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(-)
diff --git target/hexagon/translate.c target/hexagon/translate.c
index 665476ab48..58d638f734 100644
--- target/hexagon/translate.c
+++ target/hexagon/translate.c
@@ -128,14 +128,19 @@ static bool use_goto_tb(DisasContext *ctx, target_ulong dest)
return translator_use_goto_tb(&ctx->base, dest);
}
-static void gen_goto_tb(DisasContext *ctx, int idx, target_ulong dest)
+static void gen_goto_tb(DisasContext *ctx, int idx, target_ulong dest, bool
+ move_to_pc)
{
if (use_goto_tb(ctx, dest)) {
tcg_gen_goto_tb(idx);
- tcg_gen_movi_tl(hex_gpr[HEX_REG_PC], dest);
+ if (move_to_pc) {
+ tcg_gen_movi_tl(hex_gpr[HEX_REG_PC], dest);
+ }
tcg_gen_exit_tb(ctx->base.tb, idx);
} else {
- tcg_gen_movi_tl(hex_gpr[HEX_REG_PC], dest);
+ if (move_to_pc) {
+ tcg_gen_movi_tl(hex_gpr[HEX_REG_PC], dest);
+ }
tcg_gen_lookup_and_goto_ptr();
}
}
@@ -150,11 +155,11 @@ static void gen_end_tb(DisasContext *ctx)
if (ctx->branch_cond != TCG_COND_ALWAYS) {
TCGLabel *skip = gen_new_label();
tcg_gen_brcondi_tl(ctx->branch_cond, hex_branch_taken, 0, skip);
- gen_goto_tb(ctx, 0, ctx->branch_dest);
+ gen_goto_tb(ctx, 0, ctx->branch_dest, true);
gen_set_label(skip);
- gen_goto_tb(ctx, 1, ctx->next_PC);
+ gen_goto_tb(ctx, 1, ctx->next_PC, false);
} else {
- gen_goto_tb(ctx, 0, ctx->branch_dest);
+ gen_goto_tb(ctx, 0, ctx->branch_dest, true);
}
} else if (ctx->is_tight_loop &&
pkt->insn[pkt->num_insns - 1].opcode == J2_endloop0) {
@@ -165,9 +170,9 @@ static void gen_end_tb(DisasContext *ctx)
TCGLabel *skip = gen_new_label();
tcg_gen_brcondi_tl(TCG_COND_LEU, hex_gpr[HEX_REG_LC0], 1, skip);
tcg_gen_subi_tl(hex_gpr[HEX_REG_LC0], hex_gpr[HEX_REG_LC0], 1);
- gen_goto_tb(ctx, 0, ctx->base.tb->pc);
+ gen_goto_tb(ctx, 0, ctx->base.tb->pc, true);
gen_set_label(skip);
- gen_goto_tb(ctx, 1, ctx->next_PC);
+ gen_goto_tb(ctx, 1, ctx->next_PC, false);
} else {
tcg_gen_lookup_and_goto_ptr();
}
--
2.37.2
> -----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>
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_mathbern@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 <anjo@rev.ng>
--
Anton Johansson,
rev.ng Labs Srl.
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