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ash: bug fixes to avoid UBs and use bignums

References: https://todo.sr.ht/~ft/sl/11

--- a/src/cvalues.c

+++ b/src/cvalues.c

@@ -1238,6 +1238,12 @@

 	type_error("int", a);

 }

+#define sash_overflow_64(a, b, c) ( \

+	((a)<0 || (a)>(INT64_MAX>>(b))) \

+	? 1 \

+	: ((*(c)=(a)<<(b)), 0) \

+)

+

 BUILTIN("ash", ash)

 {

 	sl_fx n;

@@ -1250,11 +1256,16 @@

 	argcount(nargs, 2);

 	sl_v a = args[0];

 	n = tofixnum(args[1]);

+	mp = nil;

 	if(isfixnum(a)){

-		if(n <= 0)

+		accum = numval(a);

+		if(accum >= 0 && n > -64 && n <= 0)

 			return fixnum(numval(a)>>(-n));

-		accum = ((s64int)numval(a))<<n;

-		return fits_fixnum(accum) ? fixnum(accum) : return_from_s64(accum);

+		if(n < 0 || n >= 64 || sash_overflow_64(accum, n, &accum)){

+			mp = vtomp(accum, nil);

+			mpleft(mp, n, mp);

+		}else

+			return fits_fixnum(accum) ? fixnum(accum) : return_from_s64(accum);

 	}

 	if(iscprim(a)){

 		if(n == 0)

@@ -1283,11 +1294,16 @@

 			return a;

 		aptr = cv_data(ptr(a));

 		mp = mpnew(0);

-		if(n < 0)

-			mpright(*(mpint**)aptr, -n, mp);

-		else

-			mpleft(*(mpint**)aptr, n, mp);

-		return mk_mp(mp);

+		mpleft(*(mpint**)aptr, n, mp);

+	}

+	if(mp != nil){

+		n = mpsignif(mp);

+		if(n >= FIXNUM_BITS)

+			return mk_mp(mp);

+		accum = mptov(mp);

+		mpfree(mp);

+		assert(fits_fixnum(accum));

+		return fixnum((sl_fx)accum);

 	}

 	type_error("int", a);

 }