http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51729
Bug #: 51729
Summary: dspr2-MULT.c and dspr2-MULTU.c fail for MIPS
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: rsand...@gcc.gnu.org
dspr2-MULT.c and dspr2-MULTU.c are supposed to test that we use the
DSP accumulator registers to parallelise multiplications. They don't
work in 4.7, and I'm about to XFAIL them. There seem to be two main
problems:
* The cost of moving between DSP accumulators is greater than the cost
of moving a DSP register to or from memory. When I last looked,
this was enough to make the register allocator consider memory to
be cheaper.
This isn't a problem without -mdsp because there is then only one
accumulator register, LO+HI. (Note that we no longer allow HI and
LO to store independent values.) The cost of moving between accumulators
is therefore ignored.
On some (many?) targets, moving something out of an accumulator
and back again _is_ more expensive than storing an accumulator
to memory, so this isn't necessarily a bug in the backend.
* Even if we massage the costs to avoid that problem, each of the pseudos
that we'd like to use accumulators has one "=ka" constraint and one
"d" constraint. At one point this meant that DSP_REGS and GENERAL_REGS
had the same cost, and reg_alloc_order could be used to prefer accumulators:
http://gcc.gnu.org/ml/gcc/2010-12/msg00471.html
http://gcc.gnu.org/ml/gcc/2011-01/msg00093.html
But GENERAL_REGS now seems to have a lower cost, and since GENERAL_REGS
are much easier to spill than DSP_REGS, it's hard to argue with that.
