Hi everyone,
I have a question when it comes to optimising memory reads and writes.
What are the rules for FPC when it comes to writing to memory and then
reading from it later within a single subroutine? For example, say I had
this pair of commands:
movq %rdx,-584(%rbp)
movl -584(%rbp),%eax
That could easily be converted to "movl %edx,%eax", especially as %rbp
is likely pointing to the top of the stack. But if the reference uses
different registers, would it still be safe to make this optimisation
given that the scheduler could suspend the thread in between the two
instructions and then another thread writes to the same memory block
before control is returned?
I am aware of other examples that require caution. For example:
movslq -608(%rbp),%rdx
subl %eax,-84(%rbp,%rdx,4)
movslq -608(%rbp),%rdx
Here it might be tempting to remove the second "movslq" instruction, but
the value of %rdx could happen to be equal to -131, which would allow
the subl instruction to modify -608(%rbp), and in this situation, it's
quite likely if a malicious input is given to the program to manipulate
the value stored at -608(%rbp) and invoke a buffer overrun.
So in conclusion, theoretically, where is it perfectly safe to assume
the value in memory hasn't changed, where would it be permissible only
under -O4 and where must it not be optimised at all?
Gareth aka. Kit
P.S. Assembly examples were taken from the System unit.
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