[macstl-dev] not all rosey in the gcc-4.0.0 land

[Ilya Lipovsky]

Glen,
Looks like the expression

(u * v).sum()

produces bad code in gcc 4.0.0, if inline is NOT #defined as "inline 
__attribute__ ((always_inline)).

If, however, inline IS #defined as "inline __attribute__ 
((always_inline)), then the code is correct and the run-time results are 
OK.

This is a bug in the compiler's optimizer :( . This time I have rechecked 
myself by running gdb and looking at the actual instruction sequences. 
When reducing my -O3 option down to -O1 everything is *fine*!!!

Without redefining inline gcc's O3/O2-level optimizer "thinks" that the 
operands can be safely taken out of the loop, thus producing an empty loop 
(i.e. a bdnz onto itself!).

Another thing, since we're on the "inline" subject: it looks like 
inline almost never helps to produce more efficient code. This is in 
contrast to the situation I faced on the 3.3 compiler. Actually sometimes 
it drastically reduces performance (for example in the operator/ case).

Will be exploring all that further. Who knows, maybe 4.0.0 is not 
enough... We'll try 4.0.1, then!


-Ilya

_____________________________________________________________________

Ilya Lipovsky
Engineer, Algorithms Group
SKY Computers(R), Inc., 8 Centennial Drive M/S A-14, Peabody MA 01960