[PULL,01/11] Hexagon (translate.c): avoid redundant PC updates on COF

Commit Message

Taylor Simpson April 20, 2023, 3:26 a.m. UTC

From: Matheus Tavares Bernardino <[e-mail]>

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 <[e-mail]>
Signed-off-by: Taylor Simpson <[e-mail]>
Reviewed-by: Anton Johansson <[e-mail]>
Reviewed-by: Taylor Simpson <[e-mail]>
Message-Id: <fc059[e-mail]>
---
 target/hexagon/translate.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

Patch

diff --git a/target/hexagon/translate.c b/target/hexagon/translate.c
index 665476ab48..58d638f734 100644
--- a/target/hexagon/translate.c
+++ b/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();
     }