Hi,
I've seen GCC emit many unneeded moves in very simple cases when
handling double floating point values. I tracked this down to the
lower-subreg pass that splits DF->DF moves into DF->(2*SI)->DF moves.
I suppose that the introduced mode changes prevent the IRA from
coalescing the moves and thus I get these ugly unneeded moves.
I tried to limit the lower-subreg pass to splitting integral types by
constraining MODES_TIEABLE_P. This works for my simple test cases, but
I get some testsuite regressions of the form:
../gcc/testsuite/gcc.c-torture/execute/ieee/930529-1.c:27:1: error:
unrecognizable insn:
(insn 45 2 46 2
../gcc/testsuite/gcc.c-torture/execute/ieee/930529-1.c:2 (set
(subreg:SI (subreg:DF (reg/v:DI 136 [ d ]) 0) 0)
(const_int 2454267026 [0x92492492])) -1 (nil))
My other option is to disable subreg-splitting on my port, but I
wonder what adverse effects it could have on 64bits integral code.
What is the correct way to deal with this?
Many thanks,
Fred
