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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>
This commit is contained in:
Matheus Tavares Bernardino 2023-03-22 18:17:10 -03:00 committed by Taylor Simpson
parent 60ca584b8a
commit bee1fc56c2
1 changed files with 13 additions and 8 deletions
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21
target/hexagon/translate.c
View File

@ -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();
}