tree 941502e41d890d81e258dd8c671f2ae25c37a4ba
parent 60ca584b8af0de525656f959991a440f8c191f12
author Matheus Tavares Bernardino <quic_mathbern@quicinc.com> 1679519830 -0300
committer Taylor Simpson <tsimpson@quicinc.com> 1682094771 -0700

Hexagon (translate.c): avoid redundant PC updates on COF

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_mathbern@quicinc.com>
Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>
Reviewed-by: Anton Johansson <anjo@rev.ng>
Reviewed-by: Taylor Simpson <tsimpson@quicinc.com>
Message-Id: <fc059153c3f0526d97b7f13450c02b276b0908e1.1679519341.git.quic_mathbern@quicinc.com>