[Bug c++/41779] New: Spurious integral promotion

[Bug c++/41779] New: Spurious integral promotion

[zweije at xs4all dot nl]

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The following code produces a warning about precision loss in an int-to-float
conversion:

    $cat warn.cpp
    float f(float x, unsigned short y)
    {
        return x * y;
    }
    $g++ -Wconversion -c -o warn.o warn.cpp
    warn.cpp: In function ‘float f(float, short unsigned int)’:
    warn.cpp:3: warning: conversion to ‘float’ from ‘int’ may alter its value
    $

The code snippet does not have an int-to-float conversion.

It does have an unsigned short-to-float conversion according to the C++
standard, chapter 5 [expr] para 9: "[inapplicable note on long double and
double...] Otherwise, if either operand is float, the other shall be converted
to float [...]". Only after this rule, integral promotions are performed, which
could promote the unsigned short to an int.

Moreover, he conversion does not lose information since the 16 bits of the
unsigned short fit amply in the float's mantissa.

Not tested with other shorter-than-int types. This problem might apply there as
well...


-- 
           Summary: Spurious integral promotion
           Product: gcc
           Version: 4.4.1
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: zweije at xs4all dot nl
 GCC build triplet: x86_64-linux-gnu
  GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41779

[Bug c++/41779] Spurious integral promotion

[foo at mailinator dot com]

------- Comment #1 from foo at mailinator dot com  2009-12-29 18:02 -------
5/9 (usual arithmetic conversions) isn't relevant until after 4.5/1 (integral
promotions) has happened. Unsigned short *always* promotes to int in rvalue
contexts; then *that* int is implicitly converted to double in the context of
multiplication by a double (which is what's happening here).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41779

[Bug c++/41779] Spurious integral promotion

[zweije at xs4all dot nl]

------- Comment #2 from zweije at xs4all dot nl  2009-12-31 11:01 -------
I beg to differ. I cannot find where the standard says that 
unsigned short *always* promotes to int in rvalue contexts.

My reading in more detail is:

 1. The value of y is an lvalue of type unsigned short (5.1/7).

 2. 13.3.1.2/7 mentions (effectively) promoting the 
    operands (because built-in candidates (13.6/12) have 
    promoted arguments), but it applies only when the
    built-in multiplication has been selected by overload 
    resolution. This is not the case, because there are no
    class or enum arguments.

 3. Therefore, clause 5 applies directly (13.3.1.2/1).

 4. Clause 5 in itself does not require promoted operands
    (5.6/2), and does not distinguish lvalues from
    rvalues. Therefore the value of y is promoted directly
    to an lvalue of type float, through the usual arithmetic 
    conversion (5/9).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41779

[Bug c++/41779] Wconversion cannot see throught real*integer promotions

[manu at gcc dot gnu dot org]

------- Comment #3 from manu at gcc dot gnu dot org  2010-02-14 20:28 -------
I think this is a bug in Wconversion. It should be able to see through
promotions that the conversion cannot lead to a change of value.


-- 

manu at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu dot org
           Severity|minor                       |enhancement
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Keywords|                            |diagnostic
   Last reconfirmed|0000-00-00 00:00:00         |2010-02-14 20:28:45
               date|                            |
            Summary|Spurious integral promotion |Wconversion cannot see
                   |                            |throught real*integer
                   |                            |promotions


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41779

[Bug c++/41779] Wconversion cannot see throught real*integer promotions

[manu at gcc dot gnu dot org]

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------- Comment #4 from manu at gcc dot gnu dot org  2010-02-19 21:12 -------
Subject: Bug 41779

Author: manu
Date: Fri Feb 19 21:12:09 2010
New Revision: 156911

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=156911
Log:
2010-02-19  Manuel López-Ibáñez  <manu@gcc.gnu.org>

        PR 41779
        * c-common.c (conversion_warning): Remove widening conversions
        before checking the conversion of integers to reals.
testsuite/
        * c-c++-common/pr41779.c: New.

Added:
    trunk/gcc/testsuite/c-c++-common/pr41779.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/c-common.c
    trunk/gcc/testsuite/ChangeLog


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41779

[Bug c++/41779] Wconversion cannot see throught real*integer promotions

[manu at gcc dot gnu dot org]

------- Comment #5 from manu at gcc dot gnu dot org  2010-02-19 21:14 -------
FIXED in GCC 4.5


-- 

manu at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41779

[Bug c++/41779] Wconversion cannot see throught real*integer promotions

[hjl dot tools at gmail dot com]

-- 

hjl dot tools at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.5.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41779