Boundedly Undefined Results

Ever have those demons inside your computer that you swear are messing with your bits and producing random errors? Me too. During one of these bug chasing marathons, I wrote a FPU dumper that prints out the state of the x87 FPU whenever it is called. The program that I was working on was getting random numerical corruption on an older version of Cygwin’s GCC. I traced the corruption to a single line that had a floating point divide. The value was fine before the line was run and clobbered to a NaN after the line was run. Great, demons. The FPU dumper helped me realize what was going on. Long story short, there was a bug in newlib (Cygwin’s standard C library) which caused it to not conform to C99. The bug didn’t show up in my newer version of GCC because GCC provided a builtin for the missing C99 math function. I sent off a description to the bug to the newlib mailing list and Jeff Johnston had a patch within an hour. Thanks Jeff!

Regarding the FPU dumper, it is a header file that you include in whatever code you want to check the FPU from. If you uncomment the #define FPU_DUMP_VERBOSE line in the header or define it in the compile line (-DFPU_DUMP_VERBOSE), it prints off a bit more info on the status registers. To do the actual dumping, simply call FPUDUMP from anywhere in your code. The FPU state is saved into a struct, parsed, printed, and the FPU state is restored from the struct lest something like printf messes with the FPU in some way.

The code is licensed under the zlib license. The output format is borrowed from the excellent OllyDbg, my favorite Windows debugger. Feel free to comment/criticize my code!
Here is the code!

Example code calling the dumper:

#include "fpu_dump.h"

double x = 243;
FPUDUMP;
x = x / 3.14;
FPUDUMP;

An example of the verbose output:

================== MAGICAL FPU INSPECTOR ==================
From fpu_dump.c line 20:

Status: 0x0020
Invalid: 0, Denorm: 0, Zero Div: 0
Overflow: 0, Underflow: 0, Precision: 1
Stack fault: 0, Error summary: 0, TOP: 0
C3: 0, C2: 0, C1: 0, C0: 0

Control: 0x037F
Invalid Mask: 1, Denorm Mask: 1, Zero Div Mask: 1
Overflow Mask: 1, Underflow Mask: 1, Precision Mask: 1
Rounding: NEAR, Precision: 64, Infinity: 0

3 2 1 0      E S P U O Z D I
FST 0020  Cond 0 0 0 0  Err 0 0 1 0 0 0 0 0  (GT)
FCW 037F  Prec NEAR,64  Mask    1 1 1 1 1 1

ST0  EMPTY                        NAN   REG0
FFFF 00000000 00000000

ST1  EMPTY                          0   REG1
0000 00000000 00000000

ST2  EMPTY                          0   REG2
0000 00000000 00000000

ST3  EMPTY                          0   REG3
0000 00000000 00000000

ST4  EMPTY                          0   REG4
0000 00000000 00000000

ST5  EMPTY                          0   REG5
0000 00000000 00000000

ST6  EMPTY       16045690984229361664   REG6
403E DEADBEEF BAADD800

ST7  EMPTY       16045690984229355520   REG7
403E DEADBEEF BAADC0DF

========================= GOODBYE =========================