digitalmars.D.bugs - [Issue 6505] New: Wrong code for expression involving 8 floats, only with -O
- d-bugmail puremagic.com (43/43) Aug 16 2011 http://d.puremagic.com/issues/show_bug.cgi?id=6505
- d-bugmail puremagic.com (128/128) Aug 16 2011 http://d.puremagic.com/issues/show_bug.cgi?id=6505
- d-bugmail puremagic.com (19/19) Aug 23 2011 http://d.puremagic.com/issues/show_bug.cgi?id=6505
- d-bugmail puremagic.com (31/31) Aug 23 2011 http://d.puremagic.com/issues/show_bug.cgi?id=6505
- d-bugmail puremagic.com (13/13) Aug 23 2011 http://d.puremagic.com/issues/show_bug.cgi?id=6505
http://d.puremagic.com/issues/show_bug.cgi?id=6505
Summary: Wrong code for expression involving 8 floats, only
with -O
Product: D
Version: D1 & D2
Platform: Other
OS/Version: Windows
Status: NEW
Severity: blocker
Priority: P2
Component: DMD
AssignedTo: nobody puremagic.com
ReportedBy: clugdbug yahoo.com.au
--- Comment #0 from Don <clugdbug yahoo.com.au> 2011-08-16 04:33:20 PDT ---
Found this when I removed a reference to std.random from the DMD test suite.
This bug is in test22.d. It was originally described in:
http://www.digitalmars.com/d/archives/digitalmars/D/bugs/4766.html
but wasn't reduced correctly -- it has never been fixed.
It only applies when compiling with -O: the result is -nan.
----
real randx()
{
return 1.2;
}
void test1()
{
float x10=randx();
float x11=randx();
float x20=randx();
float x21=randx();
float y10=randx();
float y11=randx();
float y20=randx();
float y21=randx();
float tmp=(
x20*x21 + y10*y10 + y10*y11 + y11*y11 +
y11*y20 + y20*y20 + y10*y21 + y11*y21 +
y21*y21);
assert(tmp > 0);
}
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http://d.puremagic.com/issues/show_bug.cgi?id=6505
bearophile_hugs eml.cc changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |bearophile_hugs eml.cc
--- Comment #1 from bearophile_hugs eml.cc 2011-08-16 06:37:50 PDT ---
Simplified a little:
double foo() {
return 1.0;
}
void main() {
double a = foo();
double b = foo();
double x = a*a + a*a + a*a + a*a + a*a + a*a + a*a +
a*b + a*b;
assert(x > 0);
}
---------------------------
Asm normal compilation:
_D4test3fooFZd comdat
fld1
ret
__Dmain comdat
L0: enter 024h,0
call near ptr _D4test3fooFZd
fstp qword ptr -018h[EBP]
call near ptr _D4test3fooFZd
fstp qword ptr -010h[EBP]
fld qword ptr -018h[EBP]
fmul qword ptr -010h[EBP]
fld qword ptr -018h[EBP]
fmul qword ptr -010h[EBP]
fld qword ptr -018h[EBP]
fmul qword ptr -018h[EBP]
fld qword ptr -018h[EBP]
fmul qword ptr -018h[EBP]
fld qword ptr -018h[EBP]
fmul qword ptr -018h[EBP]
fld qword ptr -018h[EBP]
fmul qword ptr -018h[EBP]
fld qword ptr -018h[EBP]
fmul qword ptr -018h[EBP]
fld qword ptr -018h[EBP]
fmul qword ptr -018h[EBP]
fdecstp
fstp qword ptr -024h[EBP]
fld qword ptr -018h[EBP]
fmul qword ptr -018h[EBP]
faddp ST(1),ST
faddp ST(1),ST
faddp ST(1),ST
faddp ST(1),ST
faddp ST(1),ST
faddp ST(1),ST
faddp ST(1),ST
fld qword ptr -024h[EBP]
faddp ST(1),ST
fstp qword ptr -8[EBP]
fld qword ptr -8[EBP]
ftst
fstsw AX
sahf
fstp ST
ja L7B
mov EAX,9
call near ptr _D4test8__assertFiZv
L7B: xor EAX,EAX
leave
ret
----------------------------
Asm compilation with -O:
_D4test3fooFZd comdat
fld qword ptr FLAT:_DATA[00h]
ret
__Dmain comdat
L0: sub ESP,034h
call near ptr _D4test3fooFZd
fstp qword ptr 0Ch[ESP]
call near ptr _D4test3fooFZd
fld qword ptr 0Ch[ESP]
fld qword ptr 0Ch[ESP]
fxch ST2
fstp qword ptr 014h[ESP]
fmul qword ptr 014h[ESP]
fxch ST1
fld qword ptr 0Ch[ESP]
fxch ST1
fmul qword ptr 014h[ESP]
fxch ST1
fmul ST,ST(0)
fld qword ptr 0Ch[ESP]
fmul ST,ST(0)
fld qword ptr 0Ch[ESP]
fmul ST,ST(0)
fld qword ptr 0Ch[ESP]
fmul ST,ST(0)
fld qword ptr 0Ch[ESP]
fmul ST,ST(0)
fld qword ptr 0Ch[ESP]
fmul ST,ST(0)
fdecstp
fld qword ptr 0Ch[ESP]
fxch ST1
fstp qword ptr [ESP]
fmul ST,ST(0)
faddp ST(1),ST
faddp ST(1),ST
faddp ST(1),ST
faddp ST(1),ST
faddp ST(1),ST
faddp ST(1),ST
faddp ST(1),ST
fld qword ptr [ESP]
faddp ST(1),ST
ftst
fstsw AX
fstp ST
sahf
ja L84
mov EAX,9
call near ptr _D4test8__assertFiZv
L84 add ESP,034h
xor EAX,EAX
ret
------------------------
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http://d.puremagic.com/issues/show_bug.cgi?id=6505 --- Comment #2 from Don <clugdbug yahoo.com.au> 2011-08-23 01:52:36 PDT --- Thanks bearophile, that reduction is very helpful. The code loads 8 values on the x87 stack. The eighth one would cause an overflow, so in cg87.c, push87(), it inserts: fdecstp; fstp [EBP - 24]; The next line is: fld [EBP - 18]; The code passes to the scheduler, where there's a bug: it converts the code to: fdecstp; fld [EBP - 18]; fxch ST(1); fstp [EBP - 24]; This is an x87 stack overflow, since the load happens before space has been made in the save. The x87 returns a NaN. -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Aug 23 2011
http://d.puremagic.com/issues/show_bug.cgi?id=6505
--- Comment #3 from Don <clugdbug 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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Walter Bright <bugzilla digitalmars.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
CC| |bugzilla digitalmars.com
Resolution| |FIXED
--- Comment #4 from Walter Bright <bugzilla digitalmars.com> 2011-08-23
15:23:43 PDT ---
https://github.com/D-Programming-Language/dmd/commit/628640f45bbc2bed3e84e835e19cf03ee2c4eafe
https://github.com/D-Programming-Language/dmd/commit/a92f6bf42e547c6978e4e317b4feb2c6610cf518
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