# `cargo-call-stack` > Static, whole program stack usage analyzer

Call graph with a cycle

Other examples: [Embedded CoAP / IPv4 server](https://japaric.github.io/cargo-call-stack/ipv4.svg) ([source](https://github.com/japaric/jnet/tree/b5bd70cb998b1e01236f6b07ebc516a0359fde3d/firmware#ipv4)) ["Hello, world!"](https://japaric.github.io/cargo-call-stack/hello.svg) **HEADS UP**: This tool relies on an experimental feature (`-Z stack-sizes`) and implementation details of `rustc` (like symbol mangling) and could stop working with a nightly toolchain at any time. You have been warned! Last tested nightly: 2023-11-13. **NOTE**: This tool main use case are embedded (microcontroller) programs that lack, or have very little, indirect function calls and recursion. This tool is of very limited use -- specially its stack usage analysis -- on programs that link to the standard library as even the smallest program that links to the standard library will contain a large number of indirect function calls (like trait object dynamic dispatch specially in `core::fmt`) and potential recursion (specially in panicking branches). ## Features - The tool produces the full call graph of a program as a [dot file]. [dot file]: https://www.graphviz.org/doc/info/lang.html - A [start point](#start-point) can be specified to analyze only the call graph that begins at that function. - Each node (function) in the call graph includes the local stack usage of the function, *if* available (see [`-Z emit-stack-sizes`]). [`-Z emit-stack-sizes`]: https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/emit-stack-sizes.html - The *maximum stack usage* of each function is also computed, or at least a lower bound is provided. Maximum stack usage of a function here refers to the stack usage that includes the stack used by functions that the function may invoke. - The tool has *imperfect* support for calls through function pointers (`fn()`) and dynamic dispatch (`dyn Trait`). You *will* get a call graph from programs that do indirect calls but it will likely be missing edges or contain incorrect edges. It's best to use this tool on programs that only do direct function calls. ## Known limitations - Dynamically linked binaries are not supported. As a portion of the call graph is injected at runtime by the dynamic linker is not possible to produce a complete call graph or compute an upper bound of the program's stack usage. ## Installation ``` console $ # NOTE always use the latest stable release $ cargo +stable install cargo-call-stack $ rustup +nightly component add rust-src ``` ## Example usage > **NOTE** this tool requires that your Cargo project is configured to use *fat LTO* when Cargo uses > the release profile. This is not the default configuration (as of Rust 1.63) so you'll need to add > this to your `Cargo.toml`: ``` toml [profile.release] # `lto = true` should also work lto = 'fat' ``` The tool builds your program in release mode with LTO enabled, analyses it and then prints a dot file to stdout. See `cargo call-stack -h` for a list of build options (e.g. `--features`). [`cortex-m-rt`]: https://crates.io/crates/cortex-m-rt > **NOTE** if you have *not* set a compilation target in e.g. `.cargo/config.toml` then you'll need > to pass the `--target` flag to `cargo-call-stack` even if you are *not* cross compiling > **NOTE** the analysis corresponds to the newly produced binary, > which won't be the same as the binary produced by `cargo +nightly build --release` ``` console $ cargo +nightly call-stack --example app > cg.dot warning: assuming that llvm_asm!("") does *not* use the stack warning: assuming that llvm_asm!("") does *not* use the stack ``` Graphviz's `dot` can then be used to generate an image from this dot file. ``` console $ dot -Tsvg cg.dot > cg.svg ``` [![Call graph with direct function calls](assets/direct.png)](https://japaric.github.io/cargo-call-stack/direct.svg) Each node in this graph represents a function, which could be a free function, an inherent method or a trait method. Each directed edge indicates a "calls" relationship. For example, in the above graph `Reset` calls both `main` and `DefaultPreInit`. Each node also contains its `local` stack usage in bytes and its `max`-imum stack usage, also in bytes. The maximum stack usage includes the stack usage of all the other functions that the function could invoke. This is the `no_std` program used to generate the call graph shown above. ``` rust #![feature(llvm_asm)] #![no_main] #![no_std] extern crate panic_halt; use core::ptr; use cortex_m_rt::{entry, exception}; #[entry] fn main() -> ! { foo(); bar(); loop {} } #[inline(never)] fn foo() { // spill variables onto the stack unsafe { llvm_asm!("" : : "r"(0) "r"(1) "r"(2) "r"(3) "r"(4) "r"(5)) } } #[inline(never)] fn bar() { unsafe { llvm_asm!("" : : "r"(0) "r"(1) "r"(2) "r"(3) "r"(4) "r"(5) "r"(6) "r"(7)) } } #[exception] fn SysTick() { bar(); } #[inline(never)] fn baz() { let x = 0; unsafe { // force `x` to be on the stack ptr::read_volatile(&&x); } } ``` > In the previous example the call graph contained disconnected subgraphs. The > reason for that is *exceptions* (also known as interrupts). `SysTick`, for > example, is an exception handler that can preempt any function called from > `Reset`. This exception handler is never called from software but can be > invoked by the hardware at any time. These exception handlers can appear as > the roots of disconnected subgraphs. ## Start point In some cases you may be interested in the maximum stack usage of a particular function. The tool lets you specify a *start point* which will be used to filter the call graph to only include nodes reachable from that function. If we invoke the tool on the previous program but select `main` as the start point we get this call graph: ``` console $ cargo +nightly call-stack --example app main > cg.dot warning: assuming that llvm_asm!("") does *not* use the stack warning: assuming that llvm_asm!("") does *not* use the stack ```

