Hi!
Recently I got somewhat confused by Cortex-A57 pipeline description in
GCC and
I would be grateful if you could help me understand a few unclear points.
Particularly I am interested in how memory operations (loads/stores) are
scheduled. It seems that according to the cortex-a57.md file, firstly, two
memory operations may never be scheduled at the same cycle and,
secondly, two
loads may never be scheduled at two consecutive cycles:
;; 5. Two pipelines for load and store operations: LS1, LS2. The most
;; valuable thing we can do is force a structural hazard to split
;; up loads/stores.
(define_cpu_unit "ca57_ls_issue" "cortex_a57")
(define_cpu_unit "ca57_ldr, ca57_str" "cortex_a57")
(define_reservation "ca57_load_model" "ca57_ls_issue,ca57_ldr*2")
(define_reservation "ca57_store_model" "ca57_ls_issue,ca57_str")
However, the Cortex-A57 Software Optimization Guide states that the core
is able
to execute one load operation and one store operation every cycle. And that
agrees with my experiments. Indeed, a loop consisting of 10 loads, 10
stores and
several arithmetic operations takes on average about 10 cycles per
iteration,
provided that the instructions are intermixed properly.
So, what is the purpose of additional restrictions imposed on the
scheduler in
cortex-a57.md file? It doesn't look like an error. Rather, it looks like a
deliberate decision.
Thanks,
Nikolai
