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cc: promote integer constants according to c99 spec.

C99 integer constants with no type suffix promote differently
depending on the way that they're written: hex and oct consts
promote as int => uint => long => ulong => vlong => uvlong.
Decimal constants are always signed.

We used to promote all values to uint on overflow, and never
went wider. This change fixes that, and adds a warning when
a decimal constant that would have been promoted to uint in
the past gets promoted to int.

--- a/sys/src/cmd/8c/txt.c

+++ b/sys/src/cmd/8c/txt.c

@@ -865,7 +865,7 @@

 		gmove(f, &fregnode0);

 		gins(AFADDD, nodfconst(-2147483648.), &fregnode0);

 		gins(AFMOVLP, f, &nod);

-		gins(ASUBL, nodconst(-2147483648), &nod);

+		gins(ASUBL, nodconst(-0x80000000), &nod);

 		gmove(&nod, t);

 		return;

--- a/sys/src/cmd/cc/lex.c

+++ b/sys/src/cmd/cc/lex.c

@@ -444,7 +444,7 @@

 yylex(void)

 {

 	vlong vv;

-	long c, c1, t;

+	long c, c1, t, w;

 	char *cp;

 	Rune rune;

 	Sym *s;

@@ -844,7 +844,16 @@

 		yyerror("overflow in constant");

 	vv = yylval.vval;

-	if(c1 & Numvlong) {

+	/*

+	 * c99 is silly: decimal constants stay signed,

+	 * hex and octal go unsigned before widening.

+	 */

+	w = 32;

+	if((c1 & (Numdec|Numuns)) == Numdec)

+		w = 31;

+	if(c1 & Numvlong || (c1 & Numlong) == 0 && (uvlong)vv >= 1ULL<<w){

+		if((c1&(Numdec|Numvlong)) == Numdec && vv < 1ULL<<32)

+			warn(Z, "int constant widened to vlong: %s", symb);

 		if((c1 & Numuns) || convvtox(vv, TVLONG) < 0) {

 			c = LUVLCONST;

 			t = TUVLONG;