[RFC PATCH 3/8] accel/tcg: skip the can_do_io stores in user-only builds

[Matt Turner <mattst88@gmail.com>]

Every translation block stores to cpu->neg.can_do_io twice: false before
the first instruction, true before the last one. Nothing reads it in a
user-only build. There is no memory-mapped I/O in linux-user, and every
reader is in system_ss: cputlb.c, watchpoint.c, icount-common.c and
tcg-accel-ops-icount.c.

Two stores per TB is not much on its own, but TBs are short. An emulated
alpha gcc 16.2.0 compiling the SQLite 3.45.1 amalgamation (255k lines,
-O2) executes 34.2 billion TBs at 6.04 guest instructions each, so this is
68 billion stores for nothing.

Measured on an x86-64 host, LTO build, on top of the preceding two
patches:

    before: 1,469,729,281,442 instructions
    after:  1,402,816,253,499 instructions   -4.55%

    before: 120.97s wall clock
    after:  115.75s wall clock              -4.32%

The emulated compiler produces byte-identical output.

Signed-off-by: Matt Turner <mattst88@gmail.com>
---
 accel/tcg/translator.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git ./accel/tcg/translator.c ./accel/tcg/translator.c
index cd7d079fe0..29e609b2ec 100644
--- ./accel/tcg/translator.c
+++ ./accel/tcg/translator.c
@@ -21,12 +21,14 @@
 #include "disas/disas.h"
 #include "tb-internal.h"
 
+#ifndef CONFIG_USER_ONLY
 static void set_can_do_io(DisasContextBase *db, bool val)
 {
     QEMU_BUILD_BUG_ON(sizeof_field(CPUState, neg.can_do_io) != 1);
     tcg_gen_st8_i32(tcg_constant_i32(val), tcg_env,
                     offsetof(CPUState, neg.can_do_io) - sizeof(CPUState));
 }
+#endif
 
 bool translator_io_start(DisasContextBase *db)
 {
@@ -210,17 +212,25 @@ void translator_loop(CPUState *cpu, TranslationBlock *tb, int *max_insns,
     /*
      * Manage can_do_io for the translation block: set to false before
      * the first insn and set to true before the last insn.
+     *
+     * Nothing reads can_do_io in user-only builds.  There is no MMIO
+     * there, and every reader (cputlb.c, watchpoint.c, icount) is in
+     * system_ss, so skip the two stores per TB entirely.
      */
     if (db->num_insns == 1) {
         tcg_debug_assert(first_insn_start == db->insn_start);
     } else {
         tcg_debug_assert(first_insn_start != db->insn_start);
+#ifndef CONFIG_USER_ONLY
         tcg_ctx->emit_before_op = first_insn_start;
         set_can_do_io(db, false);
+#endif
     }
+#ifndef CONFIG_USER_ONLY
     tcg_ctx->emit_before_op = db->insn_start;
     set_can_do_io(db, true);
     tcg_ctx->emit_before_op = NULL;
+#endif
 
     /* May be used by disas_log or plugin callbacks. */
     tb->size = db->pc_next - db->pc_first;
-- 
2.54.0