tcg: Return the TB pointer from the rx region from exit_tb

This produces a small pc-relative displacement within the
generated code to the TB structure that preceeds it.

Reviewed-by: Joelle van Dyne <j@getutm.app>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

tcg/tcg-op.c

@@ -2666,7 +2666,18 @@ void tcg_gen_extr32_i64(TCGv_i64 lo, TCGv_i64 hi, TCGv_i64 arg)
 
 void tcg_gen_exit_tb(const TranslationBlock *tb, unsigned idx)
 {
-    uintptr_t val = (uintptr_t)tb + idx;
+    /*
+     * Let the jit code return the read-only version of the
+     * TranslationBlock, so that we minimize the pc-relative
+     * distance of the address of the exit_tb code to TB.
+     * This will improve utilization of pc-relative address loads.
+     *
+     * TODO: Move this to translator_loop, so that all const
+     * TranslationBlock pointers refer to read-only memory.
+     * This requires coordination with targets that do not use
+     * the translator_loop.
+     */
+    uintptr_t val = (uintptr_t)tcg_splitwx_to_rx((void *)tb) + idx;
 
     if (tb == NULL) {
         tcg_debug_assert(idx == 0);