http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46993
Summary: Optimization on i386 lead to user-visible traps during
floating-point operations
Product: gcc
Version: 4.5.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: arseny.klimov...@gmail.com
Version:
$ g++-4.5.2 -v
Using built-in specs.
COLLECT_GCC=g++-4.5.2
COLLECT_LTO_WRAPPER=/opt/gcc-4.5.2/libexec/gcc/x86_64-unknown-linux-gnu/4.5.2/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-4.5.2/configure --prefix /opt/gcc-4.5.2/
Thread model: posix
gcc version 4.5.2 (GCC)
Command:
$ g++ -m32 -fsignaling-nans -O1 -c -S -o simple.s simple.cpp
Consider a simple function, which copies one value from double array c to
double array a.
#include <string.h>
void copy (double * a, double * c)
{
double b[100];
memcpy (b, c, sizeof (double));
memcpy (a, b, sizeof (double));
}
This will generate code
.cfi_startproc
pushl %ebp
.cfi_def_cfa_offset 8
movl %esp, %ebp
.cfi_offset 5, -8
.cfi_def_cfa_register 5
movl 12(%ebp), %eax
fldl (%eax)
movl 8(%ebp), %eax
fstpl (%eax)
popl %ebp
.cfi_restore 5
.cfi_def_cfa 4, 4
ret
.cfi_endproc
Here, if c[0] is a NaN, and FPU exception mask is set, an FPE will occur.
In gcc 4.4 and earlier I could use memcpy instead of operator= to be sure, that
no floating point operations will be generated.
Now I don't know how to safely copy values from local double buffer (or a
single variable) to some address without floating point operations.
