http://d.puremagic.com/issues/show_bug.cgi?id=4380

           Summary: Poor optimisation of x*x, where x is real
           Product: D
           Version: D1 & D2
          Platform: Other
        OS/Version: Windows
            Status: NEW
          Keywords: performance
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: clugdbug@yahoo.com.au


--- Comment #0 from Don <clugdbug@yahoo.com.au> 2010-06-24 03:02:44 PDT ---
// TEST CASE
int main(string[] args) {
    real x = args.length == 2 ? 6.0 : 4.0; // just to defeat the optimiser
    real y = x*x;
    return cast(int)y;
}
----

With double x, produces:
                fstp    qword ptr [ESP] // store x
                fld     qword ptr [ESP]
                fmul    ST,ST(0)
Does the same for float x.

With real x, produces
                fstp    tbyte ptr [ESP] // store x
                fld     tbyte ptr [ESP]
                fld     tbyte ptr [ESP] // Why is it loading this twice???
                fmulp   ST(1),ST

The last two lines should just be fmul ST, ST(0)

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Don <clugdbug@yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement


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hsteoh@quickfur.ath.cx changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hsteoh@quickfur.ath.cx


--- Comment #1 from hsteoh@quickfur.ath.cx 2013-08-17 12:16:37 PDT ---
This is still happening on git HEAD.

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yebblies <yebblies@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |yebblies@gmail.com


--- Comment #2 from yebblies <yebblies@gmail.com> 2013-08-18 14:02:45 EST ---
In cg87, line 1661 (load87) it has this code:

if ((ty == TYldouble || ty == TYildouble) &&
    op != -1 && e->Eoper != OPd_ld)
    goto Ldefault;

op is 0 here (OPvar), and ty == TYldouble, so all the normal var cse checking
is skipped.


Down at 1887, we have

#if 1           /* Do this instead of codelem() to avoid the freenode(e).
                   We also lose CSE capability  */
                if (e->Eoper == OPconst)
                {
                    c = load87(e, 0, &retregs, NULL, -1);
                }
                else
                    c = (*cdxxx[e->Eoper])(e,&retregs);
#else
                c = codelem(e,&retregs,FALSE);
#endif

So it looks like this was intentional to avoid some kind of compiler internal bug.  Removing both conditions results in the correct code, but who knows what else it breaks.

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