tcg: add a CONST flag to TCG helpers

A const function only reads its arguments and does not use TCG
globals variables. Hence a call to such a function does not
save TCG globals variabes back to their canonical location.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@7008 c046a42c-6fe2-441c-8c8c-71466251a162

2 files changed, 12 insertions, 4 deletions

diff --git a/tcg/tcg.c b/tcg/tcg.c
index e5508507e2..c64043f690 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -1122,9 +1122,11 @@ static void tcg_liveness_analysis(TCGContext *s)
                         dead_temps[arg] = 1;
                     }
                     
-                    /* globals are live (they may be used by the call) */
-                    memset(dead_temps, 0, s->nb_globals);
-                    
+                    if (!(call_flags & TCG_CALL_CONST)) {
+                        /* globals are live (they may be used by the call) */
+                        memset(dead_temps, 0, s->nb_globals);
+                    }
+
                     /* input args are live */
                     dead_iargs = 0;
                     for(i = 0; i < nb_iargs; i++) {
@@ -1821,7 +1823,9 @@ static int tcg_reg_alloc_call(TCGContext *s, const TCGOpDef *def,
     
     /* store globals and free associated registers (we assume the call
        can modify any global. */
-    save_globals(s, allocated_regs);
+    if (!(flags & TCG_CALL_CONST)) {
+        save_globals(s, allocated_regs);
+    }
 
     tcg_out_op(s, opc, &func_arg, &const_func_arg);
     
diff --git a/tcg/tcg.h b/tcg/tcg.h
index cb4911cde6..c9b001e004 100644
--- a/tcg/tcg.h
+++ b/tcg/tcg.h
@@ -178,6 +178,10 @@ typedef int TCGv_i64;
    and cannot raise exceptions. Hence a call to a pure function can be
    safely suppressed if the return value is not used. */
 #define TCG_CALL_PURE           0x0010 
+/* A const function only reads its arguments and does not use TCG
+   globals variables. Hence a call to such a function does not
+   save TCG globals variabes back to their canonical location. */
+#define TCG_CALL_CONST          0x0020
 
 /* used to align parameters */
 #define TCG_CALL_DUMMY_TCGV     MAKE_TCGV_I32(-1)