Filtered call graph

Notice that `SysTick` and `baz` don't appear in this call graph since they are not reachable from `main`. ## Cycles The tool can, in some cases, compute the maximum stack usage of programs that involve recursion. Recursion appears as cycles in the call graph. Consider the following example: ``` rust #![feature(llvm_asm)] #![no_main] #![no_std] extern crate panic_halt; use core::sync::atomic::{AtomicBool, Ordering}; use cortex_m_rt::{entry, exception}; static X: AtomicBool = AtomicBool::new(true); #[inline(never)] #[entry] fn main() -> ! { foo(); quux(); loop {} } // these three functions form a cycle that breaks when `SysTick` runs #[inline(never)] fn foo() { if X.load(Ordering::Relaxed) { bar() } } #[inline(never)] fn bar() { if X.load(Ordering::Relaxed) { baz() } } #[inline(never)] fn baz() { if X.load(Ordering::Relaxed) { foo() } } #[inline(never)] fn quux() { // spill variables onto the stack unsafe { llvm_asm!("" : : "r"(0) "r"(1) "r"(2) "r"(3) "r"(4) "r"(5)) } } #[exception] fn SysTick() { X.store(false, Ordering::Relaxed); } ``` It produces the following call graph:

Call graph with a cycle

The functions `foo`, `bar` and `baz` use zero stack space thus the cycle formed by them also uses zero stack space. In this particular case the maximum stack usage of `main` can be computed. For the curious this is the disassembly of the "cyclic" program: ``` armasm 08000400 : 8000400: f240 0000 movw r0, #0 8000404: f2c2 0000 movt r0, #8192 ; 0x2000 8000408: 7800 ldrb r0, [r0, #0] 800040a: 0600 lsls r0, r0, #24 800040c: bf18 it ne 800040e: f000 b801 bne.w 8000414 8000412: 4770 bx lr 08000414 : 8000414: f240 0000 movw r0, #0 8000418: f2c2 0000 movt r0, #8192 ; 0x2000 800041c: 7800 ldrb r0, [r0, #0] 800041e: 0600 lsls r0, r0, #24 8000420: bf18 it ne 8000422: f000 b801 bne.w 8000428 8000426: 4770 bx lr 08000428 : 8000428: f240 0000 movw r0, #0 800042c: f2c2 0000 movt r0, #8192 ; 0x2000 8000430: 7800 ldrb r0, [r0, #0] 8000432: 0600 lsls r0, r0, #24 8000434: bf18 it ne 8000436: f7ff bfe3 bne.w 8000400 800043a: 4770 bx lr 0800043c : 800043c: b580 push {r7, lr} 800043e: f04f 0c00 mov.w ip, #0 8000442: f04f 0e01 mov.w lr, #1 8000446: 2202 movs r2, #2 8000448: 2303 movs r3, #3 800044a: 2004 movs r0, #4 800044c: 2105 movs r1, #5 800044e: bd80 pop {r7, pc} 08000450
: 8000450: f7ff ffd6 bl 8000400 8000454: f7ff fff2 bl 800043c 8000458: e7fe b.n 8000458 ``` And yes, the estimated maximum stack usage is correct as shown in this debug session: ``` console (gdb) b app::foo (gdb) b app::bar (gdb) b app::baz (gdb) c Continuing. Breakpoint 3, main () at src/main.rs:16 16 foo(); (gdb) p $sp $1 = (void *) 0x20005000 (gdb) c Continuing. halted: PC: 0x08000400 Breakpoint 4, app::foo () at src/main.rs:31 31 if X.load(Ordering::Relaxed) { (gdb) p $sp $2 = (void *) 0x20005000 (gdb) c Continuing. halted: PC: 0x0800040c Breakpoint 5, app::bar () at src/main.rs:38 38 if X.load(Ordering::Relaxed) { (gdb) p $sp $3 = (void *) 0x20005000 (gdb) c Continuing. halted: PC: 0x08000420 Breakpoint 6, app::baz () at src/main.rs:45 45 if X.load(Ordering::Relaxed) { (gdb) p $sp $4 = (void *) 0x20005000 (gdb) c Continuing. halted: PC: 0x08000434 Breakpoint 4, app::foo () at src/main.rs:31 31 if X.load(Ordering::Relaxed) { (gdb) p $sp $5 = (void *) 0x20005000 ``` ## Trait object dispatch > NOTE as of ~nightly-2022-09-20 there's no distinction between function pointers