[LLVMdev] basicaa result

[Haopeng Liu]

It makes sense. Does llvm provide alias analysis on c/c++ which can 
figure out whether two different pointers in local stack point to same 
memory?


On 2/19/15 8:17 PM, Nick Lewycky wrote:
> On 19 February 2015 at 16:07, Haopeng Liu <hyliuhp at gmail.com 
> <mailto: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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