How can I get VRP information for an RTX?

[David Daney]

I am looking at how the MIPS backend handles division.  For the compiler 
configuration in question (mipsel-linux) division operations trap on 
division by zero.  This is handled in mips_output_division in mips.c 
where we unconditionally emit a conditional trap.

I would like to change it so that if the divisor is not zero, the 
conditional trap would be omitted.  I am looking at this small test program:

int divtest(int x, int y)
{
   if (y == 0)
       return 45;
   else
       return x / y;
}

If I set a breakpoint in mips_output_division, I can print out the 
operands for the division operation:
(gdb) p operands[2]
$6 = (rtx) 0x2aaaae155aa0
(gdb) pr
(reg/v:SI 5 $5 [orig:196 y ] [196])

Q1: Is it possible to get to the VRP information from this rtx?  How?

If the VRP information is available, it would be nice to |define_expand 
for the division/conditional trap and perhaps let the compiler schedule 
the trap.  However the define_expand documentation states that the 
condition for the define_expand cannot depend on the data in the insn 
being matched.  That would seem to imply that the expansion cannot be 
dependent on the VRP information.

Q2:  Is that correct, and why?

Q3: Would it be better to do this at the tree level instead of rtl?


Thanks in advance,
David Daney
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