RE: [PATCH] Hexagon (translate.c): avoid redundant PC updates on COF

[Taylor Simpson]

> -----Original Message-----
> From: Matheus Tavares Bernardino <quic_mathbern@quicinc.com>
> Sent: Wednesday, March 22, 2023 3:17 PM
> To: qemu-devel@nongnu.org
> Cc: Taylor Simpson <tsimpson@quicinc.com>; richard.henderson@linaro.org;
> anjo@rev.ng
> Subject: [PATCH] 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>
> ---
>  target/hexagon/translate.c | 21 +++++++++++++--------
>  1 file changed, 13 insertions(+), 8 deletions(-)

Reviewed-by: Taylor Simpson <tsimpson@quicinc.com>