* cast of float to unsigned char bug on ARM with 3.4.5 - when was it fixed?
@ 2008-01-09 16:29 Sam Ravnborg
2008-01-10 15:23 ` John Love-Jensen
0 siblings, 1 reply; 3+ messages in thread
From: Sam Ravnborg @ 2008-01-09 16:29 UTC (permalink / raw)
To: gcc-help
The following sample code produces different result depending on
the gcc version and target:
gcc version target output
3.4.5 i386 n1: 226 n2: -30.33
4.1.2 (RedHat) i386 n1: 226 n2: -30.33
3.4.5 arm-softfloat n1: 0 n2: -30.33
4.1.1 arm n1: 226 n2: -30.33
From the sample code above it is obvious that gcc 3.4.5
on arm-softfloat differs from the rest.
The sample code:
====================
#include <stdio.h>
int main(void)
{
unsigned char n1;
float n2;
n2 = -30.33F;
n1 = (unsigned char)n2;
printf("n1: %d n2: %3.2f\n", n1, n2);
return 0;
}
====================
I have not yet tried with a arm-softfloat gcc > 3.4.5 because Dan Kegel's
crosstool do not have it out-of-the box.
I assume this is a know bug that got fixed in later gcc's.
I have tried searching the gcc bug database with no luck.
Can anyone give a hint of when it was fixed or how I can better look it
up myself?
Thanks in advance,
Sam
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: cast of float to unsigned char bug on ARM with 3.4.5 - when was it fixed?
2008-01-09 16:29 cast of float to unsigned char bug on ARM with 3.4.5 - when was it fixed? Sam Ravnborg
@ 2008-01-10 15:23 ` John Love-Jensen
2008-01-10 19:57 ` Sam Ravnborg
0 siblings, 1 reply; 3+ messages in thread
From: John Love-Jensen @ 2008-01-10 15:23 UTC (permalink / raw)
To: Sam Ravnborg, MSX to GCC
Hi Sam,
> I assume this is a know bug that got fixed in later gcc's.
> I have tried searching the gcc bug database with no luck.
It's not a GCC bug. Your code a C bug (if I may be so presumptuous to
describe using undefined C behavior as a "bug").
Since your code is performing undefined behavior, the output of all four
examples is correct.
From "The C Programming Language" A.6.3 "The result is undefined if the
value will not fit in the space provided". After truncation, -30 does not
fit in 0 ... 255 range (assuming your unsigned char is 8-bit), hence
undefined behavior.
One way you could fix your code to have more consistent behavior is to use
an intermediate int.
The sample code:
====================
#include <stdio.h>
int main(void)
{
int i;
unsigned char n1;
float n2;
n2 = -30.33F;
i = (int)n2;
n1 = (unsigned char)i;
printf("n1: %d n2: %3.2f\n", n1, n2);
return 0;
}
====================
As long as an int is the same bit-size and is 2's complement represented on
the platforms being compared, that should product the same output.
HTH,
--Eljay
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: cast of float to unsigned char bug on ARM with 3.4.5 - when was it fixed?
2008-01-10 15:23 ` John Love-Jensen
@ 2008-01-10 19:57 ` Sam Ravnborg
0 siblings, 0 replies; 3+ messages in thread
From: Sam Ravnborg @ 2008-01-10 19:57 UTC (permalink / raw)
To: John Love-Jensen; +Cc: MSX to GCC
On Wed, Jan 09, 2008 at 07:51:49AM -0600, John Love-Jensen wrote:
> Hi Sam,
>
> > I assume this is a know bug that got fixed in later gcc's.
> > I have tried searching the gcc bug database with no luck.
>
> It's not a GCC bug. Your code a C bug (if I may be so presumptuous to
> describe using undefined C behavior as a "bug").
I would name it a bug relying on undefined behaviour.
Thanks for clarifyng this!
Sam
^ permalink raw reply [flat|nested] 3+ messages in thread
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2008-01-09 16:29 cast of float to unsigned char bug on ARM with 3.4.5 - when was it fixed? Sam Ravnborg
2008-01-10 15:23 ` John Love-Jensen
2008-01-10 19:57 ` Sam Ravnborg
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