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Visitor gwendal.hays
Registered: ‎11-17-2017

Microblaze FPU conversion u32 to float error

Hi everyone,


I'm working with a Kintex7 evaluation board, in which a Microblaze is implemented.


After having trouble with conversion from u32 to float, I wrote a simple code that explains my problem :


u32 test0=2159;
float test1=(float)test0;
float test2=test0*1.0f;

Let's say my data is 2159 (u32 : 0x0000086F), but I want to things like sines and cosines, for which I want to work with floats.

I am supposed to have for test1 and test2 0x4506F000, which is the hex representation of 2159.0f.


When I run this code in debug, I get 3.025E-42, which is 0x0000086F read as a float. 


The strangest thing is that when I set the compiler to use -O1 optimization, the error doesn't exist anymore, and the values are correct for this code, but later in the program it comes back.


There are some of my microblaze's settings :




I had to change manually these settings in the compiler : (same for linker)




Ask for more informations if needed !


Thank you very much :)

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Visitor gwendal.hays
Registered: ‎11-17-2017

Re: Microblaze FPU conversion u32 to float error

After looking at the disassembly, it looks like the instruction "flt" (used for conversion form int to float by the FPU) does nothing.


Here is the disassembly : 


I can see that the value of the r6 register does not change when the flt instruction is run ...


Maybe it can help ?

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