Omitted conversion float -> double in x86 back-end

[Roberto COSTA <roberto dot costa at st dot com>]

Hello,
I've just noticed that, in a sequence of conversions signed long long -> 
float -> double, x86 back-end eats the intermediate conversion away.
In this case, the conversion to float isn't redundant because it lowers 
the precision of the computation (which is something a programmer may 
want to do).
It looks to me this this is a bug; I'm not completely sure because of my 
ignorance of what C standards state about it.
Before filing a report, I'd like to hear other opinions.

Here is a simple example:

------------- float_conv.c ------------
double foo(signed long long l)
{
    return (float)l;
}
---------------------------------------

Files float_conv.c.026t.fixupcfg and float_conv.s are the outcome of the 
following compilation:
    gcc -O0 float_conv.c -S -fdump-tree-all

------ float_conv.c.026t.fixupcfg -----
;; Function foo (foo)

foo (l)
{
  float D.1524;
  double D.1523;

<bb 2>:
  D.1524 = (float) l;
  D.1523 = (double) D.1524;
  return D.1523;
}
---------------------------------------

------------- float_conv.s ------------
        .file   "float_conv.c"
        .text
.globl foo
        .type   foo, @function
foo:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $8, %esp
        movl    8(%ebp), %eax
        movl    %eax, -8(%ebp)
        movl    12(%ebp), %eax
        movl    %eax, -4(%ebp)
        fildll  -8(%ebp)
        leave
        ret
        .size   foo, .-foo
        .ident  "GCC: (GNU) 4.2.0 20060826 (experimental)"
        .section        .note.GNU-stack,"",@progbits
---------------------------------------

The conversion is still there at the end of GIMPLE passes, which leads 
me to the conclusion that it is removed in RTL compilation.

Comments?

Cheers,
Roberto