## Why FPC may be a better compiler than Delphi

By A.Bouchez on 2015, Sunday June 21, 20:10 - Pascal Programing - Permalink

Almost every time I'm debugging some core part of our framework, I like to
see the generated asm, and trying to optimize the pascal code for better speed
- when it is worth it, of course!

I just made a nice observation, when comparing the assembler generated by
Delphi to FPC's output.

Imagine you compile the following lines (extracted from SynCommons.pas), to convert some number into ASCII characters:

... c100 := val div 100; dec(val,c100*100); PWord(P)^ := TwoDigitLookupW[val]; ...

This divides a number by 100, then computes the modulo in `val`

,
to store two digits at a time. We did not use `val := val mod 100`

here, since `mod`

would do another division, so we rely on a simple
multiplication to compute the modulo.

You may know that for today's CPUs, integer multiplication is very optimized,
taking a cycle (or less, thanks to its pipelines), whereas a division is much
more expensive - if you have some spare time, take a look at this
document, and you will find out that a `div`

opcode could use 10
times more cycles then a `mul`

- even with the latest CPU
architectures.

Let's see how our two beloved compilers do their homework (with optimization
enabled, of course)...

Delphi generates the following code for ```
c100 := val div
100
```

:

005082AB 8BC1 mov eax,ecx 005082AD BE64000000 mov esi,$00000064 005082B2 33D2 xor edx,edx 005082B4 F7F6div esi

Whereas FPC generates the following:

0043AC48 8b55f8 mov -0x8(%ebp),%edx 0043AC4B b81f85eb51 mov $0x51eb851f,%eax 0043AC50 f7e2mul %edx0043AC52 c1ea05shr $0x5,%edx0043AC55 8955f0 mov %edx,-0x10(%ebp)

Even if you are assembler agnostic, and once you did get rid of the asm
textual representation (Delphi uses Intel's, whereas FPC/GDB follows AT&T),
you can see that Delphi generates a classic (and slow) `div esi`

opcode, whereas FPC uses a single multiplication, followed by a bit shift.

This optimization is known as "Reciprocal Multiplication", and I would
let you read this
article for mathematical reference - or this one.

It multiplies (`mul`

) the number by the power of two reciprocal of
100 (which is the hexadecimal `51eb851f`

value), followed by a right
shift (`shr`

) of 5 bits.

Thanks to 32 bit rounding of the integer operations, this would in fact divide
the number per 100.

Even it consists in two assembler opcodes, a `mul`

+
`shr`

is in fact faster than a single `div`

.

It is a shame that the Delphi compiler did not include this very common optimization, which is clearly a win for some very common tasks.

Of course, the LLVM back-end used on the NextGen compiler can do it, be we
may expect this classic optimization be part of the decades-old Delphi
compiler.

And I'm still not convinced about the performance of the NextGen generated
code, since the associated RTL is known to be slow, so won't benefit of LVVM
optimization - which takes a LOT of time to compile, by the way (much
more than FPC).

Congrats, FPC folks!