[PATCH 7/8] Hexagon (target/hexagon) Use direct block chaining for direct jump/branch

[Taylor Simpson]

Direct block chaining is documented here
https://qemu.readthedocs.io/en/latest/devel/tcg.html#direct-block-chaining

Recall that Hexagon allows packets with multiple jumps where only the first
one with a true predicate will actually jump.  So, we can only use direct
block chaining when the packet contains a single PC-relative jump.  We add
the following to DisasContext in order to perform direct block chaining at
the end of packet commit (in gen_end_tb)
    has_single_direct_branch
        Indicates that we can use direct block chaining
    branch_cond
        The condition under which the branch is taken
    branch_dest
        The destination of the branch

Signed-off-by: Taylor Simpson <tsimp...@quicinc.com>
---
 target/hexagon/translate.h |  3 +++
 target/hexagon/genptr.c    | 13 ++++++-------
 target/hexagon/translate.c | 39 +++++++++++++++++++++++++++++++++++++-
 3 files changed, 47 insertions(+), 8 deletions(-)

diff --git a/target/hexagon/translate.h b/target/hexagon/translate.h
index eae358cf33..e60dbf0e7a 100644
--- a/target/hexagon/translate.h
+++ b/target/hexagon/translate.h
@@ -54,6 +54,9 @@ typedef struct DisasContext {
     bool qreg_is_predicated[NUM_QREGS];
     int qreg_log_idx;
     bool pre_commit;
+    bool has_single_direct_branch;
+    TCGv branch_cond;
+    target_ulong branch_dest;
 } DisasContext;
 
 static inline void ctx_log_reg_write(DisasContext *ctx, int rnum)
diff --git a/target/hexagon/genptr.c b/target/hexagon/genptr.c
index fba76d3b38..07b4326e56 100644
--- a/target/hexagon/genptr.c
+++ b/target/hexagon/genptr.c
@@ -505,15 +505,14 @@ static void gen_write_new_pc_pcrel(DisasContext *ctx, 
Packet *pkt,
             gen_set_label(pred_false);
         }
     } else {
-        TCGLabel *pred_false = NULL;
+        /* Defer this jump to the end of the TB */
+        g_assert(ctx->branch_cond == NULL);
+        ctx->has_single_direct_branch = true;
         if (pred != NULL) {
-            pred_false = gen_new_label();
-            tcg_gen_brcondi_tl(TCG_COND_EQ, pred, 0, pred_false);
-        }
-        tcg_gen_movi_tl(hex_gpr[HEX_REG_PC], dest);
-        if (pred != NULL) {
-            gen_set_label(pred_false);
+            ctx->branch_cond = tcg_temp_local_new();
+            tcg_gen_mov_tl(ctx->branch_cond, pred);
         }
+        ctx->branch_dest = dest;
     }
 }
 
diff --git a/target/hexagon/translate.c b/target/hexagon/translate.c
index 71ad2da682..29e2caaf0f 100644
--- a/target/hexagon/translate.c
+++ b/target/hexagon/translate.c
@@ -116,10 +116,44 @@ static void gen_exec_counters(DisasContext *ctx)
                     hex_gpr[HEX_REG_QEMU_HVX_CNT], ctx->num_hvx_insns);
 }
 
+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)
+{
+    if (use_goto_tb(ctx, dest)) {
+        tcg_gen_goto_tb(idx);
+        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);
+        tcg_gen_lookup_and_goto_ptr();
+    }
+}
+
 static void gen_end_tb(DisasContext *ctx)
 {
     gen_exec_counters(ctx);
-    tcg_gen_exit_tb(NULL, 0);
+
+    if (ctx->has_single_direct_branch) {
+        if (ctx->branch_cond != NULL) {
+            TCGLabel *skip = gen_new_label();
+            tcg_gen_brcondi_tl(TCG_COND_EQ, ctx->branch_cond, 0, skip);
+            gen_goto_tb(ctx, 0, ctx->branch_dest);
+            gen_set_label(skip);
+            gen_goto_tb(ctx, 1, ctx->next_PC);
+            tcg_temp_free(ctx->branch_cond);
+            ctx->branch_cond = NULL;
+        } else {
+            gen_goto_tb(ctx, 0, ctx->branch_dest);
+        }
+    } else {
+        tcg_gen_lookup_and_goto_ptr();
+    }
+
+    g_assert(ctx->branch_cond == NULL);
     ctx->base.is_jmp = DISAS_NORETURN;
 }
 
@@ -803,6 +837,9 @@ static void hexagon_tr_init_disas_context(DisasContextBase 
*dcbase,
 
 static void hexagon_tr_tb_start(DisasContextBase *db, CPUState *cpu)
 {
+    DisasContext *ctx = container_of(db, DisasContext, base);
+    ctx->has_single_direct_branch = false;
+    ctx->branch_cond = NULL;
 }
 
 static void hexagon_tr_insn_start(DisasContextBase *dcbase, CPUState *cpu)
-- 
2.17.1