Hi,
Now I want use llvm to do pointer analysis, and I use -mem2reg to get the
SSA IR, but when I analysis source code with pointer, the IR will not in
SSA format. For example, the following C code:
void test() {
int a, b, c, d;
int *p, *q, *m, *n;
p = &a;
get2(&p);
q = &b;
get2(&q);
m = &c;
get2(&m);
n = &d;
get2(&n);
p = q;
m = p;
p = n;
get2(&n);
}
I use the following command to get IR result:
clang -S -emit-llvm -Xclang -disable-O0-optnone -c new.c -o new.ll
opt -S -mem2reg new.ll -o new.opt.ll
It's IR:
define dso_local void @test() #0 {
entry:
%a = alloca i32, align 4
%b = alloca i32, align 4
%c = alloca i32, align 4
%d = alloca i32, align 4
%p = alloca i32*, align 4
%q = alloca i32*, align 4
%m = alloca i32*, align 4
%n = alloca i32*, align 4
store i32* %a, i32** %p, align 4
call void @get2(i32** %p)
store i32* %b, i32** %q, align 4
call void @get2(i32** %q)
store i32* %c, i32** %m, align 4
call void @get2(i32** %m)
store i32* %d, i32** %n, align 4
call void @get2(i32** %n)
%0 = load i32*, i32** %q, align 4
store i32* %0, i32** %p, align 4
%1 = load i32*, i32** %p, align 4
store i32* %1, i32** %m, align 4
%2 = load i32*, i32** %n, align 4
store i32* %2, i32** %p, align 4
call void @get2(i32** %n)
ret void
}
I want to distinguish varitable p in p = q;p = n, like p0 = q;p1 = n. How
can I get the SSA IR that variables defined multiple times in the original
representation will be split into separate instances?
Also, when I use -mem2reg to get IR, they would do some optimization, such
as remove some unnecessary stack allocation. Can I get SSA IR without other
optimization?
Thank you,
panqaq
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Hi Panqaq, On Wed, 20 Mar 2019 at 10:02, Qiuhong Pan via llvm-dev <llvm-dev at lists.llvm.org> wrote:> I want to distinguish varitable p in p = q;p = n, like p0 = q;p1 = n. How can I get the SSA IR that variables defined multiple times in the original representation will be split into separate instances?LLVM doesn't try to model memory in any kind of SSA form, so I don't think it even has the representation for what you're asking unless it can see through the memory accesses to p entirely. But &p has escaped via the first call to get2 so LLVM has very little flexibility to do that (get2 could have saved &p somewhere and later calls could access it, and the data stored there). What IR would you expect to see for this example if LLVM was doing what you wanted?> Also, when I use -mem2reg to get IR, they would do some optimization, such as remove some unnecessary stack allocation. Can I get SSA IR without other optimization?As far as I know there's nothing more minimal than mem2reg for this purpose, but on the other hand removing stack allocations is exactly what mem2reg is designed for so I'm not really sure what you're after. Cheers. Tim.
Qiuhong Pan via llvm-dev <llvm-dev at lists.llvm.org> writes:> Now I want use llvm to do pointer analysis, and I use -mem2reg to get > the SSA IR, but when I analysis source code with pointer, the IR will > not in SSA format.LLVM IR is always in SSA form. All of the temporaries (the %<name> values) are assigned exactly once in the static representation.> It's IR: > define dso_local void @test() #0 { > entry: > %a = alloca i32, align 4 > %b = alloca i32, align 4 > %c = alloca i32, align 4 > %d = alloca i32, align 4 > %p = alloca i32*, align 4 > %q = alloca i32*, align 4 > %m = alloca i32*, align 4 > %n = alloca i32*, align 4 > store i32* %a, i32** %p, align 4 > call void @get2(i32** %p) > store i32* %b, i32** %q, align 4 > call void @get2(i32** %q) > store i32* %c, i32** %m, align 4 > call void @get2(i32** %m) > store i32* %d, i32** %n, align 4 > call void @get2(i32** %n) > %0 = load i32*, i32** %q, align 4 > store i32* %0, i32** %p, align 4 > %1 = load i32*, i32** %p, align 4 > store i32* %1, i32** %m, align 4 > %2 = load i32*, i32** %n, align 4 > store i32* %2, i32** %p, align 4 > call void @get2(i32** %n) > ret void > } > > I want to distinguish varitable p in p = q;p = n, like p0 = q;p1 > n. How can I get the SSA IR that variables defined multiple times in > the original representation will be split into separate instances? > > Also, when I use -mem2reg to get IR, they would do some optimization, > such as remove some unnecessary stack allocation. Can I get SSA IR > without other optimization?allocas in LLVM IR aren't really stack locations (though isel may translate them to such), they are just a way to generate a pointer value for something that lives in memory. mem2reg attempts to promote loads and stores of "well-known" locations like allocas to temporaries and introduces phi nodes to handle the data flow through control structures. By its nature it will remove the allocas because the temporaries no longer live in memory and so their addresses are meaningless. -David