[Bug target/29963] New: could speed up variable access with different object layout

[Bug target/29963] New: could speed up variable access with different object layout

[amylaar at gcc dot gnu dot org]

Currently, variables are generally placed in the data or bss segment;
they cannot be addressed directly, hence their address needs to be
loaded into a register first.  Moreover, this address load is
done with a pc-relative load, which incurs extra latency.

If the variable was in the text segment within pc-relative load range,
integer loads could be done with pc-relative loads, while stores
and floating point loads could obtain their address using the mova
instruction.
Using a pc-relative load saves the extra latency of the address load
and/or the register that is used to hold the address.
Using mova saves the address load latency, and by making the address
cheper to obtain, makes it more feasible to recompute it in a loop
so that harder to reload values can be kept in a register.

For static variables, the compiler can determine if it is possible
to place the variable within reach when the translation unit is compiled.
(There are some issues with asms using directives like .rept or .org
 throwing off the instruction length calculations, but these are also
 a problem with other compiler activities requiring length calculation,
 i.e. branch shortening and constant pool layout, and programmers are
 well-advised to write their assembler templates in a way that their size
 will not underestimated, e.g. by passing with extra lines.)
For variables with global / namespace linkage, the best placement can
only be determined at link time.  This could either be done using the
-mrelax mechanism ( but that would need fixing first, see binutils/3298
http://sourceware.org/bugzilla/show_bug.cgi?id=3298 ), or by integrating
this optimization in the LTO framework.


-- 
           Summary: could speed up variable access with different object
                    layout
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: amylaar at gcc dot gnu dot org
GCC target triplet: sh-*-*
OtherBugsDependingO 29842
             nThis:


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29963