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>
diff --git a/target/hexagon/translate.c b/target/hexagon/translate.c
index 665476a..58d638f 100644
--- a/target/hexagon/translate.c
+++ b/target/hexagon/translate.c
@@ -128,14 +128,19 @@
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 @@
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 @@
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();
}
