[Fwd: Disabling local optimizations with GCC]

[Christopher Bahns <chris at bahns dot com>]

Thanks for the help all that responded. I'm new to embedded stuff and knew that
the volatile keyword existed but did not know what it was for. NOW I KNOW. All is
working now with optimizations enabled. COOL.

Chris

Christopher Bahns wrote:

> Hello,
>
> Anyone know how to take advantage of most of GCC's optimizations without
> it performing certain local optimizations? I read through the -O and -f
> options in the gcc manual and did not see anything specifically just for
> local optimizations. There is the "-fgcse" (global common
> subexpressions) but I guess this would not apply to commonizing
> subexpressions locally within a function... don't really know. I did not
> see any "-flcse" or similar option.
>
> Here's my problem:
> I'm using a cross-compiler under Windows 98/NT for an m68k-coff target
> (MC68306). I have certain symbols mapped to specific hardware addresses
> so that I can interact with the hardware (as I assume most would do it).
> In some cases I assign different values to the same address in
> succession, or intermixed with the assignment of values to other
> addresses. It is important for all assignments to occur because each one
> causes the hardware to perform a particular action, and all actions need
> to occur in the order I've placed them in the C-language functions. The
> problem is that with any GCC optimization level  (-O, -O1, -O2, or -O3),
> it "optimizes" out all but the last assignment to a given location. This
> causes my program not to work. Everything seems to be fine if I disable
> all optimizations, but some parts of my program are time-critical and I
> may need some of the optimizations for the program to work right.
>
> Of course, I can choose to disable all optimizations except the ones
> that I specify explicitly with "-f" options, but I'd prefer to have as
> many enabled as possible without breaking my program.
>
> Here is some example code:
> _____________________________________________________________
> /* duart's base i/o address */
> #define IoPortAddr   ((BYTE*)0xa0001)
>
> void InitDuart681(void)
> {
>    /* disable transmitter and receiver */
>    IoPortAddr[DUCRA] = ((0x0 << 4) | (0x2 << 2) | 0x2);
>    IoPortAddr[DUCRB] = ((0x0 << 4) | (0x2 << 2) | 0x2);
>
>    /* reset UartA transmitter and receiver */
>    IoPortAddr[DUCRA] = (0x2 << 4);
>    IoPortAddr[DUCRA] = (0x3 << 4);
>
>    /* make sure we're not in break */
>    IoPortAddr[DUCRA] = (0x7 << 4);
>    IoPortAddr[DUCRA] = (0x5 << 4);
>
>    /* reset error status */
>    IoPortAddr[DUCRA] = (0x4 << 4);
>
>    /* set for no N81, normal operation */
>    IoPortAddr[DUCRA] = (0x1 << 4);
>    IoPortAddr[DUMRA] = 0x13;
>    IoPortAddr[DUMRA] = 0x7;
>
>    /* set port speed-- 9600 bps */
>    IoPortAddr[DUCRA] = (0x1 << 4);
>    IoPortAddr[DUACR] = (DUacr |= 0x80);
>    IoPortAddr[DUCSRA] = 0xbb;
>
>    /* init counter- counter mode, external clock source */
>    IoPortAddr[DUACR] = (DUacr |= 0x30);
> }
> _____________________________________________________________
>
> This is not the complete function, but you can see that it writes values
> to the same location more than once, without ever accessing values from
> that location. This tells the compiler that all assignments except the
> last one (to a given location) are meaningless and can be discarded. I
> have verified this by examining the assembler listing generated by GCC.
>
> Any suggestions? I've gotta think someone else has run into this before.
>
> Thanks for any help!
> Chris
>
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