CombSortA fails

[jj2007]

CombSortA fails with an access violation for this snippet:

Code Select Expand

include \masm32\MasmBasic\MasmBasic.inc

FakeStruct STRUCT
TheR4 REAL4 ?
FakeStruct ENDS

.data
align 16
R4Array REAL4 123.456, 99.0, -3000.0, 3000.0, 0.0, 3.0e-20, 3.0e20, 1.000001, 0.999999, 1.00001e2, 0.99999e2, 1.0

.code
start:
; void Str$(0)   ; <<<<<<<<<<<<<<<<< sets FPU to down, 64
mov esi, offset R4Array
mov ebx, sizeof R4Array/4
; finit     ; workaround: reset the FPU
invoke CombSortA, esi, ebx
.Repeat
Print Str$(ebx), Str$("\t%f\n", [esi.FakeStruct.TheR4+4*ebx])
dec ebx
.Until Sign?
Inkey "done"
Exit
end start


Olly told me the culprit sits here:

Code Select Expand

CPU Disasm
Address              Hex dump               Command                        Comments
00401415             ³> ÚDB45 FC            Úfild dword ptr [local.1]
00401418             ³. ³D835 E0404000      ³fdiv dword ptr [4040E0]       ; float 1.300000
0040141E             ³. ³DB5D FC            ³fistp dword ptr [local.1]
00401421             ³. ³FF4D FC            ³dec dword ptr [local.1]


If the FPU happens to be in a virgin state, CombSortA works just fine, and even sorts the REAL4 array in the right order ;-)

[clive]

Maybe I'm missing something but it doesn't crash when built with 6.15 and current MasmBasic.

Uncommenting the void str$ gets it stuck in a loop.

[jj2007]

Strange.
With void Str$(0) and no finit, I get a straight GPF.
With void Str$(0) and finit, no GPF.
Without Str$(0) no GPF.
What I find equally worrying is that my assumption that REAL4's can be sorted like integers might be wrong. It works for mostallvalues, but a dec ebx before the .Repeat is needed ::)

.data
R4Array   REAL4 123.456, 3.3e30, 99.0, -3000.0, 3000.0, 0.0, 3.0e-20, 3.0e20, 1.000001, 0.999999, 1.00001e2, 0.99999e2, 1.0, 33.44, 44.55, 55.66, 66.77, 77.88, 88.99
Output:
19      1.953342e+23
18      3.300000e+30
17      3.000000e+20
16      3000.000
15      123.4560
14      100.0010
13      99.99900
12      99.00000
11      88.99000
10      77.88000
9       66.77000
8       55.66000
7       44.55000
6       33.44000
5       1.000001
4       1.000000
3       0.9999990
2       3.000000e-20
1       0.0
0       -3000.000