shell/math: reduce stack usage

function                                             old     new   delta
arith_apply                                         1123    1134     +11
arith_lookup_val                                     140     145      +5
evaluate_string                                     1053    1047      -6
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/1 up/down: 16/-6)              Total: 10 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 36 insertions, 30 deletions

diff --git a/shell/math.c b/shell/math.c
index 727c29467..af8e691fe 100644
--- a/shell/math.c
+++ b/shell/math.c
@@ -236,14 +236,19 @@ typedef struct {
          * then '?' selects one of them based on its left side.
          */
         arith_t second_val;
-        char second_val_present;
-        /* If NULL then it's just a number, else it's a named variable */
-        char *var;
+#define SECOND_VAL_VALID ((char*)(intptr_t)-1)
+        /* If NULL then it's just a number, if SECOND_VAL_VALID,
+         * it's a result of "expr : expr", else it's a named variable.
+         * (We use SECOND_VAL_VALID instead of a bit flag to keep
+         * var_or_num_t smaller, we allocate a lot of them on stack).
+         */
+        char *var_name;
 } var_or_num_t;
 
+
 typedef struct remembered_name {
         struct remembered_name *next;
-        const char *var;
+        const char *var_name;
 } remembered_name;
 
 
@@ -253,17 +258,17 @@ evaluate_string(arith_state_t *math_state, const char *expr);
 static const char*
 arith_lookup_val(arith_state_t *math_state, var_or_num_t *t)
 {
-        if (t->var) {
-                const char *p = math_state->lookupvar(t->var);
+        if (t->var_name && t->var_name != SECOND_VAL_VALID) {
+                const char *p = math_state->lookupvar(t->var_name);
                 if (p) {
                         remembered_name *cur;
-                        remembered_name cur_save;
+                        remembered_name remember;
 
                         /* did we already see this name?
                          * testcase: a=b; b=a; echo $((a))
                          */
                         for (cur = math_state->list_of_recursed_names; cur; cur = cur->next) {
-                                if (strcmp(cur->var, t->var) == 0) {
+                                if (strcmp(cur->var_name, t->var_name) == 0) {
                                         /* Yes */
                                         return "expression recursion loop detected";
                                 }
@@ -271,9 +276,9 @@ arith_lookup_val(arith_state_t *math_state, var_or_num_t *t)
 
                         /* push current var name */
                         cur = math_state->list_of_recursed_names;
-                        cur_save.var = t->var;
-                        cur_save.next = cur;
-                        math_state->list_of_recursed_names = &cur_save;
+                        remember.var_name = t->var_name;
+                        remember.next = cur;
+                        math_state->list_of_recursed_names = &remember;
 
                         /* recursively evaluate p as expression */
                         t->val = evaluate_string(math_state, p);
@@ -286,7 +291,7 @@ arith_lookup_val(arith_state_t *math_state, var_or_num_t *t)
                 /* treat undefined var as 0 */
                 t->val = 0;
         }
-        return 0;
+        return NULL;
 }
 
 /* "Applying" a token means performing it on the top elements on the integer
@@ -303,7 +308,7 @@ arith_apply(arith_state_t *math_state, operator op, var_or_num_t *numstack, var_
 
         /* There is no operator that can work without arguments */
         if (NUMPTR == numstack)
-                goto err;
+                goto syntax_err;
 
         top_of_stack = NUMPTR - 1;
 
@@ -326,15 +331,15 @@ arith_apply(arith_state_t *math_state, operator op, var_or_num_t *numstack, var_
         else if (op != TOK_UPLUS) {
                 /* Binary operators */
                 arith_t right_side_val;
-                char bad_second_val;
+                int bad_second_val;
 
                 /* Binary operators need two arguments */
                 if (top_of_stack == numstack)
-                        goto err;
+                        goto syntax_err;
                 /* ...and they pop one */
                 NUMPTR = top_of_stack; /* this decrements NUMPTR */
 
-                bad_second_val = top_of_stack->second_val_present;
+                bad_second_val = (top_of_stack->var_name == SECOND_VAL_VALID);
                 if (op == TOK_CONDITIONAL) { /* ? operation */
                         /* Make next if (...) protect against
                          * $((expr1 ? expr2)) - that is, missing ": expr" */
@@ -363,8 +368,10 @@ arith_apply(arith_state_t *math_state, operator op, var_or_num_t *numstack, var_
                                 /* Protect against $((expr : expr)) */
                                 return "malformed ?: operator";
                         }
-                        top_of_stack->second_val_present = op;
+                        top_of_stack->val = rez;
                         top_of_stack->second_val = right_side_val;
+                        top_of_stack->var_name = SECOND_VAL_VALID;
+                        return NULL;
                 }
                 else if (op == TOK_BOR || op == TOK_OR_ASSIGN)
                         rez |= right_side_val;
@@ -439,13 +446,13 @@ arith_apply(arith_state_t *math_state, operator op, var_or_num_t *numstack, var_
         if (is_assign_op(op)) {
                 char buf[sizeof(arith_t)*3 + 2];
 
