Re: c/10226: unsigned short promotion with bitwise inversion

[Glen Nakamura <glen@imodulo.com>]

The following reply was made to PR c/10226; it has been noted by GNATS.

From: Glen Nakamura <glen@imodulo.com>
To: Falk Hueffner <falk.hueffner@student.uni-tuebingen.de>
Cc: Michael Marks <Michael.Marks@internetmachines.com>,
	gcc-gnats@gcc.gnu.org
Subject: Re: c/10226: unsigned short promotion with bitwise inversion
Date: Wed, 26 Mar 2003 12:30:54 -1000

 On Wed, Mar 26, 2003 at 11:19:58PM +0100, Falk Hueffner wrote:
 > Glen Nakamura <glen@imodulo.com> writes:
 > > How about one of these:
 > > warning: comparison of promoted ~unsigned with unsigned is always false
 > > warning: comparison of ~(promoted unsigned) with unsigned is always false
 > 
 > Yes, that makes it even clearer. I'd prefer the second variant.  (BTW,
 > funny, the optimizer doesn't seem to realize it's always false...)
 > 
 > -- 
 > 	Falk
 
 It does on my system (GCC 3.3):
 
 .LC1:
         .string "Fail"
         .text
         .p2align 4,,15
 .globl main
         .type   main, @function
 main:
         pushl   %ebp
         movl    %esp, %ebp
         subl    $8, %esp
         andl    $-16, %esp
         movl    $.LC1, (%esp)
         call    puts
         movl    %ebp, %esp
         xorl    %eax, %eax
         popl    %ebp
         ret
 
 But, as I mentioned before my suggestion won't work for other comparisons...
 the warning probably needs to be generic enough for all of the following cases:
 unsigned (char|short) (==|!=|<|<=|>|>=) ~unsigned (char|short)
 which is *not* always false but could have unexpected results.
 
 - glen