In a previous post I described the current implementation of cargo-call-stack , a static stack usage analysis tool. In the second part of that post I described the problems the tool runs into when dealing with indirect function calls (both function pointer calls and dynamic dispatch) and proposed improving the output of the tool by having rustc inject type information in the LLVM IR it produces.

While discussing that idea with rustc developers they noted that the compiler could emit information that’s more relevant to call graph analysis than just the signatures of functions and trait methods. In this post I’ll describe the call graph metadata I’m currently implementing in rustc and how it will help tools like cargo-call-stack . By the way, I have a feature branch of cargo-call-stack that uses this upcoming rustc feature if you want to try it out but you’ll need to build a custom rustc .

The problem

First, let me rehash the problem cargo-call-stack has with indirect function calls. Consider the following program, which contains a function pointer call:

static X: AtomicBool = AtomicBool::new(false); fn main() { let mut x: fn() -> i32 = foo; // .. if X.load(Ordering::Acquire) { x = bar; } // function pointer call x(); // .. } fn foo() -> i32 { // .. } fn bar() -> i32 { // .. } fn baz() -> u32 { // .. }

The tool ( cargo-call-stack ) computes the call graph from the LLVM IR of a Rust program (reminder: we use LLVM IR because it’s pretty close in structure to the optimized machine code and it’s relatively stable, unlike the rustc API and MIR). The relevant parts of the LLVM IR of this program are shown below:

; app::main define internal void @_ZN3app4main17h2788607e78870dd4E() { ; .. ; function pointer call %3 = call i32 %2(), !dbg !140 ; .. } ; app::foo define internal i32 @_ZN3app3foo17h9efc21ae5e8e5b5aE() { ; .. } ; app::bar define internal i32 @_ZN3app3bar17h1ce8c48ecc8a3452E() { ; .. } ; app::baz define internal i32 @_ZN3app3baz17hbb47c4b31a6d6735E() { ; .. }

From the LLVM IR alone we know that main calls a function with signature i32 () through a pointer. i32 () is the LLVM IR version of the Rust type fn() -> i32 . The problem in this example is that in LLVM IR integers are always signed so the signature of function baz also becomes i32 () in the LLVM IR. This causes the tool to wrongly consider baz as a potential callee for the function pointer call in main .

Integers are not the only problem; LLVM will usually optimize away arguments and return values causing, for example, functions with signatures fn(i32) -> i32 or fn() -> i32 to become just () in LLVM IR, which is equivalent to Rust’s fn() type.

The original proposal

My original idea to deal with this type conversion problem was to add the Rust signature to all function definitions and function pointer calls using LLVM IR metadata. Thus, the previously shown LLVM IR would become:

; app::main define internal void @_ZN3app4main17h2788607e78870dd4E() !rust !0 { ; .. ^^^^^^^^ ; function pointer call %3 = call i32 %2(), !dbg !140, !rust !1 ; ^^^^^^^^ ; .. } ; app::foo define internal i32 @_ZN3app3foo17h9efc21ae5e8e5b5aE() !rust !1 { ; .. ^^^^^^^^ } ; app::bar define internal i32 @_ZN3app3bar17h1ce8c48ecc8a3452E() !rust !1 { ; .. ^^^^^^^^ } ; app::baz define internal i32 @_ZN3app3baz17hbb47c4b31a6d6735E() !rust !2 { ; .. ^^^^^^^^ } ; .. !0 = !"fn()" !1 = !"fn() -> u32" !2 = !"fn() -> i32" ; ..

With this extra information the tool would be able to narrow down the list of potential callees of the function pointer call in main to just foo and bar since all of them have the same metadata: !rust !1 , which is an alias for !"fn() -> u32" .

This works well enough in this particular example but we can do even better!

Function pointers

One of the rustc passes that runs before codegen (the MIR -> LLVM-IR pass) analyzes the MIR of the crate to decide which functions to codegen and it can easily detect which functions are casted / coerced into function pointers.

In the previous example, we coerced the anonymous function type fn() -> u32 {foo} , which is zero sized, into a function pointer fn() -> u32 , which is pointer sized (8 bytes on x86_64); then we did the same with the function bar . If we tweak the metadata generation logic to only add the !rust metadata to functions that are converted into function pointers the LLVM IR of that example would become:

; app::main define internal void @_ZN3app4main17h2788607e78870dd4E() { ; .. ^^^^^^^^ ; function pointer call %3 = call i32 %2(), !dbg !140, !rust !0 ; ^^ ; .. } ; app::foo define internal i32 @_ZN3app3foo17h9efc21ae5e8e5b5aE() !rust !0 { ; .. ^^ } ; app::bar define internal i32 @_ZN3app3bar17h1ce8c48ecc8a3452E() !rust !0 { ; .. ^^ } ; app::baz define internal i32 @_ZN3app3baz17hbb47c4b31a6d6735E() { ; .. ^^^^^^^^ } ; .. ; yes, the format of the metadata changed a bit here !0 = !{!"fn", !"fn() -> u32"} ; ..

