On 3/22/23 22:17, Matheus Tavares Bernardino wrote:
When there is a conditional change of flow or an endloop instruction, we
preload HEX_REG_PC with ctx->next_PC at gen_start_packet(). Nonetheless,
we still generate TCG code to do this update again at gen_goto_tb() when
the condition for the COF is not met, thus producing redundant
instructions. This can be seen with the following packet:
0x004002e4: 0x5c20d000 { if (!P0) jump:t PC+0 }
Which generates this TCG code:
---- 004002e4
-> mov_i32 pc,$0x4002e8
and_i32 loc9,p0,$0x1
mov_i32 branch_taken,loc9
add_i32 pkt_cnt,pkt_cnt,$0x2
add_i32 insn_cnt,insn_cnt,$0x2
brcond_i32 branch_taken,$0x0,ne,$L1
goto_tb $0x0
mov_i32 pc,$0x4002e4
exit_tb $0x7fb0c36e5200
set_label $L1
goto_tb $0x1
-> mov_i32 pc,$0x4002e8
exit_tb $0x7fb0c36e5201
set_label $L0
exit_tb $0x7fb0c36e5203
Note that even after optimizations, the redundant PC update is still
present:
---- 004002e4
-> mov_i32 pc,$0x4002e8 sync: 0 dead: 0 1 pref=0xffff
mov_i32 branch_taken,$0x1 sync: 0 dead: 0 1 pref=0xffff
add_i32 pkt_cnt,pkt_cnt,$0x2 sync: 0 dead: 0 1 pref=0xffff
add_i32 insn_cnt,insn_cnt,$0x2 sync: 0 dead: 0 1 2 pref=0xffff
goto_tb $0x1
-> mov_i32 pc,$0x4002e8 sync: 0 dead: 0 1 pref=0xffff
exit_tb $0x7fb0c36e5201
set_label $L0
exit_tb $0x7fb0c36e5203
With this patch, the second redundant update is properly discarded.
Note that we need the additional "move_to_pc" flag instead of just
avoiding the update whenever `dest == ctx->next_PC`, as that could
potentially skip updates from a COF with met condition, whose
ctx->branch_dest just happens to be equal to ctx->next_PC.
Signed-off-by: Matheus Tavares Bernardino <quic_mathb...@quicinc.com>
---
target/hexagon/translate.c | 21 +++++++++++++--------
1 file changed, 13 insertions(+), 8 deletions(-)
diff --git target/hexagon/translate.c target/hexagon/translate.c
index 665476ab48..58d638f734 100644
--- target/hexagon/translate.c
+++ target/hexagon/translate.c
Looks good, I appreciate the thorough motivation for this patch!
Reviewed-by: Anton Johansson <a...@rev.ng>
--
Anton Johansson,
rev.ng Labs Srl.
