tcg/optimize: fix known-zero bits optimization

Known-zero bits optimization is a great idea that helps to generate more
optimized code. However the current implementation only works in very few
cases as the computed mask is not saved.

Fix this to make it really working.
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>
Signed-off-by: NRichard Henderson <rth@twiddle.net>

tcg/optimize.c

@@ -691,7 +691,8 @@ static TCGArg *tcg_constant_folding(TCGContext *s, uint16_t *tcg_opc_ptr,
             break;
         }
 
-        /* Simplify using known-zero bits */
+        /* Simplify using known-zero bits. Currently only ops with a single
+           output argument is supported. */
         mask = -1;
         affected = -1;
         switch (op) {
@@ -1149,6 +1150,11 @@ static TCGArg *tcg_constant_folding(TCGContext *s, uint16_t *tcg_opc_ptr,
             } else {
                 for (i = 0; i < def->nb_oargs; i++) {
                     reset_temp(args[i]);
+                    /* Save the corresponding known-zero bits mask for the
+                       first output argument (only one supported so far). */
+                    if (i == 0) {
+                        temps[args[i]].mask = mask;
+                    }
                 }
             }
             for (i = 0; i < def->nb_args; i++) {