000007FE6EB423A9 nop
000007FE6EB423AA xor r8d,r8d
000007FE6EB423AD mov dword ptr [rbp-1Ch],r8d
000007FE6EB423B1 cmp dword ptr [rbp-1Ch],0
000007FE6EB423B5 je 000007FE6EB42409
state <- innerExists input.[i] illegalChars
000007FE6EB423B7 mov r8d,dword ptr [rbp-18h]
000007FE6EB423BB mov rdx,qword ptr [rbp+18h]
000007FE6EB423BF cmp r8,qword ptr [rdx+8]
000007FE6EB423C3 jb 000007FE6EB423CA
000007FE6EB423C5 call 000007FECD796850
000007FE6EB423CA lea rdx,[rdx+r8*2+10h]
000007FE6EB423CF movzx r8d,word ptr [rdx]
000007FE6EB423D3 mov rdx,qword ptr [rbp+10h]
000007FE6EB423D7 mov rdx,qword ptr [rdx+8]
000007FE6EB423DB mov r9,qword ptr [rbp+20h]
000007FE6EB423DF mov rcx,7FE6EEE0640h
000007FE6EB423E9 call 000007FE6EB41E40
000007FE6EB423EE mov dword ptr [rbp-20h],eax
000007FE6EB423F1 mov eax,dword ptr [rbp-20h]
000007FE6EB423F4 movzx eax,al
000007FE6EB423F7 mov dword ptr [rbp-14h],eax
i <- i + 1
000007FE6EB423FA mov eax,dword ptr [rbp-18h]

