https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66282
Bug ID: 66282
Summary: Missing x86 floating point conversion on explicit
casts violates C standard
Product: gcc
Version: 4.7.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: timo.kreuzer at reactos dot org
Target Milestone: ---
When compiling floating point code, the compiler omits to convert an
intermediate computation result, which is cast to double, to 64 bit precision
and instead keeps the internal 80 bit precision.
Example code:
void bugbug()
{
double x = 4.4;
double y;
y = (1 / x);
if ( y != ((double)(1 / x)) )
{
printf("Standard violation!\n");
}
}
compiled with "gcc.exe -Wall -g -c fpu.c -o fpu.o"
The expression on the right side inside the if() is not converted to double
precision, which is in violation of the C standard.
Quote:
"Implementations employing wide registers have to take care to honor
appropriate
semantics. Values are independent of whether they are represented in a register
or in memory. For example, an implicit spilling of a register is not permitted
to alter the value. Also, an explicit store and load is required to round to
the precision of the storage type. In particular, casts and assignments are
required to perform their specified conversion."
Emphasis is on the last sentence.
This was tested on a mingw version of the compiler, but I do not think this is
mingw specific.
