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:
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#include "fpu_dump.h"
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double x = 243;
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FPUDUMP;
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x = x / 3.14;
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FPUDUMP;
An example of the verbose output:
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================== MAGICAL FPU INSPECTOR ==================
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From fpu_dump.c line 20:
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Status: 0×0020
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Invalid: 0, Denorm: 0, Zero Div: 0
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Overflow: 0, Underflow: 0, Precision: 1
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Stack fault: 0, Error summary: 0, TOP: 0
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C3: 0, C2: 0, C1: 0, C0: 0
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Control: 0×037F
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Invalid Mask: 1, Denorm Mask: 1, Zero Div Mask: 1
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Overflow Mask: 1, Underflow Mask: 1, Precision Mask: 1
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Rounding: NEAR, Precision: 64, Infinity: 0
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3 2 1 0 E S P U O Z D I
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FST 0020 Cond 0 0 0 0 Err 0 0 1 0 0 0 0 0 (GT)
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FCW 037F Prec NEAR,64 Mask 1 1 1 1 1 1
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ST0 EMPTY NAN REG0
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FFFF 00000000 00000000
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ST1 EMPTY 0 REG1
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0000 00000000 00000000
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ST2 EMPTY 0 REG2
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0000 00000000 00000000
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ST3 EMPTY 0 REG3
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0000 00000000 00000000
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ST4 EMPTY 0 REG4
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0000 00000000 00000000
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ST5 EMPTY 0 REG5
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0000 00000000 00000000
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ST6 EMPTY 16045690984229361664 REG6
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403E DEADBEEF BAADD800
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ST7 EMPTY 16045690984229355520 REG7
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403E DEADBEEF BAADC0DF
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========================= GOODBYE =========================
I am not a true nerd. I don’t understand even 90% of this post. I love it.
1 | Alex September 12th, 2007 at 8:35 pmNeed update! Have not had on in many months!
2 | Austin January 15th, 2008 at 9:50 pmOh yea. What happened to PPH?
3 | Austin January 15th, 2008 at 9:51 pm