[Bug target/11180] [avr-gcc] Optimization decrease performance of struct assignment.

[andrewhutchinson at cox dot net]

------- Additional Comments From andrewhutchinson at cox dot net  2005-03-27 14:33 -------
The problem here is that gcc is using  a DImode register to handle 6 byte
(int+long) structure. Why I have no idea!

Since the target has no insn for DI move, gcc turns this into individual QImode
byte moves (subregs all over the place!). 

The 'stacked' 6 byte structure is 'popped' into DI register (6 bytes ). Two
other byte registers are explicitely cleared (making our 8 byte DI register) 

What then  follows is a large amount of shuffling. i.e. Moving from intermediate
virtual DI register (8 bytes) into correct place for a 6 byte return. Which
seems to surpass the abilities of the register allocator (DI and return
registers overlap).

Smaller structures (<=4 bytes) are optimally handled. Larger structure >8 are
also much better since they are returned in memory.

So in summary, it would appear that the root cause is allocation of a DI mode
register for structures >4 and <=8 bytes.

A secondary factor is the use of QImode moves (when SI,HImode are available and
more  efficient) 

The problem can be partially alleviated by defining DImode moves (that a hell of
a change though). Poor code still remains - for example clearing unused padding
bytes and extra register usage.

PS -fpack-struct does not change this bug.



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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11180