naked functions

src/attributes.md

@@ -291,6 +291,7 @@ The following is an index of all built-in attributes.
 - Code generation
   - [`inline`] --- Hint to inline code.
   - [`cold`] --- Hint that a function is unlikely to be called.
+  - [`naked`] - Prevent the compiler from emitting a function prologue.
   - [`no_builtins`] --- Disables use of certain built-in functions.
   - [`target_feature`] --- Configure platform-specific code generation.
   - [`track_caller`] --- Pass the parent call location to `std::panic::Location::caller()`.
@@ -367,6 +368,7 @@ The following is an index of all built-in attributes.
 [`macro_export`]: macros-by-example.md#path-based-scope
 [`macro_use`]: macros-by-example.md#the-macro_use-attribute
 [`must_use`]: attributes/diagnostics.md#the-must_use-attribute
+[`naked`]: attributes/codegen.md#the-naked-attribute
 [`no_builtins`]: attributes/codegen.md#the-no_builtins-attribute
 [`no_implicit_prelude`]: names/preludes.md#the-no_implicit_prelude-attribute
 [`no_link`]: items/extern-crates.md#the-no_link-attribute

src/attributes/codegen.md

@@ -51,6 +51,46 @@ r[attributes.codegen.cold]
 The *`cold` [attribute]* suggests that the attributed function is unlikely to
 be called.
 
+## The `naked` attribute
+
+r[attributes.codegen.naked]
+
+r[attributes.codegen.naked.intro]
+The *`naked` [attribute]* prevents the compiler from emitting a function prologue and
+epilogue for the attributed function.
+
+r[attributes.codegen.naked.body]
+The [function body] must consist of exactly one [`naked_asm!`] macro invocation, which
+may be enclosed within an [unsafe block].
+
+r[attributes.codegen.naked.prologue-epilogue]
+No function prologue or epilogue are generated for the attributed function: the contents
+of the `naked_asm!` invocation make up the full body of a naked function.
+
+r[attributes.codegen.naked.call-stack]
+The asm code will have a valid call stack and register state on entry as per the signature and calling convention of the function.
+
+r[attributes.codegen.naked.no-duplication]
+The asm code may not be duplicated by the compiler, except when monomorphizing polymorphic functions.
+This property is important for naked functions that define symbols in the assembly code.
+
+r[attributes.codegen.naked.unsafe-function]
+A naked function that makes use of registers in a way that does not conform
+to the specified calling convention imposes additional safety invariants on its caller,
+and therefore must be marked as an [unsafe function].
+
+r[attributes.codegen.naked.unused-variables]
+The [`unused_variables`] lint is suppressed within naked functions.
+
+r[attributes.codegen.naked.inline]
+A naked function cannot be attributed by the [`inline`](#the-inline-attribute) attribute.
+
+r[attributes.codegen.naked.track_caller]
+A naked function cannot be attributed by the [`track_caller`](#the-track_caller-attribute) attribute.
+
+r[attributes.codegen.naked.testing]
+A naked function cannot be attributed by [the testing attributes](testing.md).
+
 ## The `no_builtins` attribute
 
 r[attributes.codegen.no_builtins]
@@ -469,14 +509,22 @@ trait object whose methods are attributed.
 [`-C target-feature`]: ../../rustc/codegen-options/index.html#target-feature
 [`is_x86_feature_detected`]: ../../std/arch/macro.is_x86_feature_detected.html
 [`is_aarch64_feature_detected`]: ../../std/arch/macro.is_aarch64_feature_detected.html
+[`naked_asm!`]: ../inline-assembly.md
+[`inline`]: #the-inline-attribute
+[`track_caller`]: #the-track-caller-attribute
 [`target_feature` conditional compilation option]: ../conditional-compilation.md#target_feature
+[`unused_variables`]: ../../rustc/lints/listing/warn-by-default.html#unused-variables
 [attribute]: ../attributes.md
 [attributes]: ../attributes.md
+[FFI-safe]: ../../rustc/lints/listing/warn-by-default.html#improper-ctypes-definitions
+[function body]: ../items/functions.md#function-body
 [functions]: ../items/functions.md
+[rules for inline assembly]: ../inline-assembly.md#rules-for-inline-assembly
 [target architecture]: ../conditional-compilation.md#target_arch
 [trait]: ../items/traits.md
 [undefined behavior]: ../behavior-considered-undefined.md
-[unsafe function]: ../unsafe-keyword.md
+[unsafe block]: ../unsafe-blocks.md
+[unsafe function]: ../unsafe-functions.md
 [rust-abi]: ../items/external-blocks.md#abi
 [`Location`]: core::panic::Location
 

src/inline-assembly.md

@@ -3,10 +3,11 @@
 r[asm]
 
 r[asm.intro]
-Support for inline assembly is provided via the [`asm!`] and [`global_asm!`] macros.
+Support for inline assembly is provided via the [`asm!`], [`naked_asm!`] and [`global_asm!`] macros.
 It can be used to embed handwritten assembly in the assembly output generated by the compiler.
 
