[Issue 6505] Wrong code for expression involving 8 floats, only with -O

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=6505



--- Comment #3 from Don <clugdbug at yahoo.com.au> 2011-08-23 12:19:44 PDT ---
Here's the problem. The scheduler keeps track of the number of
used x87 registers via the "fpustackused" variable.
Each instruction has a "fpuadjust" value which says how many x87
registers it uses.
The problem is CALL instructions. They have fpuadjust = 0.
But, a function returning float returns it in ST(0) --- so fpuadjust
should be +1.
And if it's a complex result, fpuadjust should be +2.
I think it should be possible to put an assert in cgsched.c, line 2491:

           if (tbl[j])
           {
               fpu += tbl[j]->fpuadjust;
+              assert(fpu >= 0);
               if (fpu >= 8)                   // if FPU stack overflow

but this would fail at the moment, because the total goes negative
after a call followed by an FSTP.
So it needs to distinguish between each type of call, depending on how
it affects the FPU stack.
I think the bug lies in funccall() in cod1.c; it should be setting
something in 'code' to record how many x87 registers are returned.

Then, there'll be another minor problem: the number of FPU values CAN
reach 8. This is because of the code in push87() in cg87.c, which does
fdecstp; fstp; when the stack is full. So the condition above will
need to change to:
               if (fpu > 8)                   // if FPU stack overflow

This is too invasive a change for me to make and test.

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