Notes on the LLVM Demo
======================

llvm.org/docs/LangRef.html

To try the C program:

> clang -o demo demo.c
> ./demo 10

Do not forget the command line argument!

Translate demo.c into LLVM IR:

> clang -c -emit-llvm -o demo.bc demo.c

Interpret the code:

> lli demo.bc 10

Translate to textual form:

> llvm-dis -o demo.ll demo.bc
> less demo.ll

Note:

  * Allocation of global variables, string constants.
  * Note three address code.
  * Note types.
  * Note SSA form, but no phi-functions as all always
    updating the *global* variables.
  * Note control flow graph ("preds = ...") explicit.

Optimize using O1:

> opt -O1 -o demo-O1.bc demo.bc
> llvm-dis -o demo-O1.ll demo-O1.bc
> less demo-O1.ll

Note:

  * Keeping variables in registers to a much larger extent.
  * The phi-functions and how explicitly identifies relationship
    between control flow and argument to choose.
  * How the final value of i is determined *if* the loop is
    executed at least once:
    %9 contains the result of m > i + 1
    %smax is picked to be m if true, i + 1 (= 1) otherwise
    This result is then written to i, rather than %14 which is
    the register containing i. 

------------------------------------------------------------------------------

From the manual:

The ~icmp~ instruction takes three operands. The first operand is the
condition code indicating the kind of comparison to perform. It is not a
value, just a keyword. The possible condition codes are:

    eq: equal
    ne: not equal
    ugt: unsigned greater than
    uge: unsigned greater or equal
    ult: unsigned less than
    ule: unsigned less or equal
    sgt: signed greater than
    sge: signed greater or equal
    slt: signed less than
    sle: signed less or equal

----------

nuw and nsw stand for ~No Unsigned Wrap~ and ~No Signed Wrap~, respectively. 
If the nuw and/or nsw keywords are present, the result value of the add is a
poison value if unsigned and/or signed overflow, respectively, occurs.

----------

Poison values are similar to undef values, however they also represent the
fact that an instruction or constant expression that cannot evoke side effects
has nevertheless detected a condition that results in undefined behavior.

There is currently no way of representing a poison value in the IR; they only
exist when produced by operations such as add with the nsw flag.

Poison values have the same behavior as undef values, with the additional
effect that any instruction that has a dependence on a poison value has
undefined behavior.