Hello,
I am working with a VLIW processor and GCC-4.0-20040911. There is a
problem in the dependency calculation of GCC. GCC is giving
write-after-read a higher priority than write-after-write. Thus, as in
the following code, GCC gives a write-after-read dependency between
the 2 instructions. Due to this the 2 instructions are scheduled
together.
stw --b15,*a15 ;; pre-decrement b15 and store its contents in memory
add 4,a15,b15 ;; b15 = a15+4
The first instruction reads b15 and then writes b15(pre-decrement).
The second writes to b15. Thus there exists a write-after-read and a
write-after-write dependency from the second instruction on the first.
But GCC keeps only the more restrictive dependency which is determined
by reg-notes.def (line 98). The updation of dependency takes place is
sched-deps.c (line 304). Accordingly, GCC puts a write-after-read
dependency between the 2 instructions. Due to this, the 2 instructions
are scheduled for execution in parallel. This results in 2 writes to
the same register on the same clock cycle.
As of now, I have the interchanged the 2 lines in reg-notes.def (line
98 and 99).