Description Sylvain Pion 2004-04-30 15:41:53 UTC

Consider the following function :

// Forces a double to memory
double force2memdouble(double x) {
  asm("" : "=m"(x) : "m"(x));
  return x;
}

This is used to force a double value in memory for platforms like x86
where the FP registers have too much precision.

It gets compiled by GCC (3.4 and 3.3.3, with -O2 -fomit-frame-pointer) to :

force2memdouble:
        subl    $12, %esp
        fldl    16(%esp)
        fstpl   (%esp)
        fldl    (%esp)
        addl    $12, %esp
        ret

I believe that the store/load is useless, and that the function could be
compiled as simply :

force2memdouble:
        fldl    4(%esp)
        ret


It is interesting to note that when using "int", "float" or "long double"
instead of "double", then there is no such useless store/load.


So, is this something that could be improved in GCC ?

Maybe the code responsible for asm() adds too many requirements
on the operands for doubles ?

Comment 1 Sylvain Pion 2004-04-30 15:51:07 UTC

I still have gcc 2.95 around, so I have tested with it as well.
It produces the better code that I expected :

force2memdouble:
#APP
#NO_APP
        fldl 4(%esp)
        ret


So, if the code is correct, then this problem is a regression on code quality
compared to 2.95.


I want to mention that this asm() is critical when implementing interval
arithmetic.  Also, GMP uses that kind of asm() in mpz_get_d_2exp.

Comment 2 Andrew Pinski 2004-04-30 15:57:08 UTC

Note before 3.3 and after 2.95.3, the following sequence was used to copy the FP:
        subl    $12, %esp
        movl    16(%esp), %edx
        movl    20(%esp), %ecx
        movl    %edx, (%esp)
        movl    %ecx, 4(%esp)

Confirmed as a regression.

Comment 3 Gabriel Dos Reis 2004-05-16 22:11:39 UTC

Will not fix in 3.3.x.

-- Gaby

Comment 4 Mark Mitchell 2004-05-31 22:10:04 UTC

Richard --

Would you please take a look at this?

Thanks,

-- Mark

Comment 5 Richard Henderson 2004-06-07 19:27:33 UTC

Testing a patch.

Comment 8 Richard Henderson 2004-06-09 23:52:49 UTC

Fixed.