Handle a function returning bool, char or short

x86-64 psABI says that only the least significant 8 bits are
significant if RAX has a return value of a function returning bool.
Likewise, it looks like only the least significant 1 or 2 bytes
are guaranteed to have a correct value for char and short return
values, respectively.

codegen.c

@@ -291,6 +291,21 @@ static void gen_expr(Node *node) {
       println("  call %s", node->funcname);
       println("  add $8, %%rsp");
     }
+
+    // It looks like the most significant 48 or 56 bits in RAX may
+    // contain garbage if a function return type is short or bool/char,
+    // respectively. We clear the upper bits here.
+    switch (node->ty->kind) {
+    case TY_BOOL:
+      println("  movzx %%al, %%eax");
+      return;
+    case TY_CHAR:
+      println("  movsbl %%al, %%eax");
+      return;
+    case TY_SHORT:
+      println("  movswl %%ax, %%eax");
+      return;
+    }
     return;
   }
   }

parse.c

@@ -479,7 +479,7 @@ static Type *declspec(Token **rest, Token *tok, VarAttr *attr) {
   return ty;
 }
 
-// func-params = ("void" | param ("," param)*)? ")"
+// func-params = ("void" | param ("," param)*?)? ")"
 // param       = declspec declarator
 static Type *func_params(Token **rest, Token *tok, Type *ty) {
   if (equal(tok, "void") && equal(tok->next, ")")) {

test/common

@@ -1,3 +1,4 @@
+#include <stdarg.h>
 #include <stdio.h>
 #include <stdlib.h>
 
@@ -16,3 +17,8 @@ int *ext2 = &ext1;
 int ext3 = 7;
 int ext_fn1(int x) { return x; }
 int ext_fn2(int x) { return x; }
+
+int false_fn() { return 512; }
+int true_fn() { return 513; }
+int char_fn() { return (2<<8)+3; }
+int short_fn() { return (2<<16)+5; }

test/function.c

@@ -65,6 +65,11 @@ void ret_none() {
   return;
 }
 
+_Bool true_fn();
+_Bool false_fn();
+char char_fn();
+short short_fn();
+
 int main() {
   ASSERT(3, ret3());
   ASSERT(8, add2(3, 5));
@@ -106,6 +111,11 @@ int main() {
 
   ret_none();
 
+  ASSERT(1, true_fn());
+  ASSERT(0, false_fn());
+  ASSERT(3, char_fn());
+  ASSERT(5, short_fn());
+
   printf("OK\n");
   return 0;
 }

test/test.h

@@ -2,5 +2,6 @@
 
 void assert(int expected, int actual, char *code);
 int printf();
+int sprintf();
 int strcmp(char *p, char *q);
 int memcmp(char *p, char *q, long n);