By mistake I sent this to the original author only, not to gcc-help. -------- Original Message -------- Subject: Re: Change default sizeof(long double) to 128-bit Date: Sun, 27 Mar 2011 15:33:23 +0100 From: Dr. David Kirkby <david.kirkby@xxxxxxxxxx> To: bd satish <bdsatish@xxxxxxxxx> On 03/27/11 01:29 PM, bd satish wrote:
Hi, I am trying to build gcc and I would like gcc to map (by default) "long double" to 128-bit. What option should I pass to "configure" script to get this behaviour ? I'm running a 32-bit OS [ Ubuntu 10.10 ] on intel core i3 processor. Currently sizeof( long double) prints 12 bytes, i.e., 96 bits. PS: I do not like to specify the flag "-m128bit-long-double" to every C program I compile. Instead, I want gcc to use 16-byte (128-bit) by default without the end user having to specify this flag. Is this possible ? For sake of completeness, [gnu]:15 $ gcc -v Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/home/god/gcc/libexec/gcc/i786-pc-linux-gnu/4.7.0/lto-wrapper Target: i786-pc-linux-gnu Configured with: ./configure --prefix=/home/god/gnu/installed/gcc --enable-languages=c,fortran --build=i786-pc-linux-gnu --enable-checking=release --disable-libmudflap --enable-libgomp --disable-shared --disable-bootstrap Thread model: posix gcc version 4.7.0 20110319 (experimental) (GCC) Thanks !
I don't know if this works on any platform (read the docs), but on the gcc I downloaded for the AIX operating system, there was an option to the 'configure' script which might do what you want. Here's how gcc was built on AIX -bash-4.1$ gcc -v Using built-in specs. Target: powerpc-ibm-aix5.3.0.0 Configured with: ../stage/gcc-4.2.4/configure --disable-shared --enable-threads=posix --prefix=/opt/pware --with-long-double-128 --with-mpfr=/opt/pware --with-gmp=/opt/pware Thread model: aix gcc version 4.2.4 -bash-4.1$ I assume 128 bit long doubles will be much slower, as this would have to be implemented in sofware, whereas I think the 12-byte long double can be done in hardware on the x86 chips, as the floating point processor uses 80 bits internally. -- A: Because it messes up the order in which people normally read text. Q: Why is top-posting such a bad thing? A: Top-posting. Q: What is the most annoying thing in e-mail? Dave
