gcc 4.3.0, -Wconversion against -O1

Andriy Gapon avg@icyb.net.ua
Fri Sep 19 16:57:00 GMT 2008

There is a very simple function:
/******** func.c **********/
#include <netinet/in.h>

uint16_t my_htons(uint16_t hostshort)
         return htons(hostshort);

$ cc func.c -Wconversion -Werror -c -o func.o
Exit 0

$ cc func.c -O1 -Wconversion -Werror -c -o func.o
cc1: warnings being treated as errors
func.c: In function ‘my_htons’:
func.c:5: error: conversion to ‘short unsigned int’ from ‘int’ may alter 
its value
Exit 1

Adding -E flag I get the following code:
# 2 "func.c" 2

uint16_t my_htons(uint16_t hostshort)
  return (__extension__ ({ register unsigned short int __v, __x = 
(hostshort); if (__builtin_constant_p (__x)) __v = ((((__x) >> 8) & 
0xff) | (((__x) & 0xff) << 8)); else __asm__ ("rorw $8, %w0" : "=r" 
(__v) : "0" (__x) : "cc"); __v; }));

Is there anything (smart) that either I or gcc or glibc/linux can do 
about this?
No type-casts in *my* code help.

The warning seems to be uncalled for in this case, because I don't see 
explicit 'int' type anywhere in the expanded code.

P.S. the code seems to be coming from /usr/include/bits/byteswap.h, 
__bswap_16() macro in this way:

======= netinet/in.h =======
#ifdef __OPTIMIZE__
/* We can optimize calls to the conversion functions.  Either nothing has
    to be done or we are using directly the byte-swapping functions which
    often can be inlined.  */
/* The host byte order is the same as network byte order,
    so these functions are all just identity.  */
# define ntohl(x)       (x)
# define ntohs(x)       (x)
# define htonl(x)       (x)
# define htons(x)       (x)
# else
#   define ntohl(x)     __bswap_32 (x)
#   define ntohs(x)     __bswap_16 (x)
#   define htonl(x)     __bswap_32 (x)
#   define htons(x)     __bswap_16 (x)
#  endif
# endif

Andriy Gapon

More information about the Gcc mailing list