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
From: "Haopeng Liu" <hyliuhp@gmail.com>
To: LLVMdev@cs.uiuc.edu
Sent: Thursday, February 19, 2015 6:07:10 PM
Subject: [LLVMdev] basicaa resultHi,
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.bcHowever, 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
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
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?
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?
No. LLVM knows nothing about C/C++ at all. Clang also does not provide such
an analysis, but if there were one in the llvm project it would go under
clang.
Nick