Support local register variables

Similar to GCC a local asm register variable enforces the use of a
specified register in asm operands (and doesn't otherwise
matter).  Works only if the variable is directly mentioned as
operand.  For that we now generally store a backpointer from
an SValue to a Sym when the SValue was the result of unary()
parsing a symbol identifier.

i386-asm.c

@@ -1185,6 +1185,28 @@ static const char *skip_constraint_modifiers(const char *p)
     return p;
 }
 
+/* If T (a token) is of the form "%reg" returns the register
+   number and type, otherwise return -1.  */
+ST_FUNC int asm_parse_regvar (int t)
+{
+    const char *s;
+    Operand op;
+    if (t < TOK_IDENT)
+        return -1;
+    s = table_ident[t - TOK_IDENT]->str;
+    if (s[0] != '%')
+        return -1;
+    t = tok_alloc(s+1, strlen(s)-1)->tok;
+    unget_tok(t);
+    unget_tok('%');
+    parse_operand(tcc_state, &op);
+    /* Accept only integer regs for now.  */
+    if (op.type & OP_REG)
+        return op.reg;
+    else
+        return -1;
+}
+
 #define REG_OUT_MASK 0x01
 #define REG_IN_MASK  0x02
 
@@ -1227,6 +1249,11 @@ ST_FUNC void asm_compute_constraints(ASMOperand *operands,
                 tcc_error("cannot reference twice the same operand");
             operands[k].input_index = i;
             op->priority = 5;
+	} else if ((op->vt->r & VT_VALMASK) == VT_LOCAL
+		   && op->vt->sym
+		   && (reg = op->vt->sym->r & VT_VALMASK) < VT_CONST) {
+	    op->priority = 1;
+	    op->reg = reg;
         } else {
             op->priority = constraint_priority(str);
         }
@@ -1274,6 +1301,12 @@ ST_FUNC void asm_compute_constraints(ASMOperand *operands,
         } else {
             reg_mask = REG_IN_MASK;
         }
+	if (op->reg >= 0) {
+	    if (is_reg_allocated(op->reg))
+	        tcc_error("asm regvar requests register that's taken already");
+	    reg = op->reg;
+	    goto reg_found;
+	}
     try_next:
         c = *str++;
         switch(c) {

tcc.h

@@ -372,7 +372,8 @@ typedef struct SValue {
     unsigned short r2;     /* second register, used for 'long long'
                               type. If not used, set to VT_CONST */
     CValue c;              /* constant, if VT_CONST */
-    struct Sym *sym;       /* symbol, if (VT_SYM | VT_CONST) */
+    struct Sym *sym;       /* symbol, if (VT_SYM | VT_CONST), or if
+    			      result of unary() for an identifier. */
 } SValue;
 
 struct Attribute {
@@ -407,7 +408,7 @@ typedef struct Sym {
         int scope;  /* scope level for locals */
     };
     union {
-        long r;    /* associated register */
+        long r;    /* associated register or VT_CONST/VT_LOCAL and LVAL type */
         struct Attribute a;
     };
     union {
@@ -416,7 +417,7 @@ typedef struct Sym {
     };
     CType type;    /* associated type */
     union {
-        struct Sym *next; /* next related symbol */
+        struct Sym *next; /* next related symbol (for fields and anoms) */
         long jnext; /* next jump label */
     };
     struct Sym *prev; /* prev symbol in stack */
@@ -1542,6 +1543,7 @@ ST_FUNC void gen_expr32(ExprValue *pe);
 ST_FUNC void gen_expr64(ExprValue *pe);
 #endif
 ST_FUNC void asm_opcode(TCCState *s1, int opcode);
+ST_FUNC int asm_parse_regvar(int t);
 ST_FUNC void asm_compute_constraints(ASMOperand *operands, int nb_operands, int nb_outputs, const uint8_t *clobber_regs, int *pout_reg);
 ST_FUNC void subst_asm_operand(CString *add_str, SValue *sv, int modifier);
 ST_FUNC void asm_gen_code(ASMOperand *operands, int nb_operands, int nb_outputs, int is_output, uint8_t *clobber_regs, int out_reg);

tccgen.c

@@ -386,7 +386,7 @@ ST_FUNC Sym *sym_push2(Sym **ps, int v, int t, long c)
     Sym *s;
 
     s = sym_malloc();
-    s->asm_label = 0;
+    s->scope = 0;
     s->v = v;
     s->type.t = t;
     s->type.ref = NULL;
@@ -577,6 +577,7 @@ static void vsetc(CType *type, int r, CValue *vc)
     vtop->r = r;
     vtop->r2 = VT_CONST;
     vtop->c = *vc;
+    vtop->sym = NULL;
 }
 
 /* push constant of type "type" with useless value */
@@ -4599,11 +4600,17 @@ ST_FUNC void unary(void)
             r = VT_SYM | VT_CONST;
         } else {
             r = s->r;
+	    /* A symbol that has a register is a local register variable,
+	       which starts out as VT_LOCAL value.  */
+	    if ((r & VT_VALMASK) < VT_CONST)
+	      r = (r & ~VT_VALMASK) | VT_LOCAL;
         }
         vset(&s->type, r, s->c);
-        /* if forward reference, we must point to s */
+        /* Point to s as backpointer (even without r&VT_SYM).
-        if (vtop->r & VT_SYM) {
+	   Will be used by at least the x86 inline asm parser for
+	   regvars.  */
 	vtop->sym = s;
+        if (vtop->r & VT_SYM) {
             vtop->c.i = 0;
         }
         break;
@@ -6462,6 +6469,13 @@ static void decl_initializer_alloc(CType *type, AttributeDef *ad, int r,
 #endif
         if (v) {
             /* local variable */
+#ifdef CONFIG_TCC_ASM
+	    if (ad->asm_label) {
+		int reg = asm_parse_regvar(ad->asm_label);
+		if (reg >= 0)
+		    r = (r & ~VT_VALMASK) | reg;
+	    }
+#endif
             sym_push(v, type, r, addr);
         } else {
             /* push local reference */

tests/tcctest.c

@@ -2939,6 +2939,7 @@ void asm_test(void)
 #ifdef BOOL_ISOC99
     _Bool somebool;
 #endif
+    register int regvar asm("%esi");
 
     printf("inline asm:\n");
 
@@ -3001,6 +3002,8 @@ void asm_test(void)
     val = 44;
     fancy_copy2 (&val, &val2);
     printf ("fancycpy2(%d)=%d\n", val, val2);
+    asm volatile ("mov $0x4243, %%esi" : "=r" (regvar));
+    printf ("regvar=%x\n", regvar);
     return;
  label1:
     goto label2;