Answer 1

Others pointed out to use existing function FSharp.Core to achieve the same result but I think that OP asks if in loops like the boundary check of arrays are elided (as it is checked in the loop condition).
For simple code like above the jitter should be able to elide the checks. To see this it is correct to check the assembly code but it is important to not run with the VS debugger attached as the jitter don't optimize the code then. The reason that it can be impossible to show correct values in the debugger.
First let's look at exists optimized x64:
; not state?
00007ff9`1cd37551 85c0 test eax,eax
; if state is true then exit the loop
00007ff9`1cd37553 7521 jne 00007ff9`1cd37576
; i < input.Length?
00007ff9`1cd37555 395e08 cmp dword ptr [rsi+8],ebx
; Seems overly complex but perhaps this is as good as it gets?
00007ff9`1cd37558 0f9fc1 setg cl
00007ff9`1cd3755b 0fb6c9 movzx ecx,cl
00007ff9`1cd3755e 85c9 test ecx,ecx
; if we have reached end of the array then exit
00007ff9`1cd37560 7414 je 00007ff9`1cd37576
; mov i in ebx to rcx, unnecessary but moves like these are very cheap
00007ff9`1cd37562 4863cb movsxd rcx,ebx
; input.[i] (note we don't check the boundary again)
00007ff9`1cd37565 0fb74c4e10 movzx ecx,word ptr [rsi+rcx*2+10h]
; move illegalChars pointer to rdx
00007ff9`1cd3756a 488bd7 mov rdx,rdi
; call innerExists
00007ff9`1cd3756d e8ee9affff call 00007ff9`1cd31060
; i <- i + 1
00007ff9`1cd37572 ffc3 inc ebx
; Jump top of loop
00007ff9`1cd37574 ebdb jmp 00007ff9`1cd37551
; We are done!
00007ff9`1cd37576
So the code looks a bit too complex for what it should need to be but it seems it only checks the array condition once.
Now let's look at innerExists optimized x64:
# let mutable state = false
00007ff9`1cd375a0 33c0 xor eax,eax
# let mutable i = 0
00007ff9`1cd375a2 4533c0 xor r8d,r8d
; not state?
00007ff9`1cd375a5 85c0 test eax,eax
; if state is true then exit the loop
00007ff9`1cd375a7 752b jne 00007ff9`1cd375d4
; i < items.Length
00007ff9`1cd375a9 44394208 cmp dword ptr [rdx+8],r8d
; Seems overly complex but perhaps this is as good as it gets?
00007ff9`1cd375ad 410f9fc1 setg r9b
00007ff9`1cd375b1 450fb6c9 movzx r9d,r9b
00007ff9`1cd375b5 4585c9 test r9d,r9d
; if we have reached end of the array then exit
00007ff9`1cd375b8 741a je 00007ff9`1cd375d4
; mov i in r8d to rax, unnecessary but moves like these are very cheap
00007ff9`1cd375ba 4963c0 movsxd rax,r8d
; items.[i] (note we don't check the boundary again)
00007ff9`1cd375bd 0fb7444210 movzx eax,word ptr [rdx+rax*2+10h]
; mov item in cx to r9d, unnecessary but moves like these are very cheap
00007ff9`1cd375c2 440fb7c9 movzx r9d,cx
; item = items.[i]?
00007ff9`1cd375c6 413bc1 cmp eax,r9d
00007ff9`1cd375c9 0f94c0 sete al
; state <- ?
00007ff9`1cd375cc 0fb6c0 movzx eax,al
; i <- i + 1
00007ff9`1cd375cf 41ffc0 inc r8d
; Jump top of loop
00007ff9`1cd375d2 ebd1 jmp 00007ff9`1cd375a5
; We are done!
00007ff9`1cd375d4 c3 ret
So looks overly complex for what it should be but at least it looks like it only checks the array condition once.
So finally, it looks like the jitter eliminates the array boundary checks because it can prove this has already been checked successfully in the loop condition which I believe is what the OP wondered.
The x64 code doesn't look as clean as it could but from my experimentation x64 code that is cleaned up doesn't perform that much better, I suspect the CPU vendors optimize the CPU for the crappy code jitters produce.
An interesting exercise would be to code up an equivalent program in C++ and run it through https://godbolt.org/, choose x86-64 gcc (trunk) (gcc seems to do best right now) and specify the options -O3 -march=native and see the resulting x64 code.
Update
The code rewritten in https://godbolt.org/ to allow us seeing the assembly code generated by a c++ compiler:
template<int N>
bool innerExists(char item, char const (&items)[N]) {
for (auto i = 0; i < N; ++i) {
if (item == items[i]) return true;
}
return false;
}
template<int N1, int N2>
bool exists(char const (&input)[N1], char const (&illegalCharacters)[N2]) {
for (auto i = 0; i < N1; ++i) {
if (innerExists(input[i], illegalCharacters)) return true;
}
return false;
}
char const separators[] = { '.', ',', ';' };
char const str[58] = { };
bool test() {
return exists(str, separators);
}
x86-64 gcc (trunk) with options -O3 -march=native the following code is generated
; Load the string to test into edx
mov edx, OFFSET FLAT:str+1
.L2:
; Have we reached the end?
cmp rdx, OFFSET FLAT:str+58
; If yes, then jump to the end
je .L7
; Load a character
movzx ecx, BYTE PTR [rdx]
; Comparing the 3 separators are encoded in the assembler
; because the compiler detected the array is always the same
mov eax, ecx
and eax, -3
cmp al, 44
sete al
cmp cl, 59
sete cl
; increase outer i
inc rdx
; Did we find a match?
or al, cl
; If no then loop to .L2
je .L2
; We are done!
ret
.L7:
; No match found, clear result
xor eax, eax
; We are done!
ret
Looks pretty good but what I missing in the code above is using AVX to test multiple characters at once.

Answer 2

Bound check are eliminated by the JIT compiler, so it works the same for F# as for C#. You can expect elimination for code as in your example, as well as for
for i = 0 to data.Lenght - 1 do
...
and also for tail recursive functions, which compiles down to loops.
The built in Array.contains and Array.exists (source code) are written so that the JIT compiler can eliminate bounds checks.

Answer 3

What's wrong with the Array.contains and Array.exists functions ?
let exists input illegalChars =
input |> Array.exists (fun c -> illegalChars |> Array.contains c)