[PULL,for-6.1,02/12] accel/tcg: Remove unnecessary check on icount_extra in cpu_loop_exec_tb()

Commit Message

Richard Henderson July 26, 2021, 9:47 p.m. UTC

From: Peter Maydell <[e-mail]>

In cpu_loop_exec_tb(), we decide whether to look for a TB with
exactly insns_left instructions in it using the condition
 (!cpu->icount_extra && insns_left > 0 && insns_left < tb->icount)

The check for icount_extra == 0 is unnecessary, because we just set
  insns_left = MIN(0xffff, cpu->icount_budget);
  icount_extra = icount_budget - insns_left;
and so icount_extra can only be non-zero if icount_budget > 0xffff
and insns_left == 0xffff. But in that case insns_left >= tb->icount
because 0xffff is much larger than TCG_MAX_INSNS, so the condition
will be false anyway.

Remove the unnecessary check, and instead assert:
 * that we are only going to execute a partial TB here if the
   icount budget has run out (ie icount_extra == 0)
 * that the number of insns we're going to execute does fit into
   the CF_COUNT_MASK

Signed-off-by: Peter Maydell <[e-mail]>
Reviewed-by: Richard Henderson <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Richard Henderson <[e-mail]>
---
 accel/tcg/cpu-exec.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

Patch

diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
index 6e8dc29119..5aa42fbff3 100644
--- a/accel/tcg/cpu-exec.c
+++ b/accel/tcg/cpu-exec.c
@@ -843,7 +843,9 @@  static inline void cpu_loop_exec_tb(CPUState *cpu, TranslationBlock *tb,
      * execute we need to ensure we find/generate a TB with exactly
      * insns_left instructions in it.
      */
-    if (!cpu->icount_extra && insns_left > 0 && insns_left < tb->icount)  {
+    if (insns_left > 0 && insns_left < tb->icount)  {
+        assert(insns_left <= CF_COUNT_MASK);
+        assert(cpu->icount_extra == 0);
         cpu->cflags_next_tb = (tb->cflags & ~CF_COUNT_MASK) | insns_left;
     }
 #endif