gcj non-optimization curiosity

Sun Dec 16 15:21:00 GMT 2001

Tom Tromey wrote:
>L16 is the call to throw a null pointer exception (a non-returning
>call), and L18 is the call to throw a bad array index exception
>(another non-returning call).
>>There are a few things I don't understand about this code that I would
>like to understand.
>
I'm a bit concerned about the high cost of bounds checks too. It used to 
be that --no-bounds-check made a modest improvement to performance, but 
with the current compiler the difference is huge in some cases. Some 
code runs twice as fast with --no-bounds-check now.
>* Why isn't the null-pointer check hoisted out of the loop?
> We know `foo' doesn't change during the loop, so this would be safe.
>>* Why is the length of the array ("movl 4(%eax), %edx") re-loaded each
> time through the loop?
>
I've noticed this too and it is strange. I have a vague suspicion that 
the use of SAVE_EXPR to force the bounds check to occur at the right 
time may be inhibiting the compilers ability to optimize them properly.
regards
Bryce.