llvm dev - Feb 2015 - [LLVMdev] basicaa result

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!

-Haopeng

----- Original Message -----
> From: "Haopeng Liu" <hyliuhp at gmail.com>
> To: LLVMdev at cs.uiuc.edu
> Sent: Thursday, February 19, 2015 6:07:10 PM
> Subject: [LLVMdev] basicaa result
> 
> 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.
If you've run only -O0, then you're likely looking at the aliasing of
the local stack areas assigned to 'p' and 'q', and they are
distinct.

 -Hal
> 
> Could anyone explain it? Your help is much appreciated!
> 
> -Haopeng
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> 
-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory

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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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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