[LLVMdev] Restoring SSA form

[Vladimir Prus]

Hello,
for some my use case, I'd like to temporary break SSA property and then ask 
LLVM to restore it. Here's more details:

  if (i < 0) {
 i = -i;
  }

This is a C code example. I'm trying to create a value range analysis, which 
will determine, that after this code, 'i' is non-negative. In SSA form, this 
will become

   i = 0;
   if (i < 0) {
        i.2 = -i;
   }
   i.3 = phi(i, i.2);

I'll assign value ranges to i, i.2, i.3, do some iteration and get necessary 
results. Now consider another case:

   i = 0;
   if (i > 0) {
 foo(i);
   }

Here's 'foo' is always called with positive value. However, the value range 
assigned to 'i' variable can be only [-inf, +inf], because uses of 'i' 
outside of condition can get any value. So, I'd like to convert the above to:

   i = 0;
   if (i > 0) {
     i = change_value_range(i);
     foo(i);
   }

where 'change_value_range' is some artificial instruction. After that, I'd ask 
LLVM to restore SSA property, giving:

   i = 0;
   if (i > 0) {
     i.2 = change_value_range(i);
     foo(i.2);
   }
   i.3 = phi(i, i.2)

And then will do my analysis as usual. This is a long-winded background, and 
the question is: how to I "restore SSA property"? Sure, both dominance 
frontier and dominator tree are available, so I can just try to impelement 
the SSA constructon algorithm by hand, but is there a smarter way? The 
Mem2Reg pass is too specialised to work here.

Thanks in advance,
Volodya