[LLVMdev] basicaa result

[Nick Lewycky]

On 19 February 2015 at 16:07, Haopeng Liu <hyliuhp at gmail.com> wrote:

> Hi,
>
> I'm trying to explore basicaa alias analysis.
>
> test.c:
> int main() {
>     int a;
>     int *p, *q;
>     p = &a;
>     q = p;
>     ...
> }
>
> Commands:
> clang -emit-llvm -c -g -O0 test.c -o test.bc
> opt -basicaa -print-must-aliases test.bc
>
> However, the result shows that p and q are no alias.
>
> Could anyone explain it? Your help is much appreciated!
>

LLVM's alias analysis works on LLVM IR, not on C. The conversion from C to
LLVM IR is not as straight-forward as you might at first imagine; for
instance, there is no address-of operation in LLVM. Write your examples in
LLVM IR, or use clang to produce those examples from C code, but look at
the IR it has produced and start thinking about AA results from there.

Here's the IR for test.bc:

define i32 @main() {
entry:
  %a = alloca i32, align 4
  %p = alloca i32*, align 8
  %q = alloca i32*, align 8
  store i32* %a, i32** %p, align 8
  %0 = load i32** %p, align 8
  store i32* %0, i32** %q, align 8
  ret i32 0
}

%p and %q do not alias, they are two separate pointers into the stack.

Nick
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