With this change the tool would still be able to locate the candidates for the function pointer call in main . What would be the advantage of doing it this way? We can reduce false positives with this approach. Consider this program:

static X: AtomicBool = AtomicBool::new(false); fn main() { let mut x: fn() -> u32 = foo; if X.load(Ordering::Acquire) { x = bar; } x(); baz(); } fn foo() -> u32 { // .. } fn bar() -> u32 { // .. } fn baz() -> u32 { // .. }

This time foo , bar and baz all have the same Rust signature; however, the function pointer call can only invoke foo or bar . If we apply the !rust metadata only to functions that are coerced into function pointers we would end with the following LLVM IR:

; app::main define internal void @_() { ; .. ; `x()` %2 = tail call i32 %spec.select() #8, !dbg !138, !rust !93 ; `baz()` tail call fastcc void @_ZN3app3baz17h9f37979edaee3ecdE(), !dbg !140 ; .. } ; app::foo define internal i32 @() !rust !93 { ; .. } ; app::bar define internal i32 @_() !rust !93 { ; .. } ; app::baz define internal i32 @_() { ; .. } ; .. !93 = !{!"fn", !"fn() -> u32"} ; ..

Note that baz does not have “fn” metadata. The tool would then be able to produce an accurate call graph:

The dashed node fn() -> u32 in the graph represents the function pointer call ( x() ), which can result in either foo or bar being invoked.

If we had used the original approach of adding metadata to all functions the above call graph would have ended with a false positive edge between fn() -> u32 and baz .

A requisite for safely invoking a function through a pointer is first creating a function pointer from said function. In the improved approach we are tracking this operation with the “fn” metadata: no “fn” metadata means that the function is never converted into a function pointer so we can omit it from the list of callee candidates of function pointer calls. This results in less edges (false positives) compared to my original proposal.

In theory, it is possible to invoke a function that has never be converted into a function pointer through a pointer doing something like (mem::transmute::<usize, fn()>(0x2000_0000))() but this will result in Undefined Behavior (UB) in most cases as the addresses of functions are only known after linking. If the intention is to invoke a function at a known, fixed memory address (like an interrupt handler) you are much better off using something like extern "C" { fn interrupt(); } interrupt() which does not result in UB but also it’s not a function pointer call – it’s a direct function call.

The improved approach is not perfect but it’s certainly an improvement over my original proposal. Even with the improved approach, one can fabricate false positives like this:

static X: AtomicBool = AtomicBool::new(false); fn main() { let mut x: fn() -> u32 = foo; if X.load(Ordering::Acquire) { x = bar; } x(); baz(); // never invoked or used let y: fn() -> u32 = baz; } fn foo() -> u32 { .. } fn bar() -> u32 { .. } fn baz() -> u32 { .. }

This produces the following incorrect call graph

As MIR optimizations get better the let y statement will get optimized away before the pre-codegen analysis pass where the compiler decides which functions get “fn” metadata, meaning that baz won’t have “fn” metadata in the output LLVM IR.

Of course, even with perfect MIR optimizations one can still force a false positive by doing something like this:

fn main() { let mut x: fn() -> u32 = foo; if X.load(Ordering::Acquire) { x = bar; } x(); baz(); // never invoked let y: fn() -> u32 = baz; // this prevents `y` from being optimized away both at the MIR and LLVM-IR level unsafe { core::ptr::read_volatile(&y) } } fn foo() -> u32 { .. } fn bar() -> u32 { .. } fn baz() -> u32 { .. }

This will produce a false edge between the fn() -> u32 node and baz in the call graph.

Trait objects

We have covered function pointers; trait objects and dynamic dispatch needs a slightly different treatment. In my original proposal all trait methods and dynamic dispatch sites get annotated with trait - method metadata. This means, for example, that the following program:

static X: AtomicBool = AtomicBool::new(false); fn main() { let mut to: &dyn Foo = &Bar; if X.load(Ordering::Acquire) { to = &Baz; } to.foo(); Quux.foo(); } trait Foo { // default implementation of this method fn foo(&self) -> bool; } struct Bar; impl Foo for Bar { fn foo(&self) -> bool { .. } } struct Baz; impl Foo for Baz { fn foo(&self) -> bool { .. } } struct Quux; impl Foo for Quux { fn foo(&self) -> bool { .. } }