and trait objects > at the llvm-ir so the analysis can insert an edge from a dynamic dispatch call site (caller) to a > regular function (callee) In *some* cases the tool can produce correct call graphs for programs that use trait objects -- more details about where and how it fails in the ["Known limitations"](#known-limitations) section. Here's an example: ``` rust #![feature(llvm_asm)] #![no_main] #![no_std] extern crate panic_halt; use cortex_m_rt::{entry, exception}; use spin::Mutex; // spin = "0.5.0" static TO: Mutex<&'static (dyn Foo + Sync)> = Mutex::new(&Bar); #[entry] #[inline(never)] fn main() -> ! { // trait object dispatch (*TO.lock()).foo(); Quux.foo(); loop {} } trait Foo { // default implementation of this method fn foo(&self) -> bool { // spill variables onto the stack unsafe { llvm_asm!("" : : "r"(0) "r"(1) "r"(2) "r"(3) "r"(4) "r"(5)) } false } } struct Bar; // uses the default method implementation impl Foo for Bar {} struct Baz; impl Foo for Baz { // overrides the default method fn foo(&self) -> bool { unsafe { llvm_asm!("" : : "r"(0) "r"(1) "r"(2) "r"(3) "r"(4) "r"(5) "r"(6) "r"(7)) } true } } struct Quux; impl Quux { // not a trait method! #[inline(never)] fn foo(&self) -> bool { // NOTE(llvm_asm!) side effect to preserve function calls to this method unsafe { llvm_asm!("NOP" : : : : "volatile") } false } } // this handler can change the trait object at any time #[exception] fn SysTick() { *TO.lock() = &Baz; } ``` The tool produces the following call graph:

Dynamic dispatch

Here `i1 ({}*)` denotes dynamic dispatch of a method with (Rust) signature `fn(&[mut] self) -> bool`. The dynamic dispatch can invoke either `Bar.foo`, which boils down to the default method implementation (`app::Foo::foo` in the graph), or `Baz.foo` (`::foo` in the graph). In this case the tool does *not* a draw an edge between `i1 ({}*)` and `Quux::foo`, whose signature is also `fn(&self) -> bool`, so the call graph is accurate. If you are wondering why we use LLVM notation for the function signature of the trait method: that's because the tool operates on LLVM-IR where there's no `bool` primitive and most of Rust's type information has been erased. ## Function pointers > NOTE as of ~nightly-2022-09-20 llvm discards type information associated to pointers and uses > opaque pointers in llvm-ir so functions whose signature include references (and raw pointers) will > include more callee edges that will never occur in practice. In *some* cases the tool can produce correct call graphs for programs that invoke functions via pointers (e.g. `fn()`). Here's an example: ``` rust #![feature(llvm_asm)] #![no_main] #![no_std] extern crate panic_halt; use core::sync::atomic::{AtomicPtr, Ordering}; use cortex_m_rt::{entry, exception}; static F: AtomicPtr bool> = AtomicPtr::new(foo as *mut _); #[inline(never)] #[entry] fn main() -> ! { if let Some(f) = unsafe { F.load(Ordering::Relaxed).as_ref() } { // call via function pointer f(); } loop {} } fn foo() -> bool { // spill variables onto the stack unsafe { llvm_asm!("" : : "r"(0) "r"(1) "r"(2) "r"(3) "r"(4) "r"(5)) } false } fn bar() -> bool { unsafe { llvm_asm!("" : : "r"(0) "r"(1) "r"(2) "r"(3) "r"(4) "r"(5) "r"(6) "r"(7)) } true } // this handler can change the function pointer at any time #[exception] fn SysTick() { F.store(bar as *mut _, Ordering::Relaxed); } ``` The tool produces the following call graph:

