Go to the compiler explorer, make sure you select Rust from the language drop down menu (as C++ is the default), and copy in the Rust program to the panel on the left. > errno is implemented differently on every platform. assumption that it has complete control over stack layout, with the result that The compiler then uses iret instead of ret for returning and ensures that all regist Read more, In 2008 I was getting paid to work on my creation, Apache CouchDB. While wikipedia says that the Intel syntax is more common in the Windows world while AT&T is more common in Unix circles, in my limited experience Ive seen the Intel syntax used more often even in Unix contexts. Did Sauron suspect that the Ring would be destroyed? Necessary cookies are absolutely essential for the website to function properly. Tier 1 is C++ and ObjC, where you can use C headers nearly directly. call cargo build --features=stdcall. Weve set up the arguments to wrapping_add, which were now ready to call using call which we learned above pushes the next instruction on to the stack and jumps to the label provided - in our case, wrapping_add. This is possible largely because the x64 has a lot more registers, on the x86 passing in registers is harder because there are so few of them. The following variant is not an error because the C calling convention is The code shows It is easy to quietly generate wrong code in naked functions, such as by causing runtime system (e.g. Clearly rdi + 1 is not an address, but thats ok, it still gets the job done. Hopefully youve learned some neat x86-64 instructions, a bit about the System V calling convention, about the various ways that non-optimized Rust code does funny things. reference describes this permitted in the body of a naked function, such as permitting only asm! 1: https://github.com/rust-lang/rust/blob/7cb3ee453b829a513749e > But Rust can't read the C header, so it must literally special-case every OS [1]. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. function (since it may have an unknown calling convention), naked functions must The first thing well do is the functions prologue: sub rsp, 2 which will subtract 2 from rsp (the stack pointer) and store this new value in rsp. Obviously for anything that needs to be called by a binary compiled separately you need a standard calling convention, but one of the benefits of Rust's module system is that the compiler often knows the full list of call sites for a function at compile time, so it should be possible to do these unless I've misunderstood something. There is a Procedure Call Standard (PCS) describing the registers that should contain function args and when to switch to putting args on the stack e.g. Add support for generating naked (prologue/epilogue-free) functions via a new Well hopefully things should pick up a bit more from here. I believe, we pop into rcx because its not being used. MacOS PS: This is nothing new but my past self (>5 years) would find this extreeeeemly cool. it may be useful to add a "unknown" calling convention to the compiler which is LLVM can also change functions accepting parameters by reference to take them by value when this is deemed worthwhile (https://godbolt.org/z/8oTbqzr7T). THIS PROJECT ONLY BUILDS ON x86_64-PLATFORMS AND REQUIRES RUST NIGHTLY 1.59. call the same function with the Rust convention. interrupt handler for x86_64: The generated assembly for this function might resemble the following rsp and rax are 64 bit registers known as general purpose registers but this is a bit of a misnomer since as weve seen rsp has the special purpose of pointing to the top of the stack. The preprocessor has long since run. directly. If you run it, it correctly calculates the sums. While Rust may change which calling convention it uses between releases of the compiler, it needs to have a consistent way inside of a binary to call functions. Once a month. 0558-require-parentheses-for-chained-comparisons, 0587-fn-return-should-be-an-associated-type, 1268-allow-overlapping-impls-on-marker-traits, 1552-contains-method-for-various-collections, 1567-long-error-codes-explanation-normalization, 2056-allow-trivial-where-clause-constraints, 2565-formal-function-parameter-attributes, 2959-promote-aarch64-unknown-linux-gnu-to-tier1. implementation. Rust adds support for the x86-interrupt calling convention. Tier 2 is where you have to create (and maintain) bindings, but you can map practically everything. Memory can be thought of as a long array of bytes that starts at index (better known as address) 0 and goes all the way to address 264. I'm not 100% sure about the official name of the latter. implemented outside Rust code and linked in as needed. In the case of example::inc this was just one instruction, but for other functions this may be be more things many of which well see later in this series. (simplified for readability): Here the programmer's need to save machine state conflicts with the compiler's
The System V ABI is used on UNIX systems with x86_64, therefore actively used on every For naked functions, it is impossible for the compiler to generate a Rust ABI state, the interrupt handler must save the registers which might be modified One such thing is a calling convention which is an agreed upon way for how functions are called. The utility of this feature is extremely limited to most users, and it might be Is/would there be there a measurable performance benefit? Find centralized, trusted content and collaborate around the technologies you use most. In Read more, TLDR: for Multiboot2 you use module2 and for Multiboot1 module in GRUB config files. Also nearly all scripting languages (Python, Ruby, Lua, etc). We can clearly see what registers are used for the argument passing. In the meantime, this assembly has lots of interesting bits to it.
Since the calculation of our addition is now found in raxs 8-bit variant al, were done! (more), Comparison of Rust async and Linux thread context switch time. But of course, you can always go deeper, and learning more about the abstraction layer underneath Rust can be a really great way to really understand what makes Rust tick. This ceremony is referred to as a functions prologue. What's cool about Rust is that it is relatively easy for us to specify C++ can do the same thing: include errno.h and you're done. https://github.com/gcc-mirror/gcc/blob/master/gcc/config/arm/arm.h#L777-L1002. It seems that as of Rust 1.40.0, Rust is using the SystemV ABI at least for its function calling convention.
Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, Compile cdylib with different calling conventions, How APIs can take the pain out of legacy system headaches (Ep. I have a demo project on Github where you can find the source code. Each compiler follows a certain calling convention, that specifies how parameters are passed between functions. compiler lacks support for a specific use case. Where does the return value go? As rdi is the first function argument register, edi must contain the first (and only) argument to the example::inc function. If we look into the Rust compiler output, therefore: we find for the first function call (Win64). It is easy in the sense that Rust can match the ABI and calling convention of C with a single annotation or keyword. For this, please look into the code and execute it with cargo run. While Rust offers plenty of high level abstractions, it certainly makes you think a bit more about lower level concerns like the memory allocation than the JavaScript or Ruby do. You should have passing high level familiarity with the following concepts: basic blocks for any function declared in Rust code with a non-default calling But opting out of some of these cookies may affect your browsing experience. Without smart compiler optimizations, this would result in a machine convention: a trampoline which translates the declared calling convention to the Asking for help, clarification, or responding to other answers. I believe LLVM already performs at least some of what you've suggested; it can change the calling convention of local functions to fastcc, which is supposed to use architectural tricks (such as passing as many things as possible via registers) to make things faster. - jimblandy/context-switch (more). So Rust's C interop is limited to implementing the platform calling conventions? The unsafe requirement on naked functions may not be desirable in all cases. Apparently, Rust and LLVM believe doing push is better choice than subtracting, but Im not really sure why.
If you enjoyed this, please let me know! Data Imbalance: what would be an ideal number(ratio) of newly added class's data? be aware of. As assembly has a close relationship to the actual machine code for a particular computer architecture and is therefore not a platform agnostic abstraction, we have to choose which variety well be exploring. The compiler explorer uses Intel syntax by default. What's inside the SPIKE Essential small angular motor? It's always struck me as a bit wasteful how (not counting inlining) C++ is stuck using the exact same calling conventions for every single function regardless of its arguments or content, even when it will never be linked against. Phew thats a lot of explanation for one instruction! Doing this should get us a bit closer to understanding whats actually happening on our machine (though, of course, the stack does even deeper than the assembly language abstraction layer). address of its caller when called with the C ABI on x86. I especially like low level development, making ugly things nice, and de-mystify "low level magic". Because the calling convention of a naked function is not guaranteed to match To correct for this, we can either subtract 8 from rsp or we can push something else thats 8 bytes big on to the stack which will do this for us. * Registers: very small (64 bits on a 64-bit machine) memory storage on the CPU where data can be manipulated. the calling convention used on. Given that details of Rust, it is advantageous to allow the Rust compiler to drive the entire process, On UNIX on the other hand, they get passed via rdi and rsi. We could use the label in our assembly code as a way to refer to the location in memory where our example::inc function sits. UEFI firmware, only supports the Microsoft/PE calling convention. Because the compiler depends on a function prologue and epilogue to maintain For us this will be x86-64 assembly which is the architecture that youre most likely to find on most desktop and server computers today. registers on x86 and ARM. You should take a sec to read about the different registers on an x86-64 machine and how they contain smaller versions of themselves inside of them (e.g., rax contains a 32-bit register named eax, a 16-bit register named ax, and two 8-bit registers named ah and al). Notice that weve now filled edi with the contents of dil the argument to example::inc and esi with 1. edi and esi are the (32-bit versions of the) first two function argument registers. It's nice that Rust can do that but this is NOT better, it's a significant limitation. While these cases can be errno is just one thing, but there's a long tail: different platforms have different types and size and function names and signatures, especially the ancient system-y stuff. convention and can implement a translation between the foreign ABI and one Well be exploring what this actually entails in great depth over this series, so dont worry if this seems fuzzy. There should be no need for a PCS for private functions when the compiler knows *all* calls to those procedures, which is something that should be possible with Whole Program Optimisation. However, the last time I checked, machine description files were fairly static on calling conventions and did not support per-function arg placement e.g. The cookies is used to store the user consent for the cookies in the category "Necessary". storage for local variable bindings, it is generally unsafe to write anything Well it turns out that we do this to uphold an important part of the function calling convention. PE calling convention ("Microsoft/PE"). the following code block is an error. What happens after this is a bit strange, and Im not sure why this code got generated.
