Something is _really_ missing here.
gcc will never use registers below r12 for such a code.
The code will be:
mov &muxSelect, r15
add #llo(1), r15
and #llo(7), r15
mov r15, &muxSelect
ret
However, slightly better result can be obtained if you'll write:
muxSelect++;
muxSelect &= 0x07;
~d
On Tuesday 17 December 2002 17:35, David Brown wrote:
> Hi,
>
> I am looking at the code generated for a function, and I feel there is
> often unnecessary movement of data between registers, especially r15. Is
> there some reason while small calcuations always seem to use r15? I'm
> compiling with -O2.
>
> In this particular code snippet, I have a variable "muxSelect" defined as a
> "word" (typedef for unsigned int). The code includes:
>
> void foo(void) {
> word i, j;
> word nextMux = (muxSelect + 1) & 0x07;
>
> // More code, using nextMux and muxSelect
>
> muxSelect = nextMux;
> }
>
> The "nextMux = (muxSelect + 1) & 0x07" line is compiled to:
> mov &muxSelect, r9 // Puts muxSelect in a register, as
> it will be used later
> mov r9, r15
> add #llo(1), r15
> mov r15, r7 // Why go through r15, and not
> just use r7 directly?
> and #llo(7), r7
>
> I've seen this sort of thing on several occasions - r15 is used as a
> scratch register, when the original calculation could have been done
> directly in the destination register. Am I missing something here, or is
> there the possibility of optomising this more?
>
> Just for fun, I added the "-frename-registers" flag to the compile. The
> result was that r8 was used instead of r15, but basically the same code was
> produced.
>
> mvh.
>
> David
>
>
>
>
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