Logical operators || and && are short-circuiting.asm

`true || X` short-circuits to 1 without evaluating X
`false && X` short-circuits to 0 without evaluating X

Fixes gbdev#619

include/asm/rpn.h

@@ -46,6 +46,9 @@ static inline bool rpn_isSymbol(const struct Expression *expr)
 	return expr->isSymbol;
 }
 
+void rpn_EnterShortCircuitOp(bool startShortCircuit);
+void rpn_LeaveShortCircuitOp(void);
+bool rpn_IsShortCircuited(void);
 void rpn_Symbol(struct Expression *expr, char const *tzSym);
 void rpn_Number(struct Expression *expr, uint32_t i);
 void rpn_LOGNOT(struct Expression *expr, const struct Expression *src);

src/asm/lexer.c

@@ -1371,6 +1371,9 @@ static char const *readInterpolation(void)
 		}
 	}
 
+	if (rpn_IsShortCircuited())
+		return NULL;
+
 	if (i == sizeof(symName)) {
 		warning(WARNING_LONG_STR, "Symbol name too long\n");
 		i--;

src/asm/parser.y

@@ -939,11 +939,27 @@ relocexpr_no_str : scoped_anon_id	{ rpn_Symbol(&$$, $1); }
 		| T_OP_LOGICNOT relocexpr %prec NEG {
 			rpn_LOGNOT(&$$, &$2);
 		}
-		| relocexpr T_OP_LOGICOR relocexpr {
-			rpn_BinaryOp(RPN_LOGOR, &$$, &$1, &$3);
-		}
-		| relocexpr T_OP_LOGICAND relocexpr {
-			rpn_BinaryOp(RPN_LOGAND, &$$, &$1, &$3);
+		| relocexpr T_OP_LOGICOR {
+			// If the first operand is nonzero...
+			rpn_EnterShortCircuitOp(rpn_isKnown(&$1) && $1.nVal);
+		} relocexpr {
+			// ...then short-circuit to 1 without evaluating the second operand.
+			if (rpn_IsShortCircuited())
+				rpn_Number(&$$, 1);
+			else
+				rpn_BinaryOp(RPN_LOGOR, &$$, &$1, &$4);
+			rpn_LeaveShortCircuitOp();
+		}
+		| relocexpr T_OP_LOGICAND {
+			// If the first operand is zero...
+			rpn_EnterShortCircuitOp(rpn_isKnown(&$1) && !$1.nVal);
+		} relocexpr {
+			// ...then short-circuit to 0 without evaluating the second operand.
+			if (rpn_IsShortCircuited())
+				rpn_Number(&$$, 0);
+			else
+				rpn_BinaryOp(RPN_LOGAND, &$$, &$1, &$4);
+			rpn_LeaveShortCircuitOp();
 		}
 		| relocexpr T_OP_LOGICEQU relocexpr {
 			rpn_BinaryOp(RPN_LOGEQ, &$$, &$1, &$3);
@@ -1089,7 +1105,10 @@ const		: relocexpr {
 
 string		: T_STRING
 		| T_OP_STRSUB T_LPAREN string T_COMMA uconst T_COMMA uconst T_RPAREN {
-			strsubUTF8($$, $3, $5, $7);
+			if (rpn_IsShortCircuited())
+				$$[0] = '\0';
+			else
+				strsubUTF8($$, $3, $5, $7);
 		}
 		| T_OP_STRCAT T_LPAREN T_RPAREN {
 			$$[0] = '\0';
@@ -1107,7 +1126,9 @@ string		: T_STRING
 
 strcat_args	: string
 		| strcat_args T_COMMA string {
-			if (snprintf($$, MAXSTRLEN + 1, "%s%s", $1, $3) > MAXSTRLEN)
+			if (rpn_IsShortCircuited())
+				$$[0] = '\0';
+			else if (snprintf($$, MAXSTRLEN + 1, "%s%s", $1, $3) > MAXSTRLEN)
 				warning(WARNING_LONG_STR, "STRCAT: String too long '%s%s'\n",
 					$1, $3);
 		}

src/asm/rpn.c

@@ -24,6 +24,9 @@
 #include "asm/symbol.h"
 #include "asm/warning.h"
 
+uint32_t nestedShortCircuitOps; // How many nested short-circuitable operators there currently are
+uint32_t shortCircuitAtDepth; // The nested depth at which short-circuiting occurred (0 if N/A)
+
 /* Makes an expression "not known", also setting its error message */
 #define makeUnknown(expr_, ...) do { \
 	struct Expression *_expr = expr_; \
@@ -95,6 +98,25 @@ void rpn_Free(struct Expression *expr)
 	rpn_Init(expr);
 }
 
+void rpn_EnterShortCircuitOp(bool startShortCircuit)
+{
+	nestedShortCircuitOps++;
+	if (startShortCircuit && shortCircuitAtDepth == 0)
+		shortCircuitAtDepth = nestedShortCircuitOps;
+}
+
+void rpn_LeaveShortCircuitOp(void)
+{
+	nestedShortCircuitOps--;
+	if (shortCircuitAtDepth > nestedShortCircuitOps)
+		shortCircuitAtDepth = 0;
+}
+
+bool rpn_IsShortCircuited(void)
+{
+	return shortCircuitAtDepth > 0 && shortCircuitAtDepth >= nestedShortCircuitOps;
+}
+
 /*
  * Add symbols, constants and operators to expression
  */
@@ -106,6 +128,11 @@ void rpn_Number(struct Expression *expr, uint32_t i)
 
 void rpn_Symbol(struct Expression *expr, char const *tzSym)
 {
+	if (rpn_IsShortCircuited()) {
+		rpn_Number(expr, 0);
+		return;
+	}
+
 	struct Symbol *sym = sym_FindScopedSymbol(tzSym);
 
 	if (sym_IsPC(sym) && !sect_GetSymbolSection()) {
@@ -157,6 +184,11 @@ void rpn_BankSelf(struct Expression *expr)
 
 void rpn_BankSymbol(struct Expression *expr, char const *tzSym)
 {
+	if (rpn_IsShortCircuited()) {
+		rpn_Number(expr, 0);
+		return;
+	}
+
 	struct Symbol const *sym = sym_FindScopedSymbol(tzSym);
 
 	/* The @ symbol is treated differently. */
@@ -189,6 +221,11 @@ void rpn_BankSymbol(struct Expression *expr, char const *tzSym)
 
 void rpn_BankSection(struct Expression *expr, char const *tzSectionName)
 {
+	if (rpn_IsShortCircuited()) {
+		rpn_Number(expr, 0);
+		return;
+	}
+
 	rpn_Init(expr);
 
 	struct Section *pSection = out_FindSectionByName(tzSectionName);
@@ -321,6 +358,11 @@ static bool isDiffConstant(struct Expression const *src1,
 void rpn_BinaryOp(enum RPNCommand op, struct Expression *expr,
 		  const struct Expression *src1, const struct Expression *src2)
 {
+	if (rpn_IsShortCircuited()) {
+		rpn_Number(expr, 0);
+		return;
+	}
+
 	expr->isSymbol = false;
 
 	/* First, check if the expression is known */

test/asm/short-circuiting.asm

@@ -0,0 +1,9 @@
+
+	printv 1 == 1 || undef_n == 42
+	printt "\n"
+
+	printv 1 == 2 && 0 < STRLEN("{undef_s}")
+	printt "\n"
+
+	printv (((1 == 1 || undef_a) || undef_b) || undef_c)
+	printt "\n"

test/asm/short-circuiting.out

@@ -0,0 +1,3 @@
+$1
+$0
+$1