On 8/6/24 17:36, Vineet Gupta wrote:
> I'm currently pursuing a different trail which comes form observation
> that initial model setup concludes that pressure is 28 so with 27
> allocable regs we are bound to spill one.
> More on that after I find something concrete.
(caveat: I enabled -fomit-frame-pointer for both risc-v and aarch64)
Observation: So on risc-v, sched1's very first pressure dump starts off with
initial pressure 1
;; ======================================================
;; -- basic block 2 from 6 to 174 -- before reload
;; ======================================================
;; | idx insn | mpri hght dpth prio | # model_record_pressures ()
;; | 0 6 | 0 3 0 5 | r154=high(`j') GR_REGS:[1,+1]
^^^
While on aarch64 is starts off with 0.
;; | 0 6 | 0 3 0 6 | r122=high(`j') GENERAL_REGS:[0,+1]
FP_REGS:[0,+0] PR_LO_REGS:[0,+0] PR_HI_REGS:[0,+0]
^^^
This seems to be happening because of HARD_FP (reno 8)
model_start_schedule ()
initiate_reg_pressure_info (df_get_live_in (bb))
EXECUTE_IF_SET_IN_BITMAP (live, 0, j, bi)
mark_regno_birth_or_death (.. j )
if ( ! TEST_HARD_REG_BIT (ira_no_alloc_regs, regno)))
bitmap_set_bit (live, regno)
For RISC-V, the loop above executes for regno 2 (SP), 8 (HARD_FP), 64 (FP), 65
(Arg).
The DF infra (before reload) sets up artificial usage for HARD_FP : see
df_get_regular_block_artificial_uses () hence it shows up in df_get_live_in (bb)
On RISC-V, FIXED_REGISTERS omits FP and consequently ira_no_alloc_regs doesn't
include HARD_FP. This seems sensible (at least intuitive) since reg
allocator is allowed to use HARD_FP (which due to -fomit-frame-pointer becomes
first callee reg S0).
(gdb) p/x this_target_ira_int->x_no_unit_alloc_regs
$1 = {elts = {0x1f, 0xffffffffffffffff}} <-- bit 8 for HARD_FP not set
On aarch64, HARD_FP regno 29 is marked as FIXED_REGISTERS thus is present in
ira_no_alloc_regs
(gdb) p/x this_target_ira_int->x_no_unit_alloc_regs
$1 = {elts = {0xa0000000, 0x0}}
So I don't understand 2 things:
1. Why is aarch64 reserving away HARD_FP (at least from ira) when clearly user
is saying -fomit-frame-pointer (It seems this remains even if disable
exception, asynch unwind etc)
2. On RISC-V sched1 is counter intuitively assuming HARD_FP is live due to the
weird interaction of DF infra (which always marks HARD_FP with
artificial def) and ira_no_alloc_regs.
Thx,
-Vineet