Would result in the following LLVM IR

; main define void @main() { ; .. ; dynamic dispatch %4 = tail call zeroext i1 %3({}* %2) #8, !dbg !185, !rust !0 ; ^^^^^^^^ ; call <app::Quux as app::Foo>::foo tail call fastcc void @_(), !dbg !186 ; .. } ; <app::Bar as app::Foo>::foo define internal void @_(%Bar*) unnamed_addr #1 !dbg !112 !rust !0 { ; .. ^^^^^^^^ } ; <app::Baz as app::Foo>::foo define internal void @_(%Baz*) unnamed_addr #1 !dbg !125 !rust !0 { ; .. ^^^^^^^^ } ; <app::Quux as app::Foo>::foo define internal fastcc void @_() unnamed_addr #2 !dbg !136 !rust !0 { ; .. ^^^^^^^^ } !0 = "(dyn Foo).foo"

From which the tool would produce the following call graph:

Which is not quite correct: the edge between the dynamic dispatch node, "(dyn Foo).foo" , and the Quux.foo node shouldn’t exist because Quux is never used as a trait object. rustc can help with that!

VTables

Turns out the compiler also knows which types will be used as trait objects because it has to codegen a vtable for each type - trait pair so we can do better here by only emitting trait metadata for the methods of types that are coerced / casted into trait objects.

Applying that to the previous example results in the following changes in the LLVM IR:

; main define void @main() { ; .. ; dynamic dispatch %4 = tail call zeroext i1 %3({}* %2) #8, !dbg !185, !rust !0 ; ^^^^^^^^ ; call <app::Quux as app::Foo>::foo tail call fastcc void @_(), !dbg !186 ; .. } ; <app::Bar as app::Foo>::foo define internal void @_(%Bar*) unnamed_addr #1 !dbg !112 !rust !0 { ; .. ^^^^^^^^ } ; <app::Baz as app::Foo>::foo define internal void @_(%Baz*) unnamed_addr #1 !dbg !125 !rust !0 { ; .. ^^^^^^^^ } ; <app::Quux as app::Foo>::foo define internal fastcc void @_() unnamed_addr #2 !dbg !136 { ; .. ^^^^^^^^ } ; yes, the format of the metadata changed a bit here !0 = !{!"dyn", !"Foo", !"foo"}

Which would let the tool produce an accurate call graph:

Something that I realized midway implementing this in the compiler is that you can not attach multiple metadata nodes of the same kind (e.g. !rust ) to a define item or an instruction. This is a problem because a function ( define item) can be both dynamically dispatched and invoked via a function pointer, i.e. it needs both “dyn” metadata and “fn” metadata. Consider the following program:

static X: AtomicBool = AtomicBool::new(false); fn main() { // "dyn" metadata let x: &dyn Foo = &Bar; x.foo(); // "fn" metadata let y: fn(&Bar) = Bar::foo; } trait Foo { fn foo(&self); } struct Bar; impl Foo for Bar { .. }

The solution we picked was to attach a metadata tuple that contains both pieces of metadata to the define item. Namely:

; app::main define internal void @_ZN3app4main17h63fa795f778d6e4eE() unnamed_addr #0 !dbg !141 { ; .. call void %8({}* align 1 %3), !dbg !163, !rust !164 ; .. } ; <app::Bar as app::Foo>::foo define internal void @_(%Bar*) unnamed_addr #0 !dbg !167 !rust !169 { ; ^^^^^^^^^^ single node } !164 = !{!"dyn", !"Foo", !"foo"} ; tuple with `fn` and `dyn` metadata !169 = !{!164, !170} !170 = !{!"fn", !"fn(&Bar) -> ()"}

Drop glue

Something which I had not considered at all in my previous post is the drop glue of trait objects.

Trait objects are a form of (open set) polymorphism that consists of erasing the concrete type of an object leaving you with only the trait interface; this lets you pack objects of different types, but that implement a common trait, in a collection like Vec or store them in the same memory slot as shown in the example below.

static X: AtomicBool = AtomicBool::new(false); fn main() { let mut x: Box<dyn Foo> = Box::new(Bar); if X.load(Ordering::Acquire) { // leak the initial value unsafe { ptr::write(&mut x, Box::new(Baz)) } } drop(x); } trait Foo { fn foo(&self); } struct Bar; impl Foo for Bar { .. } impl Drop for Bar { .. } struct Baz; impl Foo for Baz { .. } impl Drop for Baz { .. }

Eventually a trait object has to be destroyed; what happens at that point is that the destructor of the original type has be invoked. In the example above, drop(x) could invoke either Bar ’s or Baz ’s destructor ( Drop implementation).

