[LLVMdev] fptoui vs fptosi

[Andreas Grünwald]

Hi,
 
I hope this is the right mailing-list to post this question to. 
 
It is about the LLVM instructions fptoui and fptosi. I'm not entirely clear on the difference between these two instructions.
 
I wrote a little test-program and compiled it
#include <stdio.h>

int main(int argc, char** argv)
{
    double d = -3.5 - 4;
    unsigned int ui = (unsigned int) d;
    int si = (int) d;

    printf("unsigned %u, 0x%x\n", ui, ui);
    printf("signed   %i, 0x%x\n", si, si);      
    return 0;    
} 
as I expected,  this resulted in the generation of both kinds of instructions
...
%5 = fptoui double %4 to i32
store i32 %5, i32* %ui, align 4
%6 = load double* %d, align 8
%7 = fptosi double %6 to i32
store i32 %7, i32* %si, align 4
... 
when the program is executed, the output is 

unsigned 4294967289, 0xfffffff9

signed -7, 0xfffffff9
 
so both instructions produce the same result value. 
 
The LLVM IR documentation states:  "The ‘fptoui‘ instruction converts its floating point operand into the
  nearest (rounding towards zero) unsigned integer value."
 
Wouldn't the nearest unsigned integer for a negative value be 0? Or is this undefined behavior?
 
It would be helpful if someone could clarify for me, in which cases the two instructions produce different results.
 
 
Regards,
Andreas Gruenwald
 
 		 	   		  
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