Re: check_ext_dependent_givs

[Canqun Yang <canqun at nudt dot edu dot cn>]

Hi, all,

I do not want to modify the old loop optimizer defined 
in loop.c. I am preparing to port some improvements 
done on gcc-3.5 to gcc-4.0, and the GIV optimizations 
is one of my concerns.

On IA-64, the GIV optimizations can hardly improve the 
performance. The reason is that 
check_ext_dependent_givs can not give an exactly 
answer whether the BIVs will be wrap around or not. As
check_ext_dependent_givs can only deal with BIVs in 
constant-iteration loops or BIVs are the same as the 
loop iteration variable, and only small parts of BIVs 
satisfy this condition, that in most cases, only a 
conservative result is produced to report that the 
BIVs may overflow and the corresponding GIVs can not 
be reduced. 

I modified the code in check_ext_dependent_givs to let 
the BIVs always successfully pass the check, then 
tested the NAS benchmarks and SPEC CPF2000 benchmarks, 
excepting significant performance improvements, no 
extra errors occurred.

I have read the codes in check_ext_dependent_givs and 
the mails abount BIV overflow checking in GCC's 
mailing list written by Richard Henderson and Zdenek 
Dvorak, also tested the example Paolo Bonzini sent to 
me. But I still have some questions about this.

1. There is an option '-fwrapv' to control the 
behavior of signed overflows. Can it also be used in 
check_ext_dependent_givs?

2. If check_ext_dependent_givs has not been invoked, 
the program will give wrong result, otherwise, 
correct. Would you please send me an example to show 
this?  (FORTRAN programs are nicer).

3. For FORTRAN programs, is there any thing special. 
As I know, only signed integers in FORTRAN, also the 
counted loops in FORTRAN are more strict than in C?

4. Is it reasonable to turn off this checking at some 
optimization level or with compile options like '-
ffast-math' and '-fno-wrapv'?

5. Is there any way to extend the function of 
check_ext_dependent_givs to manage 
non-iteration-variable BIVs in non-constant-iteration 
loops. I have tried but failed. 


Best regards,

Canqun Yang
Creative Compiler Research Group.
National University of Defense Technology, China.