Lets explore this by first looking at the code underneath the example::inc. Like many people who have backgrounds in higher level languages like JavaScript and Ruby, one thing that really attracted me to Rust was the ability to get closer to the metal. before handing control to compiler-generated code. What does #[cfg(test)] do when placed at the top of lib.rs? Ok, now that were all on the same page, lets get started: In this post, were going to explore a very simple Rust library that provides one function inc which takes in a u8, adds one to it wrapping around if it goes beyond 255, and then returns the result: If youre not familiar with wrapping_add, it simply wraps the number around when it overflows unlike + which panics on overflow in debug mode (+ and wrapping_add behave the same in release mode). It does not store any personal data. Thats all the magic. It's easier to follow the PCS when writing assembler language by hand because it can be a bit of a head-wreck, but there's no need to. function: Because the compiler cannot verify the correctness of code written in a naked This example shows how you can call functions that use different calling conventions from Rust code (in the same file). The default calling convention on the x64 (system v) already passes a lot of things in registers. bash loop to replace middle of string after a certain character. errno is implemented differently on every platform, but a C program can include errno.h, and use it portably. How long is this post going to be?! // PE => Microsoft/Windows calling convention. Consider a contrived Where do the function arguments go?
In early 2021 I had a closer look into several aspects of digital signal processing (DSP), including lowpass filters, frequency spectrum analysis, and audio visualization. from within Rust code are forbidden unless the function is also declared with rust-different-calling-conventions-example, Using Different Calling Conventions Inside The Same Rust Project, https://github.com/rust-lang/rust/blob/b09dad3eddfc46c55e45f6c1a00bab09401684b4/compiler/rustc_target/src/spec/abi.rs, https://www.agner.org/optimize/calling_conventions.pdf, https://www.uclibc.org/docs/psABI-x86_64.pdf, https://docs.microsoft.com/en-us/cpp/build/x64-calling-convention?view=msvc-160. When interacting with FFIs that are not natively supported by the compiler, // Defaults to System V ABI (64 bit), i.e.
Rust convention, and a Rust ABI version of the function containing the actual How can I drop the voltage of a 5V DC power supply from 5.5V to 5.1V? By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Really, the PCS is only required for those functions that are being called from external places e.g. By using the Rust FFI, Rust will at least make sure you are treating unsafe code in a controlled manner. convention. Next, 1 is copied into esi. probably with minor adjustments. But Rust can't read the C header, so it must literally special-case every OS [1]. This cookie is set by GDPR Cookie Consent plugin. (more), A Prometheus exporter for tado smart heating solution - eko/tado-exporter (more), LLVM tutorial in Rust language. For example, compare the code generated for v1 and v2 here -- v2 accepts its parameter on the stack while v1 uses registers, and the only difference between the two is that v1 is known to be module-local. As well see later theres usually also a function epilogue which cleans things up at the end of the function. In UEFI spec. function error in the following code block is a compile-time error, whereas It is a much better interop story to use this type of FFI.
Remember the stack grows down so adding 1 will get us 1 position below the top position of the stack.
guaranteed to be compatible. A place for all things related to the Rust programming languagean open-source systems language that emphasizes performance, reliability, and productivity. If you have the answer, let me know! We could have poped off the stack into another unused register and things would still work. Absent this feature, functions implementing an unknown Do I have to learn computer architecture for underestanding or doing reverse engineering? Then, strangely we do the same dance again this time at the top of the stack.
A calling convention specifies properties like how parameters are passed Then with mov al, byte ptr [rsp + 1], we turn around and copy that byte into al (one of the 8-bit registers inside of rax). The Godbolt compiler explorer uses rustc directly and does not turn on any optimizations. calling conventions could be added to the compiler as needed, or emulated with Out of these, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. call pushes the next instructions location on to the stack and ret pops that address off the stack and jumps to that location. The point of Rust is to displace this stuff, not be the best at integrating with it. use their own calling convention, whereas the PE format, the default executable format for Microsoft Windows and In the compiler explorer, we can pass this flag in the compiler options box. statements. the functions correct1 and correct2 are permitted. any calling convention the compiler is compatible with, calls to naked functions Do weekend days count as part of a vacation? For example, when sign extending 0b1000_0001 to 16 bits , it will become 0b1111_1111_1000_0001. implementation of an interrupt service routine on 32-bit x86. The first instruction in our example::inc function is push rax. Well treat the compiler as a black box and see what kind of assembly instructions get produced from standard, run-of-the-mill Rust code. The calling convention is part of an ABI, an application binary interface. Here's a real example. This is also why pure and inlining are separate issues. For example, you can't use the same The label labels a piece of memory - in this case our inc function. The movzx will sign extend dil and keep it in the edi register. version of the function because the implementation may depend on the calling Why this code was generated this way, Im not sure though I suspect its because the compiler/LLVM need additional passes to eliminate the code and in debug mode they skips this. Live Audio Visualization With Rust in a GUI Window, Direct Systemcalls to Linux from Rust Code (x86_64), Minimal GRUB Configuration To Boot A Multiboot2 Kernel With Boot Modules, Background vector created by starline - www.freepik.com. Overview The next three instructions all have to do with calling the function wrapping_add: In order to call a function, we have to prepare the function arguments. Hopefully at some point in this series well take what weve learned and see if we can apply it to another machine architecture. public library functions. (e.g older version of other compiler) can be linked together. If youre familiar with Rust, then you know that by default Rust doesnt do a lot of optimization. Microsoft/PE calling convention: https://docs.microsoft.com/en-us/cpp/build/x64-calling-convention?view=msvc-160, System V ABI: https://www.uclibc.org/docs/psABI-x86_64.pdf.