[LLVMdev] SSA or not SSA?

[Matthieu Moy]

Patrick Meredith <pmeredit at uiuc.edu> writes:

> Memory is what the i32* points too.  The i32* itself is in a  
> register.  You can store to it as many times as you want, but you  
> can't change the address, because that would violate SSA.

Thanks,

For the record, I finally understood by making a few experiments:

This is (obviously) valid:

  define i32 @main() {
         %some-variable = add i32 1, 1   ; integer with the value 1+1
         %some-other-variable = add i32 %some-variable, 1
         ret i32 %some-other-variable
  }

This is not (the assembler complains with ``Redefinition of value
'some-variable' of type 'i32' ''):

  define i32 @main() {
         %some-variable = add i32 1, 1   ; integer with the value 1+1
         %some-variable = add i32 %some-variable, 1
         ret i32 %some-variable
  }

but this one is:

  define i32 @main() {
         %some-pointer = alloca i32 ; declares an int in memory (stack)
         store i32 42, i32* %some-pointer
         store i32 54, i32* %some-pointer
         %retval = load i32* %some-pointer
         ret i32 %retval
  }

In short, "Single Assignment" means "Single '='", not "Single
'store'".

-- 
Matthieu