Re: [Patch, AVR]: QI builtins for parity, popcount, 1<< n

["Joseph S. Myers" <joseph@codesourcery.com>]

On Fri, 17 Jun 2011, Georg-Johann Lay wrote:

> I don't see what's bat with the patch, it's straight forward.

C is a high-level language, defined in terms of high-level semantics 
rather than machine instructions.  C code should be written where possible 
using machine-independent functionality, falling into machine-dependent 
operations only where the semantics cannot readily be represented in a 
machine-independent way; the compiler should generally be responsible for 
picking optimal instructions for the source code.

C code should write "1 << n" (or "1 << (n & 7)" if that's what it wants) 
for shifts rather than __builtin_avr_pow2.  That way it's more portable 
and more readable to general C programmers who aren't familiar with all 
the machine-specific interfaces.  Similarly, code wanting to add values 
should use the "+" operator rather than each target having its own 
__builtin_<arch>_add.  And since parity and population-count operations 
are widely present and generically useful, GNU C has generic built-in 
functions for those, so code should use __builtin_parity and 
__builtin_popcount on all machines rather than machine-specific variants; 
such machine-specific variants should not exist.

The machine-independent parts of the compiler should know about the 
equivalence of (popcount X) and (popcount (zero_extend X)), (parity X) and 
(parity (zero_extend X)) and (parity X) and (parity (sign_extend X)) if 
the sign-extension is by an even number of bits - in fact, there is 
already code in simplify_unary_operation_1 that does know this (the 
assumption that all sign-extensions are by an even number of bits is 
hardcoded).  The target should just need to describe how to code these 
operations on various modes.  If the existing code doesn't suffice to 
cause popcountqi etc. patterns to be used for the generic built-in 
functions, the appropriate fix is generic rather than adding new 
target-specific built-in functions - just as if the compiler didn't 
generate your target's addition instruction from the '+' operator, the 
right fix is not to add __builtin_<arch>_add to generate that instruction 
but rather to make '+' generate it.

-- 
Joseph S. Myers
joseph@codesourcery.com