Fix UBSan error with overflowing exponent operator (#1727)

src/opmath.cpp

@@ -21,20 +21,17 @@ int32_t op_modulo(int32_t dividend, int32_t divisor) {
 }
 
 int32_t op_exponent(int32_t base, uint32_t power) {
-	int32_t result = 1;
+	uint32_t result = 1;
 
-	for (;;) {
+	for (uint32_t ubase = base; power; power /= 2) {
 		if (power % 2) {
-			result *= base;
+			result = static_cast<uint64_t>(result) * ubase;
 		}
-		power /= 2;
+		ubase = static_cast<uint64_t>(ubase) * ubase;
-		if (!power) {
-			break;
-		}
-		base *= base;
 	}
 
-	return result;
+	// Avoid arithmetic overflow runtime error
+	return result <= INT32_MAX ? result : -static_cast<int32_t>(~result) - 1;
 }
 
 int32_t op_shift_left(int32_t value, int32_t amount) {

test/asm/exponent.asm

@@ -0,0 +1,11 @@
+def n = 30
+for x, -n, n
+  for p, n
+    def v1 = x ** p
+    def v2 = 1
+    for i, p
+      def v2 *= x
+    endr
+    assert v1 == v2, "{d:x}**{d:p} = {d:v1} or {d:v2}?"
+  endr
+endr

test/asm/overflow.asm

@@ -34,6 +34,12 @@ def x = x << 1
 	dl -1 << 1
 	print_x
 
+def x = 2 ** 31
+	print_x
+
+def x = 5 ** 29
+	print_x
+
 def x = 4294967295
 def x = 4294967296
 

test/asm/overflow.err

@@ -2,7 +2,7 @@ warning: overflow.asm(23): [-Wdiv]
     Division of -2147483648 by -1 yields -2147483648
 warning: overflow.asm(24): [-Wdiv]
     Division of -2147483648 by -1 yields -2147483648
-warning: overflow.asm(38): [-Wlarge-constant]
+warning: overflow.asm(44): [-Wlarge-constant]
     Integer constant is too large
-warning: overflow.asm(41): [-Wlarge-constant]
+warning: overflow.asm(47): [-Wlarge-constant]
     Graphics constant is too long, only first 8 pixels considered

test/asm/overflow.out

@@ -4,3 +4,5 @@ $80000000
 $80000000
 $0
 $FFFFFFFE
+$80000000
+$6B90BE55