From which version of gcc does -mcmodel=large work? - gcc

Question

I have code which produces executables larger than 2GB (it's generated code).
On x64 with gcc 4.3.2 I get errors like:
crtstuff.c:(.text+0x20): relocation truncated to fit:
R_X86_64_32S against `.dtors'
So I understand i need the -mcmodel=large option. However that doesn't do anything or solve the problem on my system.
I am sure I read somewhere, that it was only supported from a particular version of gcc, and the option was ignored on versions before that. I would tell my operations team to install that version of gcc if only I knew what it was. But I just can't find any evidence right now to tell me if that hypothesis is true, and if so in which version the feature was introduced.
For example
(1) Here it is stated that the option doesn't do anything. The book in question claims to cover "GCC 4.x". The book came out 2006.
(2) Here a compiler bug is being reported against the option, therefore I conclude in that version it must do at least something. That seems to be gcc 4.6.1.
So although I can no longer find evidence of exactly in which version the feature was implemented, at least there is evidence that this has changed over time.
I have tried looking through the changelogs for all the various GCC 4.x versions to no avail (and normally they are pretty good so the lack of information there almost implies that I am wrong and nothing has changed between versions.)
Edit: This seems to imply that perhaps it did work, but I need to "recompile crtstuff.c", but I don't really know where I find that file or how I do that.

Answer 1

I believe 4.4 is the version that added support for this feature. I demonstrate below that 4.1 doesn't work while 4.4 does, on something that needs a large data block (rather than code). I'm not sure about 4.2 and 4.3, but both your example and my memory suggest 4.3 didn't have working support for this. My example should let you validate whether a particular installation works or not though, on an otherwise easy to compile bit of code.
As background, I maintain a program that's a fork of the stream benchmark, modified specially to use 64 bit structures for testing larger systems. I was plagued with these "relocation truncated to fit" errors until I started using "-mcmodel=large", and my fork won't compile/run unless that really does work. The oldest version of gcc I've definitely found my program compatible with is the 4.4.5 that ships with Debian Squeeze.
Here's a complete test case showing my fork of stream compiling and using >4GB of RAM with the large model, after failing to do so without the option:
$ gcc --version
gcc (Debian 4.4.5-8) 4.4.5
...
$ git clone https://github.com/gregs1104/stream-scaling.git
$ cd stream-scaling
$ gcc -O3 -DN=200000000 -fopenmp stream.c -o stream
/tmp/cca8rR1I.o: In function `checkSTREAMresults':
stream.c:(.text+0x34): relocation truncated to fit: R_X86_64_32S against `.bss'
...
stream.c:(.text+0x6ab): additional relocation overflows omitted from the output
collect2: ld returned 1 exit status
$ gcc -O3 -DN=200000000 -fopenmp stream.c -o stream -mcmodel=large
$ ./stream
-------------------------------------------------------------
STREAM version $Revision: 5.9 $
-------------------------------------------------------------
This system uses 8 bytes per DOUBLE PRECISION word.
-------------------------------------------------------------
Array size = 200000000, Offset = 0
Total memory required = 4577.6 MB.
...
And here's what happens on a version of gcc that doesn't have the large model, one running RedHat 5 derived software (CentOS 5.8):
$ gcc --version
gcc (GCC) 4.1.2 20080704 (Red Hat 4.1.2-52)
...
$ gcc -O3 -DN=200000000 -fopenmp stream.c -o stream -mcmodel=large
stream.c:1: sorry, unimplemented: code model ‘large’ not supported yet
So on older versions of gcc, it should throw that error out, not just ignore the option.
crtstuff is a library coming with gcc. The bug report you linked to on the gcc mailing list was from someone trying to build their own gcc for a RedHat 5 system, which as you can see in this last example ships with gcc 4.1. They rebuilt part of gcc with the large model, but it was still linking against the original, 4.1 built crtstuff library. You shouldn't run into that problem if you're using a properly packaged gcc, which is why it wasn't considered a real bug by the gcc developers. I think you just need gcc 4.4 or later.