 [`asm!`]: core::arch::asm
+[`naked_asm!`]: core::arch::naked_asm
 [`global_asm!`]: core::arch::global_asm
 
 r[asm.stable-targets]
@@ -61,6 +62,7 @@ option := "pure" / "nomem" / "readonly" / "preserves_flags" / "noreturn" / "nost
 options := "options(" option *("," option) [","] ")"
 operand := reg_operand / clobber_abi / options
 asm := "asm!(" format_string *("," format_string) *("," operand) [","] ")"
+naked_asm := "naked_asm!(" format_string *("," format_string) *("," operand) [","] ")"
 global_asm := "global_asm!(" format_string *("," format_string) *("," operand) [","] ")"
 ```
 
@@ -69,13 +71,17 @@ global_asm := "global_asm!(" format_string *("," format_string) *("," operand) [
 r[asm.scope]
 
 r[asm.scope.intro]
-Inline assembly can be used in one of two ways.
+Inline assembly can be used in one of three ways.
 
 r[asm.scope.asm]
 With the `asm!` macro, the assembly code is emitted in a function scope and integrated into the compiler-generated assembly code of a function.
 This assembly code must obey [strict rules](#rules-for-inline-assembly) to avoid undefined behavior.
 Note that in some cases the compiler may choose to emit the assembly code as a separate function and generate a call to it.
 
+r[asm.scope.naked_asm]
+With the `naked_asm!` macro, the assembly code is emitted in a function scope and constitutes the full assembly code of a function.
+The `naked_asm!` macro is only allowed in [naked functions](attributes/codegen.md#the-naked-attribute).
+
 r[asm.scope.global_asm]
 With the `global_asm!` macro, the assembly code is emitted in a global scope, outside a function.
 This can be used to hand-write entire functions using assembly code, and generally provides much more freedom to use arbitrary registers and assembler directives.
@@ -185,8 +191,11 @@ Operand expressions are evaluated from left to right, just like function call ar
 After the `asm!` has executed, outputs are written to in left to right order.
 This is significant if two outputs point to the same place: that place will contain the value of the rightmost output.
 
+r[asm.operand-type.naked_asm-restriction]
+Because `naked_asm!` defines a whole function body, it can only use `sym` and `const` operands.
+
 r[asm.operand-type.global_asm-restriction]
-Since `global_asm!` exists outside a function, it can only use `sym` and `const` operands.
+Because `global_asm!` exists outside a function, it can only use `sym` and `const` operands.
 
 ## Register operands
 
@@ -580,9 +589,13 @@ r[asm.options.checks.pure]
 r[asm.options.checks.noreturn]
 - It is a compile-time error to specify `noreturn` on an asm block with outputs.
 
+r[asm.options.naked_asm-restriction]
+`naked_asm!` only supports the `att_syntax` and `raw` options.
+The remaining options are not meaningful because the inline assembly defines the whole function body.
+
 r[asm.options.global_asm-restriction]
 `global_asm!` only supports the `att_syntax` and `raw` options.
-The remaining options are not meaningful for global-scope inline assembly
+The remaining options are not meaningful for global-scope inline assembly.
 
 ## Rules for inline assembly
 
@@ -695,6 +708,40 @@ r[asm.rules.x86-prefix-restriction]
 r[asm.rules.preserves_flags]
 > **Note**: As a general rule, the flags covered by `preserves_flags` are those which are *not* preserved when performing a function call.
 
+## Rules for naked inline assembly
+
+r[asm.naked-rules]
+
+r[asm.naked-rules.intro]
+To avoid undefined behavior, these rules must be followed when using function-scope inline assembly in naked functions (`naked_asm!`):
+
+r[asm.naked-rules.reg-not-input]
+- Any registers not used for function inputs according to the calling convention and function signature will contain an undefined value on entry to the asm block.
+  - An "undefined value" in the context of inline assembly means that the register can (non-deterministically) have any one of the possible values allowed by the architecture.
+    Notably it is not the same as an LLVM `undef` which can have a different value every time you read it (since such a concept does not exist in assembly code).
+
+r[asm.naked-rules.reg-not-output]
+- Any callee-saved registers must have the same value upon return as they had on entry, otherwise behavior is undefined.
+  - Caller-saved registes may be used freely, even if they are not used for the return value.
+
+r[asm.naked-rules.unwind]
+- Behavior is undefined if execution unwinds out of an asm block.
+  - This also applies if the assembly code calls a function which then unwinds.
+
+r[asm.naked-rules.noreturn]
+- Behavior is undefined if execution falls through to the end of the asm block.
+    - the assembly code is expected to contain a return instruction or to diverge
+
+r[asm.naked-rules.mem-same-as-ffi]
+- The set of memory locations that assembly code is allowed to read and write are the same as those allowed for an FFI function.
+  - Refer to the unsafe code guidelines for the exact rules.
+  - These rules do not apply to memory which is private to the asm code, such as stack space allocated within the asm block.
+
+r[asm.naked-rules.black-box]
+- The compiler cannot assume that the instructions in the asm are the ones that will actually end up executed.
+  - This effectively means that the compiler must treat the `naked_asm!` as a black box and only take the interface specification into account, not the instructions themselves.
+  - Runtime code patching is allowed, via target-specific mechanisms.
+
 ### Correctness and Validity
 
 r[asm.validity]