Re: m68k-elf local register allocation

[Joel Sherrill <joel dot sherrill at OARcorp dot com>]

Ray Pimm wrote:
> 
> I am noticing suboptimal register allocation produced by gcc built for the
> m68k-elf target.

The best thing you can do is prepare as small a test case as possible
that
demonstrates the problem still applies to 3.2 and submit a gcc PR.

> In some functions the compiler uses registers other than d0/d1/a0/a1, and
> goes to the trouble of saving and restoring them to/from the stack, while
> leaving unused one or more of d0/d1/a0/a1, when it could have achieved the
> same result using only registers from the d0/d1/a0/a1 set.
> 
> Then in other functions, it will use d0/d1/a0/a1 only. I cannot see any
> pattern to the register allocation.
> 
> I am using primarily gcc-2.95.3, but I notice that the same phenomenon
> occurs on gcc-3.2. I generally use optimization level -O2, but higher
> optimization levels do not seem to affect the result.
> 
> My compiler options are as follows:
> 
> CXXFLAGS =  -O2                 \
>              -mcpu32             \
>              -fno-exceptions     \
>              -fno-for-scope      \
>              -fshort-enums       \
>              -Wall               \
>              -Wno-uninitialized  \
>              -Wno-switch         \
> 
> BTW, all of my code is C++, if this is relevant.
> 
> Thanks - Ray Pimm
> 
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