Re: Using `const int' values to size stack arrays

[Daniel Towner <daniel dot towner at picochip dot com>]

Robert Dewar wrote:

> > I presume we're talking C here, and gnu C at that.
> > MAX_VAL is not an integral constant expression, so int ary[MAX_VAL]
> > must be a VLA, if it is valid.
> >
> > C89 disallows both uses,
> > C99 disallows VLAs at file scope, but allows them at function scope.
> > GNU C is similar to C99.
>
>
> Of course, since GNU C allows dynamic arrays, it does allow this
> construct, and given that it is allowed, it is perfectly reasonable
> to ask that the compiler do this efficiently and not actually treat
> it as dynamic at the implementation level (even if it is dynamic at
> the semantic level).

Absolutely. My port of gcc generates code which initialises a memory 
variable with the value 12, and then in the body of the code which 
handles the variable length array, loads that value into a register, and 
does all the appropriate arithmetic on it (some example code is shown 
below). Can the code not be optimised at the instruction level, so that 
it would recognise that the memory value will always be 12, so the load 
from memory will always be 12, and hence  copy propogation could remove 
large chunks of code?

The main requirement for such an optimisation seems to be whether the 
optimiser can recognise that the load from memory will always be a 
constant value. If the original symbol was declared as `const int', or 
`static const int', and that int is written to memory, can its value 
ever be different to 12? I think that it is allowed to change if the 
variable was marked as volatile, but in the other cases I would have 
thought it could have been optimised?

thanks,

dan

---------------------

Example code. Note that after the initial load from memory all 
subsequent operations could be removed through constant propogation:

.section .data
_MAX_VAL:
.word 12          // Can this value ever change?

.section .text

_main:
       COPY.0 &_MAX_VAL,R5          // Get address of MAX_VAL
       LDW (R5)0,R5                 // R5 now contains constant `12'
       COPY.0 0,R0
       STW R14,(FP)-4
       SUB.0 FP,-(-8),FP
       ADD.0 R5,R5,R5               // Multiply by 2
       STW R3,(FP)1
       ADD.0 R5,6,R5                // Add 6
       COPY.1 FP,R4
       AND.0 R5, -4, R5             // Mask
       ADD.0 FP,R5,FP               // Change FP by constant value.
       etc...

============================================================================
Daniel Towner
picoChip Designs Ltd., Riverside Buildings, 108, Walcot Street, BATH,
BA1 5BG
daniel.towner@picochip.com
07786 702589