Hi,
I'm working on IA64 with GCC-4.1.1; what I do is to instrument some
sensitive instructions (e.g. memory access) to do information flow
tracking. As I insert the instrumentation after register allocation,
I need to allocate general registers and predicates myself; for
corner cases in allocation, for example, once I can not find enough
general registers, I will spill some of them into memory and do the
allocation again; for predicate registers, once I cannot find enough
predicates to do instrumentation, I allocate one more general
register, say X, and use the following instruction:
mov X = pr
on IA64 to save all predicates; of course, I need to restore all
predicates at the end of instrumentation, and I use the following
instruction:
mov pr = X, -1
Thus a code sequence for this type of instrumentation may be:
mov X = pr
instrumentation prologue
sensitive instruction
instrumentation epilogue
mov pr = X, -1
However, I found GCC may not correctly handle data dependencies with
mov X=pr or mov pr = X, -1 instructions.
Consider the following instruction sequence:
mov r8=pr
...
cmp.eq p6,p7= …
(p6) ...
(p7) ...
...
mov pr=r8,-1
Obviously, there are some data dependencies there, for example, we can
not execute
mov pr = r8, -1
until we finish executing instructions using p6 and p7 predicate registers.
BUT, after the second scheduling, GCC generates code in the following sequence:
mov r8=pr
cmp.eq p6,p7=something0
(p6) something1
mov pr=r8,-1
(p7) something2
This code is absolutely wrong as p7 may hold value with nonsense !
I think the cause of the problem is that GCC doesn't handle data
dependencies between mov X = pr (or mov pr = X, -1) and other usage of
a specific predicate register (e.g. p6, p7); as GCC only emits mov X =
pr (or mov pr = X, -1) instruction at the function prologue and
epilogue, this problem doesn't exist.
So, my question becomes clear:
How to solve this problem by making GCC knows the data dependencies
between mov X = pr (or mov pr = X, -1) and other usage of a specific
predicate register (e.g. p6, p7)?
Personally speaking, I think I need to modify itanium1.md or
itanium2.md to instruct GCC to notice these dependencies (However,
these files look too much complex :-(); or is there any simpler way to
get around this problem ?
Any help is truly appreciated
Thanks !
