[fpc-devel] Proposal for 2 new compiler functions

[J. Gareth Moreton]

 Hi everyone,
 Sorry to flood the mailing list again with more ideas and experiments.

 I would like to propose introducing a new pair of in-built functions for
the compiler.

 function FLog2(X: Cardinal): Cardinal;
 function CLog2(X: Cardinal): Cardinal;

 FLog2 returns the base-2 logarithm of X, rounded down, while CLog2 returns
the base-2 logarithm of X, rounded up.

 To give examples where these functions could be useful, FLog2(X) + 1
indicates the minimum number of bits required to store X as an unsigned
integer, while CLog2(X) is equal to the maximum number of iterations
required for a binary search of an X-sized list.

 Why should they be in-built though? With the binary search example, the
size of the list is sometimes known at compile time, hence is a constant -
therefore, its logarithm is also a constant.  By pre-calculating the
logarithm using the in-built function, it can be used to aid optimization
such as loop unrolling.  It also makes them easier to inline, where
FLog2(X) on x86_64-win64 translates to a single line of assembly language:
BSR EAX, ECX (unless X is zero, in which case ZF is set and EAX is
undefined).

 If there had to be a slight nuance though, it's what happens if X = 0,
since log(0) = -oo, which cannot be stored as an integer and may cause
problems with the compiler.  I would propose it return 0 as a special
case, as this will fix most problems when it comes to loops and storage,
and also ensure that FLog2 and CLog2 are "entire functions".  To give an
optimised example of FLog(2) in x86-64 assembly language:

 XOR EDX, EDX
 BSR EAX, ECX // ZF is set if ECX is zero
 CMOVZ EAX, EDX // Zero (EDX) written to result if ZF is set.

 Some kind of deep optimization could be used if the input is known to be
non-zero and remove the instructions either side of BSR.

 (Alternative names: FILog2 and CILog2, to indicate they work on integers
and to distinguish them from versions that work with floating-point
numbers)

 Gareth
  
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