RFA: -mfpmath=sse -fpic vs double constants

[Dale Johannesen <dalej at apple dot com>]

Compiling a simple function like

double foo(double x)  {   return x+1.0;  }

on x86 with -O2 -march=pentium4 -mtune=prescott -mfpmath=sse -fpic, the 
load of 1.0 is done as

        cvtss2sd        .LC0@GOTOFF(%ecx), %xmm0

(this is Linux, the same happens on Darwin).
This is not really a good idea, as movsd of a double-precision 1.0 is 
faster.
The change from double to single precision is done in 
compress_float_constant,
and there's no cost computation there; presumably the RTL optimizers 
are expected
to change it back if that's beneficial.

Without -fpic, this does happen in cse_insn.  (mem/u/i:SF 
(symbol_ref/u:SI ("*.LC0")
gets run through fold_rtx, which recognizes this as a pool constant.  
This causes the
known equivalent CONST_DOUBLE 1.0 to be run through force_const_mem,
producing (mem/u/i:DF (symbol_ref/u:SI ("*.LC1").  Which is then tried 
in place
of the FLOAT_EXTEND, and selected as valid and cheaper.  This all seems 
to
be working as expected.

With -fpic, first, fold_rtx doesn't recognize the PIC form as 
representing a constant,
so cse_insn never tries forcing the CONST_DOUBLE into memory.   Hacking 
around
that doesn't help, because force_const_mem doesn't produce the PIC form 
of
constant reference, even though we're in PIC mode; we get the same
(mem/u/i:DF (symbol_ref/u:SI ("*.LC1"), which doesn't test as valid in 
PIC mode (correctly).

At this point I'm wondering if this is the right place to be attacking 
the problem at all.
Advice?  Thanks.