Instruction Set Specific Shared Objects

The dynamic token $ISALIST is expanded at runtime to reflect the native instruction sets executable on this platform, as displayed by the utility isalist(1). This token is available for filters, runpath definitions, and dependencies. As this token can expand to multiple objects, its use with dependencies is controlled. Dependencies obtained with dlopen(3C), can use this token with the mode RTLD_FIRST. Explicit dependencies that use this token will load the first appropriate dependency found.

Any string name that incorporates the $ISALIST token is effectively duplicated into multiple strings. Each string is assigned one of the available instruction sets.

The following example shows how the auxiliary filter libfoo.so.1 can be designed to access an instruction set specific filtee libbar.so.1.

$ LD_OPTIONS='-f /opt/ISV/lib/$ISALIST/libbar.so.1' \
cc -o libfoo.so.1 -G -K pic -h libfoo.so.1 -R. foo.c
$ elfdump -d libfoo.so.1 | egrep 'SONAME|AUXILIARY'
       [2]  SONAME            0x1                 libfoo.so.1
       [3]  AUXILIARY         0x96                /opt/ISV/lib/$ISALIST/libbar.so.1

Or alternatively the runpath can be used.

$ LD_OPTIONS='-f libbar.so.1' \
cc -o libfoo.so.1 -G -K pic -h libfoo.so.1 -R'/opt/ISV/lib/$ISALIST' foo.c
$ elfdump -d libfoo.so.1 | egrep 'RUNPATH|AUXILIARY'
       [3]  AUXILIARY         0x96                libbar.so.1
       [4]  RUNPATH           0xa2                /opt/ISV/lib/$ISALIST

In either case the runtime linker uses the platform available instruction list to construct multiple search paths. For example, the following application is dependent on libfoo.so.1 and executed on a SUNW,Ultra-2.

$ ldd -ls prog
....
  find object=libbar.so.1; required by ./libfoo.so.1
    search path=/opt/ISV/lib/$ISALIST  (RPATH from file ./libfoo.so.1)
      trying path=/opt/ISV/lib/sparcv9+vis/libbar.so.1
      trying path=/opt/ISV/lib/sparcv9/libbar.so.1
      trying path=/opt/ISV/lib/sparcv8plus+vis/libbar.so.1
      trying path=/opt/ISV/lib/sparcv8plus/libbar.so.1
      trying path=/opt/ISV/lib/sparcv8/libbar.so.1
      trying path=/opt/ISV/lib/sparcv8-fsmuld/libbar.so.1
      trying path=/opt/ISV/lib/sparcv7/libbar.so.1
      trying path=/opt/ISV/lib/sparc/libbar.so.1

Or an application with similar dependencies is executed on an MMX configured Pentium Pro.

$ ldd -ls prog
....
  find object=libbar.so.1; required by ./libfoo.so.1
    search path=/opt/ISV/lib/$ISALIST  (RPATH from file ./libfoo.so.1)
      trying path=/opt/ISV/lib/pentium_pro+mmx/libbar.so.1
      trying path=/opt/ISV/lib/pentium_pro/libbar.so.1
      trying path=/opt/ISV/lib/pentium+mmx/libbar.so.1
      trying path=/opt/ISV/lib/pentium/libbar.so.1
      trying path=/opt/ISV/lib/i486/libbar.so.1
      trying path=/opt/ISV/lib/i386/libbar.so.1
      trying path=/opt/ISV/lib/i86/libbar.so.1

Reducing Filtee Searches

The use of $ISALIST within a filter enables one or more filtees to provide implementations of interfaces defined within the filter.

Any interface defined in a filter can result in an exhaustive search of all potential filtees in an attempt to locate the required interface. If filtees are being employed to provide performance critical functions, this exhaustive filtee searching can be counterproductive.

A filtee can be built with the link-editor's -z endfiltee option to indicate that it is the last of the available filtees. This option terminates any further filtee searching for that filter. From the previous SPARC example, if the SPARCV9 filtee existed, and was tagged with -z endfiltee, the filtee searches would be as follows.

$ ldd -ls prog
....
  find object=libbar.so.1; required by ./libfoo.so.1
    search path=/opt/ISV/lib/$ISALIST  (RPATH from file ./libfoo.so.1)
      trying path=/opt/ISV/lib/sparcv9+vis/libbar.so.1
      trying path=/opt/ISV/lib/sparcv9/libbar.so.1