It should be noted that the code that’s executed on drop(x) (or when x goes out of scope) is not x ’s Drop implementation but x s drop glue and this drop glue invokes x ’s Drop implementation, if it has one. “Drop glue” is code (a function) synthesized by the compiler for all types, even for those that don’t implement the Drop trait. The reason for that is that even if the type itself doesn’t implement the Drop trait its fields might so the drop glue of the type has to run the drop glue of each field.

Going back to trait objects, trait objects store a function pointer to the drop glue of the erased type in their vtable. The drop glue of a trait object simply calls the drop glue stored in its vtable. This is basically a function pointer call but we can’t use “fn” metadata in this case. To see why consider our example: the drop glue of dyn Foo could invoke Bar ’s drop glue or Baz ’s drop glue; these two functions have different signatures ( fn(*mut Bar) vs fn(*mut Baz) ) and thus different “fn” metadata.

Thus we need a third kind of metadata to handle this case: enter “drop” metadata. The idea is to attach “drop” metadata to the drop glue of all types that are coerced / casted into trait objects. Likewise, we’ll attach “drop” metadata to the invocation of a trait object drop glue.

The LLVM IR of our example would then look like this:

; Function Attrs: noinline noreturn nounwind define void @main() unnamed_addr #2 !dbg !161 { ; .. ; `drop(x)` tail call void %2({}* nonnull align 1 inttoptr (i32 1 to {}*)) #8, !rust !131 ; ^^^^^^^^^^ ; .. } ; `Baz`'s drop glue ; core::ptr::real_drop_in_place::h7d740ebe6d682eb9 define internal void @(%Baz*) unnamed_addr #0 !dbg !120 !rust !131 { ; .. ^^^^^^^^^^ ; call <app::Baz as core::ops::drop::Drop>::drop tail call fastcc void @_(), !dbg !133 ; .. } ; `Bar`'s drop glue ; core::ptr::real_drop_in_place::hfa2e157a7c8b54b4 define internal void @_(%Bar*) unnamed_addr #0 !dbg !134 !rust !131 { ; .. ^^^^^^^^^^ ; call <app::Bar as core::ops::drop::Drop>::drop tail call fastcc void @_(), !dbg !143 ; .. } !131 = !{!"drop", !"Foo"}

With this metadata the tool would be able to produce an accurate call graph:

It’s worth nothing that since a type may implement different traits its drop glue may be called by the drop glue of different trait objects. This is reflected in the LLVM IR by attaching multiple “drop” metadata nodes to the same drop glue using the tuple syntax we saw before. An example:

fn main() { let x: Box<dyn Baz> = Box::new(Foo); let y: Box<dyn Quux> = Box::new(Foo); } struct Foo; trait Bar { fn bar(&self); } impl Bar for Foo { .. } trait Baz: Bar { fn baz(&self); } impl Baz for Foo { .. } trait Quux { fn quux(&self); } impl Quux for Foo { .. }

; core::ptr::real_drop_in_place define internal void _(%Foo* nonnull align 1) unnamed_addr #0 !dbg !99 !rust !107 { ; .. } !107 = !{!108, !109} !108 = !{!"drop", !"Quux"} !109 = !{!"drop", !"Baz"}

Note that Foo ’s drop glue doesn’t have the Bar trait within its “drop” metadata because it’s never coerced into a Bar trait object. However, the Foo.bar method does have “dyn” metadata because the bar method can be invoked through a dyn Baz trait object due to the supertrait relationship between Baz and Bar .

; <app::Foo as app::Bar>::bar define internal void @_(%Foo*) unnamed_addr #0 !dbg !169 !rust !166 { ; .. } !166 = !{!"dyn", !"Bar", !"bar"}

Summary

In summary, I’m adding three kinds of LLVM IR metadata (behind an unstable -Z flag) to improve tools that perform call graph analysis of Rust programs:

“fn” metadata to map function pointer calls to the functions that could be invoked at runtime. The potential callees are chosen (that is tagged with metadata) based on the function signature and only functions that are coerced / casted into function pointers are included in the list.

“dyn” metadata to map the dynamic dispatch of methods to the functions that could be invoked at runtime. The potential callees are chosen based on the trait and method being dispatched and only types that casted / coerced into a trait object are included in the list.

“drop” metadata to map trait objects’ drop glue to the functions that could be invoked at runtime. Here the candidates are limited to the types that are casted / coerced into trait objects.

Tools like cargo-call-stack will be able to use this extra information to build more accurate call graphs. The PR for this experimental feature has recently entered the review phase; before landing it, we’ll have to confirm that the changes don’t (significantly) impact the compilation times of people that are not using this feature.

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