Hi all,
does the compiler have the optimization to avoid reloading into a register of an address (or a value) whether it is already present?
I ask this because of this code fragment compiled with the -O4 option:
.Lj380:
# Register eax allocated
# [726] T := T-1;
subw $1,2(%ebx)
# Register ax released
# Register edx allocated
# [727] H6 := @STK^[T];
movl U_$GLOBALS_$$_STK,%edx
# Register eax allocated
movzwl 2(%ebx),%eax
leal -8(%edx,%eax,8),%eax
# Register edx released
movl %eax,-256(%ebp)
# [728] H7 := H6;
movl %eax,-260(%ebp) <-(1)
# Register eax released
# [729] Inc(H7, 1);
addl $8,-260(%ebp)
# Register eax allocated
# [730] H6^.I := H6^.I+H7^.I;
movl -256(%ebp),%eax <-(2)
# Register ecx allocated
movswl 4(%eax),%ecx
movl -260(%ebp),%eax
# Register edx allocated
movswl 4(%eax),%edx
leal (%ecx,%edx),%eax
# Register ecx released
movw %ax,%dx
movl -256(%ebp),%eax <-(3)
movw %dx,4(%eax)
# Register eax,dx released
In (1) the compiler assumes that eax has the correct address but in (2) does not omit to reload it (eax released too soon).
Both seem to be of the same level of the analysis complexity, more deeper should be takes into account of (3) with the use of a supplementar registers as si or di (they are used in other parts of the code).
Thank you for any response, Marco Borsari