[Bug c++/56908] New: Spurious warning when XOR-ing uint8_t values

[Bug c++/56908] New: Spurious warning when XOR-ing uint8_t values

[hniksic at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56908

             Bug #: 56908
           Summary: Spurious warning when XOR-ing uint8_t values
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: hniksic@gmail.com


g++ -Wconversion displays what I believe is a spurious warning in my code:

warning: conversion to 'uint8_t {aka unsigned char}' from 'int' may alter its
value [-Wconversion]

-Wconversion is mandated by the project I'm working on.  Since I'm striving for
warningless compilation, I looked into the function. Even after a careful
examination, I cannot find fault with it, nor an obvious way to get rid of the
warning without compromising the code's readability.  The minimal example that
warns is:

#include <stdint.h>
#include <stddef.h>

void
xorblock(uint8_t* dest, const uint8_t* src, size_t len)
{
  while (len--)
    *dest++ ^= *src++;
}

Compiled with g++ -Wconversion a.cc, it prints the warning as:

a.cc: In function 'void xorblock(uint8_t*, const uint8_t*, size_t)':
a.cc:8:20: warning: conversion to 'uint8_t {aka unsigned char}' from 'int' may
alter its value [-Wconversion]

Both operands have the same type, which is appropriate for the calculation.
Casting the right-hand side of the compound assignment to either int, uint8_t,
or unsigned int fails to silence the warning.  The warning does not occur with
gcc, only with g++.

The only way I found to remove the warning is to rewrite the assignment as a
non-compound assignment.  But this precludes the use of the post-increment
operator and consequently makes the function harder to follow.  (In this case
shorter is more readable, as *dest++ = *src++ is one of the most widely
understood C idioms.)

The warning happens with g++ 4.7 and 4.8, but not with g++ 4.6 series.

[Bug c++/56908] Spurious warning when XOR-ing uint8_t values

[manu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56908

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-07-09
                 CC|                            |manu at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
Confirmed, perhaps a regression. More analysis here:
https://gcc.gnu.org/ml/gcc-help/2014-07/msg00042.html
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From: "paolo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/60686] message "=?UTF-8?Q? only declarations of constructors can be ‘explicit’ ?=" now conflicting with C++11
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id`686

--- Comment #3 from paolo at gcc dot gnu.org <paolo at gcc dot gnu.org> ---
Author: paolo
Date: Wed Jul  9 22:44:42 2014
New Revision: 212415

URL: https://gcc.gnu.org/viewcvs?rev!2415&root=gcc&view=rev
Log:
/cp
2014-07-09  Paolo Carlini  <paolo.carlini@oracle.com>

    PR c++/60686
    * decl.c (grokdeclarator): Adjust error messages about 'explicit'
    outside class declaration, in friend declaration, and neither on
    constructor nor conversion operator.

/testsuite
2014-07-09  Paolo Carlini  <paolo.carlini@oracle.com>

    PR c++/60686
    * g++.dg/cpp0x/explicit8.C: New.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/explicit8.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/decl.c
    trunk/gcc/testsuite/ChangeLog

[Bug c++/56908] Spurious warning when XOR-ing uint8_t values

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56908

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|NEW                         |RESOLVED
   Target Milestone|---                         |10.0
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=91993

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Fixed in GCC 10.