[Bug target/43745] [avr] g++ puts VTABLES in SRAM

["tfrancuz at mp dot pl" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #2 from tfrancuz at mp dot pl  2010-04-14 20:39 -------
(In reply to comment #1)
> What is your suggestion?

Because VTABLES are generated during compile time and they are const data there
is no need to copy them into SRAM. Appropriate addresses of virtual methods can
be read directly from FLASH and used to make indirect call as usual. As far as
I know the problem is that AVR port of gcc doesn’t have appropriate isns
to do that. The problem is extremely important because for example my program
uses 964 bytes of SRAM, from which 860 bytes are used for VTABLES (wasted).
This is a typical situation for any C++ program on AVR.
The only problem is speed penalty, LD instruction on AVR takes 1 cycle, whereas
LPM instruction needed for FLASH access 3 cycles, which gives total 2 or 6
cycles to read virtual method address respectively. So probably the best
solution is to add another gcc option (or pragma) to choose between storing
VTABLES only in FLASH (slower code execution) or moving it to SRAM (faster
execution for the cost of higher SRAM memory occupation).


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tfrancuz at mp dot pl changed:

           What    |Removed                     |Added
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                 CC|                            |tfrancuz at mp dot pl


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43745