Re: [PATCH] Don't set TEXT_START_ADDR for MIPS n64

["Maciej W. Rozycki" <macro at linux-mips dot org>]

On Sat, 28 Jun 2008, Richard Sandiford wrote:

> In:
> 
>     http://sourceware.org/ml/binutils/2003-05/msg00636.html
> 
> Maciej increased the n64 TEXT_START_ADDR beyond the 2GB limit:
> 
>     Setting text segment's start address above 4 GB for 64-bit
>     executables aids in finding unportable software -- if a pointer
>     is cropped to 32 bits, instead of a possible memory corruption a
>     segmentation fault is sent as nothing is mapped below text normally.
> 
> That's certainly a useful thing to want to test.  However,
> as linux has proved, you can get big size savings using -msym32
> for n64 code.  The n64 definition prevents -msym32 working by default.
> 
> Of the two, I think allowing -msym32 is the better default choice.
> Anyone wanting to smoke out invalid assumptions could still use
> -Ttext to do so.
> 
> Tested on mips64-linux-gnu and mips64el-linux-gnu.  OK to install?

 Richard, I have no doubt about the usefulness of -msym32, but unlike with
the Linux kernel which uses non-PIC code and where merely shorter
instruction sequences are used omitting references to the %higher() and
%highest() parts of relocations, here this option is an externally visible
change to the ABI.  So why don't you simply use a separate set of linker
scripts for this new emulation?  For pure n64 binaries there is a clear
benefit from locating text beyond the first 4GB and no negative impact
whatsoever.

  Maciej