Remove undefined behavior from shifts

`asl` and `asr` in `src/link/patch.c` courtesy of @pinobatch, and rearranged in RGBASM
evaluators.

src/asm/constexpr.c

@@ -205,28 +205,52 @@ void constexpr_BinaryOp(struct ConstExpression *expr,
 			result = value1 & value2;
 			break;
 		case T_OP_SHL:
-			if (value1 < 0)
-				warning(WARNING_SHIFT, "Left shift of negative value: %d",
-					value1);
-
-			if (value2 < 0)
-				fatalerror("Shift by negative value: %d",
-					   value2);
-			else if (value2 >= 32)
-				fatalerror("Shift by too big value: %d",
-					   value2);
-
-			result = (uint32_t)value1 << value2;
-			break;
 		case T_OP_SHR:
 			if (value2 < 0)
-				fatalerror("Shift by negative value: %d",
-					   value2);
-			else if (value2 >= 32)
-				fatalerror("Shift by too big value: %d",
-					   value2);
+				warning(WARNING_SHIFT_AMOUNT, "Shifting %s by negative amount %d",
+					op == T_OP_SHL ? "left" : "right",
+					value2);
+			if (op == T_OP_SHR) {
+				value2 = -value2; /* Right shift == neg left */
 
-			result = value1 >> value2;
+				if (value1 < 0)
+					warning(WARNING_SHIFT, "Shifting negative value %d",
+						value1);
+			}
+
+			if (value2 >= 0) {
+				// Shift left
+				if (value2 >= 32) {
+					warning(WARNING_SHIFT_AMOUNT, "Shifting left by large amount %d",
+						value2);
+					result = 0;
+				} else {
+					/*
+					 * Use unsigned to force a bitwise shift
+					 * Casting back is OK because the types
+					 * implement two's complement behavior
+					 */
+					result = (uint32_t)value1 << value2;
+				}
+			} else {
+				// Shift right
+				value2 = -value2;
+				if (value2 >= 32) {
+					warning(WARNING_SHIFT_AMOUNT, "Shifting right by large amount %d",
+						value2);
+					result = value1 < 0 ? -1 : 0;
+				} else if (value1 >= 0) {
+					result = value1 >> value2;
+				} else {
+					/*
+					 * The C standard leaves shifting right
+					 * negative values undefined, so use a
+					 * left shift manually sign-extended
+					 */
+					result = (uint32_t)value1 >> value2 |
+						-((uint32_t)1 << (32 - value2));
+				}
+			}
 			break;
 		case T_OP_MUL:
 			result = (uint32_t)value1 * (uint32_t)value2;

src/asm/rpn.c

@@ -409,22 +409,59 @@ void rpn_AND(struct Expression *expr, const struct Expression *src1,
 	expr->nRPNPatchSize++;
 }
 
+static int32_t shift(int32_t shiftee, int32_t amount)
+{
+	if (shiftee < 0)
+		warning(WARNING_SHIFT, "Shifting negative value %d", shiftee);
+
+	if (amount >= 0) {
+		// Left shift
+		if (amount >= 32) {
+			warning(WARNING_SHIFT_AMOUNT, "Shifting left by large amount %d",
+				amount);
+			return 0;
+
+		} else {
+			/*
+			 * Use unsigned to force a bitwise shift
+			 * Casting back is OK because the types implement two's
+			 * complement behavior
+			 */
+			return (uint32_t)shiftee << amount;
+		}
+	} else {
+		// Right shift
+		amount = -amount;
+		if (amount >= 32) {
+			warning(WARNING_SHIFT_AMOUNT, "Shifting right by large amount %d",
+				amount);
+			return shiftee < 0 ? -1 : 0;
+
+		} else if (shiftee >= 0) {
+			return shiftee >> amount;
+
+		} else {
+			/*
+			 * The C standard leaves shifting right negative values
+			 * undefined, so use a left shift manually sign-extended
+			 */
+			return (uint32_t)shiftee >> amount
+				| -((uint32_t)1 << (32 - amount));
+		}
+	}
+}
+
 void rpn_SHL(struct Expression *expr, const struct Expression *src1,
 	     const struct Expression *src2)
 {
 	mergetwoexpressions(expr, src1, src2);
 
 	if (!expr->isReloc) {
-		if (src1->nVal < 0)
-			warning(WARNING_SHIFT, "Left shift of negative value: %d",
-				src1->nVal);
-
 		if (src2->nVal < 0)
-			fatalerror("Shift by negative value: %d", src2->nVal);
-		else if (src2->nVal >= 32)
-			fatalerror("Shift by too big value: %d", src2->nVal);
+			warning(WARNING_SHIFT_AMOUNT, "Shifting left by negative value: %d",
+				src2->nVal);
 
-		expr->nVal = ((uint32_t)src1->nVal << src2->nVal);
+		expr->nVal = shift(src1->nVal, src2->nVal);
 	}
 
 	pushbyte(expr, RPN_SHL);
@@ -438,11 +475,10 @@ void rpn_SHR(struct Expression *expr, const struct Expression *src1,
 
 	if (!expr->isReloc) {
 		if (src2->nVal < 0)
-			fatalerror("Shift by negative value: %d", src2->nVal);
-		else if (src2->nVal >= 32)
-			fatalerror("Shift by too big value: %d", src2->nVal);
+			warning(WARNING_SHIFT_AMOUNT, "Shifting right by negative value: %d",
+				src2->nVal);
 
-		expr->nVal = (src1->nVal >> src2->nVal);
+		expr->nVal = shift(src1->nVal, -src2->nVal);
 	}
 
 	pushbyte(expr, RPN_SHR);

src/asm/warning.c

@@ -36,6 +36,7 @@ static enum WarningState const defaultWarnings[NB_WARNINGS] = {
 	WARNING_DISABLED, /* Obsolete things */
 	WARNING_DISABLED, /* Shifting undefined behavior */
 	WARNING_ENABLED,  /* User warnings */
+	WARNING_DISABLED, /* Strange shift amount */
 };
 
 static enum WarningState warningStates[NB_WARNINGS];
@@ -70,6 +71,7 @@ static char const *warningFlags[NB_WARNINGS_ALL] = {
 	"obsolete",
 	"shift",
 	"user",
+	"shift-amount",
 
 	/* Meta warnings */
 	"all",
@@ -106,6 +108,7 @@ static uint8_t const _weverythingCommands[] = {
 	WARNING_OBSOLETE,
 	WARNING_SHIFT,
 	WARNING_USER,
+	WARNING_SHIFT_AMOUNT,
 	META_WARNING_DONE
 };