I'm porting to a 16 bit micro and noticed that when optimization is
enabled (function.c:2101), most (non-volatile, non-addressed) stack
variables are copied into virtual registers. I assume this is so the
register allocator will attempt to allocate them to physical registers.
Unfortunately, in those cases where there are a lot of stack parameters
and those virtual registers are spilled to local variables a lot of code
and stack space is wasted copying an image of the stack parameters into
a spill slot instead of the overflow dropping back to use the original
stack parameter space.
For example ...
add sp,-20
st r0,[sp]
st r1,[sp+2]
ld r0,[sp+24]
st r0,[sp+4]
ld r2,[sp+26]
st r2,[sp+6]
ld r3,[sp+28]
st r3,[sp+8]
ld r0,[sp+30]
st r0,[sp+10]
I'm aware that there are cases where the stack parameters are unsuitable
as spill locations (due to partial values, alignment etc) but the
majority of cases I see such as the example above the stack parameters
are viable spill locations. In the case illustrated above the required
stack space has blown out significantly and there are a host of
redundant load/store operations that could be eliminated if the spill
were to the stack parameters.
For small micros such as MSP430 & friends and many of the Renasis MCUs,
some of which only have 2K or ram on board, this could be a real issue.
Although I reproduced the same behaviour on i386 using a stock compiler,
I was wondering if there was something missing in my port that prevents
spilling to stack locations, or a cleanup pass that addresses this issue
?
Thanks, Paul
