[Qemu-devel] [PATCH] tcg: Implement CPU_LOG_TB_NOCHAIN during expansion

[Richard Henderson]

Rather than test NOCHAIN before linking, do not emit the
goto_tb opcode at all.  We already do this for goto_ptr.

Previously, nochain seemed more or less a hack to debug code gen.
But there's a real use case when it comes to tracing, so spend a
little more effort on what we produce.

Signed-off-by: Richard Henderson <address@hidden>
---
 accel/tcg/cpu-exec.c | 2 +-
 tcg/tcg-op.c         | 9 ++++++++-
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
index 6bcb6d99bd..870027d435 100644
--- a/accel/tcg/cpu-exec.c
+++ b/accel/tcg/cpu-exec.c
@@ -416,7 +416,7 @@ static inline TranslationBlock *tb_find(CPUState *cpu,
     }
 #endif
     /* See if we can patch the calling TB. */
-    if (last_tb && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
+    if (last_tb) {
         tb_add_jump(last_tb, tb_exit, tb);
     }
     return tb;
diff --git a/tcg/tcg-op.c b/tcg/tcg-op.c
index daa416a143..7a8015c5a9 100644
--- a/tcg/tcg-op.c
+++ b/tcg/tcg-op.c
@@ -2586,6 +2586,10 @@ void tcg_gen_exit_tb(TranslationBlock *tb, unsigned idx)
            seen this numbered exit before, via tcg_gen_goto_tb.  */
         tcg_debug_assert(tcg_ctx->goto_tb_issue_mask & (1 << idx));
 #endif
+        /* When not chaining, exit without indicating a link.  */
+        if (qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
+            val = 0;
+        }
     } else {
         /* This is an exit via the exitreq label.  */
         tcg_debug_assert(idx == TB_EXIT_REQUESTED);
@@ -2603,7 +2607,10 @@ void tcg_gen_goto_tb(unsigned idx)
     tcg_debug_assert((tcg_ctx->goto_tb_issue_mask & (1 << idx)) == 0);
     tcg_ctx->goto_tb_issue_mask |= 1 << idx;
 #endif
-    tcg_gen_op1i(INDEX_op_goto_tb, idx);
+    /* When not chaining, we simply fall through to the "fallback" exit.  */
+    if (!qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
+        tcg_gen_op1i(INDEX_op_goto_tb, idx);
+    }
 }
 
 void tcg_gen_lookup_and_goto_ptr(void)
-- 
2.17.1