Function pointers

The node `i1 ()*` represents a call via function pointer -- the LLVM type `i1 ()*` is equivalent to Rust's `fn() -> bool`. This indirect call could invoke `foo` or `bar`, the only functions with signature `fn() -> bool`. ## Known limitations ### Lossy type information To reason about indirect function calls the tool uses the type information available in the LLVM-IR of the program. This information does not exactly match Rust's type information and leads to mislabeling of functions. For example, consider this program: ``` rust #![feature(llvm_asm)] #![no_main] #![no_std] extern crate panic_halt; use core::{ ptr, sync::atomic::{AtomicPtr, Ordering}, }; use cortex_m_rt::{entry, exception}; static F: AtomicPtr u32> = AtomicPtr::new(foo as *mut _); #[inline(never)] #[entry] fn main() -> ! { if let Some(f) = unsafe { F.load(Ordering::Relaxed).as_ref() } { // call via function pointer f(); } let x = baz(); unsafe { // NOTE(volatile) used to preserve the return value of `baz` ptr::read_volatile(&x); } loop {} } // this handler can change the function pointer at any time #[exception] fn SysTick() { F.store(bar as *mut _, Ordering::Relaxed); } fn foo() -> u32 { // spill variables onto the stack unsafe { llvm_asm!("" : : "r"(0) "r"(1) "r"(2) "r"(3) "r"(4) "r"(5)) } 0 } fn bar() -> u32 { 1 } #[inline(never)] fn baz() -> i32 { unsafe { llvm_asm!("" : : "r"(0) "r"(1) "r"(2) "r"(3) "r"(4) "r"(5) "r"(6) "r"(7)) } F.load(Ordering::Relaxed) as usize as i32 } ``` The tool produces the following call graph:

Lossy types

Note that the node that represents the indirect function call has type `i32 ()*` (`fn() -> i32`), not `u32 ()*`. The reason is that there's no `u32` type in LLVM, there are only signed integers. This leads the tool to wrongly add an edge between `i32 ()*` and `baz`. If the tool had Rust's type information then this edge would have not been added. ### Opaque pointers in llvm-ir As of ~nightly-2022-09-20 llvm is discarding type information associated to pointers and instead using opaque pointers (`ptr`) in llvm-ir. This results in even greater loss of type information, for example: - the signature of `fn f(x: &i32) -> bool` becomes `fn(ptr) -> i1` in llvm-ir - the signature of `impl Foo { fn f(&self) -> bool }` also becomes `fn(ptr) -> i1` in llvm-ir so the two are callee candidates for both a function pointer call with signature `fn(&T) -> bool` and for dynamic dispatch of a method with signature `fn(&self) -> bool` ### Miscellaneous Inline assembly breaks LLVM's stack usage analysis. LLVM does *not* consider inline assembly in its analysis and reports an incorrect number. In this case, `cargo-call-stack` will use its own stack usage analysis based on machine code, which only supports the ARM Cortex-M architecture. Hardware exceptions, like `SysTick` on Cortex-M devices, appear as disconnected nodes in the call graph. At the moment, `cargo-call-stack` cannot compute the whole program maximum stack usage when exceptions are present. The tool only supports ELF binaries because `-Z emit-stack-sizes` only supports the ELF format. ## License Licensed under either of - Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0) - MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT) at your option. ### Contribution Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.