https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71460
Bug ID: 71460
Summary: Copying structs can trap (on x86-32)
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: ch3root at openwall dot com
Target Milestone: ---
Members of structs that have a floating-point type are sometimes copied as
floating-point numbers instead of just blocks of memory (presumably due to SRA
optimization but it could happens even without optimizations -- see the example
below). There are several consequences on x86-32/64:
- padding in long double is not copied. Probably ok (in the same way as padding
in substructures is not always copied);
- sNaNs are turned into qNaNs on x86-32 if traps are turned off. Probably ok if
sNaNs and qNaNs are considered different representations of the same value;
- sNaNs trap on x86-32 if traps are turned on. This seems to be a problem as
C11, 6.2.6.1p6, specifically says that "[t]he value of a structure or union
object is never a trap representation, even though the value of a member of the
structure or union object may be a trap representation." This is especially
problematic for copying partially initialized structs that are not fully
specified in a standard.
Example for the last item above:
----------------------------------------------------------------------
#define _GNU_SOURCE
#include <fenv.h>
#include <stdio.h>
int main()
{
feenableexcept(FE_INVALID);
struct s { double d; };
struct s x = {0};
((unsigned char *)&x)[7] = 0x7f;
((unsigned char *)&x)[6] = 0xf0; // sNaN
((unsigned char *)&x)[0] = 0x01;
struct s y;
y = x;
printf("%02x\n", ((unsigned char *)&y)[6]);
}
----------------------------------------------------------------------
Results:
----------------------------------------------------------------------
$ gcc -std=c11 -pedantic -Wall -Wextra -fsignaling-nans -lm -m32 test.c &&
./a.out
Floating point exception
----------------------------------------------------------------------
gcc version: gcc (GCC) 7.0.0 20160608 (experimental)