-                if (top_of_stack->var == NULL) {
+                if (!top_of_stack->var_name || top_of_stack->var_name == SECOND_VAL_VALID) {
                         /* Hmm, 1=2 ? */
-                        goto err;
+                        goto syntax_err;
                 }
                 /* Save to shell variable */
                 sprintf(buf, ARITH_FMT, rez);
-                math_state->setvar(top_of_stack->var, buf);
+                math_state->setvar(top_of_stack->var_name, buf);
                 /* After saving, make previous value for v++ or v-- */
                 if (op == TOK_POST_INC)
                         rez--;
@@ -455,9 +462,9 @@ arith_apply(arith_state_t *math_state, operator op, var_or_num_t *numstack, var_
 
         top_of_stack->val = rez;
         /* Erase var name, it is just a number now */
-        top_of_stack->var = NULL;
+        top_of_stack->var_name = NULL;
         return NULL;
- err:
+ syntax_err:
         return "arithmetic syntax error";
 #undef NUMPTR
 }
@@ -660,12 +667,11 @@ evaluate_string(arith_state_t *math_state, const char *expr)
                 if (p != expr) {
                         /* Name */
                         size_t var_name_size = (p - expr) + 1;  /* +1 for NUL */
-                        numstackptr->var = alloca(var_name_size);
-                        safe_strncpy(numstackptr->var, expr, var_name_size);
+                        numstackptr->var_name = alloca(var_name_size);
+                        safe_strncpy(numstackptr->var_name, expr, var_name_size);
 //bb_error_msg("var:'%s'", numstackptr->var);
                         expr = p;
  num:
-                        numstackptr->second_val_present = 0;
                         numstackptr++;
                         lasttok = TOK_NUM;
                         continue;
@@ -673,7 +679,7 @@ evaluate_string(arith_state_t *math_state, const char *expr)
 
                 if (isdigit(*expr)) {
                         /* Number */
-                        numstackptr->var = NULL;
+                        numstackptr->var_name = NULL;
                         errno = 0;
                         numstackptr->val = strto_arith_t(expr, (char**) &expr);
                         /* A number can't be followed by another number, or a variable name.
@@ -702,7 +708,7 @@ evaluate_string(arith_state_t *math_state, const char *expr)
                 if ((expr[0] == '+' || expr[0] == '-')
                  && (expr[1] == expr[0])
                 ) {
-                        if (numstackptr == numstack || !numstackptr[-1].var) { /* not a VAR++ */
+                        if (numstackptr == numstack || !numstackptr[-1].var_name) { /* not a VAR++ */
                                 char next = skip_whitespace(expr + 2)[0];
                                 if (!(isalpha(next) || next == '_')) { /* not a ++VAR */
                                         //bb_error_msg("special %c%c", expr[0], expr[0]);
@@ -795,14 +801,14 @@ evaluate_string(arith_state_t *math_state, const char *expr)
 //bb_error_msg("op == TOK_RPAREN");
                                         if (prev_op == TOK_LPAREN) {
 //bb_error_msg("prev_op == TOK_LPAREN");
-//bb_error_msg("  %p %p numstackptr[-1].var:'%s'", numstack, numstackptr-1, numstackptr[-1].var);
-                                                if (numstackptr[-1].var) {
+//bb_error_msg("  %p %p numstackptr[-1].var_name:'%s'", numstack, numstackptr-1, numstackptr[-1].var_name);
+                                                if (numstackptr[-1].var_name && numstackptr[-1].var_name != SECOND_VAL_VALID) {
                                                         /* Expression is (var), lookup now */
                                                         errmsg = arith_lookup_val(math_state, &numstackptr[-1]);
                                                         if (errmsg)
                                                                 goto err_with_custom_msg;
                                                         /* Erase var name: (var) is just a number, for example, (var) = 1 is not valid */
-                                                        numstackptr[-1].var = NULL;
+                                                        numstackptr[-1].var_name = NULL;
                                                 }
                                                 /* Any operator directly after a
                                                  * close paren should consider itself binary */