i386-gen: use EBX as 4th register

May be enabled/disabled by changing this line:
    #define USE_EBX 1

i386-gen.c

@@ -21,7 +21,7 @@
 #ifdef TARGET_DEFS_ONLY
 
 /* number of available registers */
-#define NB_REGS         4
+#define NB_REGS         5
 #define NB_ASM_REGS     8
 
 /* a register can belong to several classes. The classes must be
@@ -33,6 +33,8 @@
 #define RC_ST0     0x0008 
 #define RC_ECX     0x0010
 #define RC_EDX     0x0020
+#define RC_EBX     0x0040
+
 #define RC_IRET    RC_EAX /* function return: integer register */
 #define RC_LRET    RC_EDX /* function return: second integer register */
 #define RC_FRET    RC_ST0 /* function return: float register */
@@ -42,6 +44,7 @@ enum {
     TREG_EAX = 0,
     TREG_ECX,
     TREG_EDX,
+    TREG_EBX,
     TREG_ST0,
     TREG_ESP = 4
 };
@@ -89,10 +92,14 @@ enum {
 /******************************************************/
 #include "tcc.h"
 
+/* define to 1/0 to [not] have EBX as 4th register */
+#define USE_EBX 1
+
 ST_DATA const int reg_classes[NB_REGS] = {
     /* eax */ RC_INT | RC_EAX,
     /* ecx */ RC_INT | RC_ECX,
     /* edx */ RC_INT | RC_EDX,
+    /* ebx */ (RC_INT | RC_EBX) * USE_EBX,
     /* st0 */ RC_FLOAT | RC_ST0,
 };
 
@@ -518,9 +525,9 @@ ST_FUNC void gfunc_call(int nb_args)
 }
 
 #ifdef TCC_TARGET_PE
-#define FUNC_PROLOG_SIZE 10
+#define FUNC_PROLOG_SIZE (10 + USE_EBX)
 #else
-#define FUNC_PROLOG_SIZE 9
+#define FUNC_PROLOG_SIZE (9 + USE_EBX)
 #endif
 
 /* generate function prolog of type 't' */
@@ -646,6 +653,7 @@ ST_FUNC void gfunc_epilog(void)
         o(0x585a); /* restore returned value, if any */
     }
 #endif
+    o(0x5b * USE_EBX); /* pop ebx */
     o(0xc9); /* leave */
     if (func_ret_sub == 0) {
         o(0xc3); /* ret */
@@ -669,10 +677,11 @@ ST_FUNC void gfunc_epilog(void)
         o(0xe58955);  /* push %ebp, mov %esp, %ebp */
         o(0xec81);  /* sub esp, stacksize */
         gen_le32(v);
-#if FUNC_PROLOG_SIZE == 10
+#ifdef TCC_TARGET_PE
         o(0x90);  /* adjust to FUNC_PROLOG_SIZE */
 #endif
     }
+    o(0x53 * USE_EBX); /* push ebx */
     ind = saved_ind;
 }