[Bug target/35507] [avr] 4.3.0: size of small funcion increases from 2 to 29 words

[hutchinsonandy at aim dot com]

------- Comment #1 from hutchinsonandy at aim dot com  2008-03-09 04:35 -------
I can confirms this regression.

There appears to be something strange in commutation of operands before RTL is
created which may well explain why it used to work.

BThe  default expander are creating calls to commutative functions that have
the opposite argument order from normal function parameters. Functionally this
does not matter, but it twists up the data flow.

The argument order presented by the RTL is backwards from the ideal. This
happens with both target AVR expander and the default expander (the original PR
was using a default expander). Clearly this is avoidable.

It is not a function of the original code - gcc is purposely ordering the
operands. Regardless of operand ordering (x*y or y*x), the RTL is always
backwards. 


The other factor is the lack of forward propagation in the RTL stages. If this
was effective, RTL ordering would not matter. The limited propagation that is
present avoids hard registers - so never connects the arguments with the
library function. Also combine can't exploit the commutation of the operands.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35507