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Support using const pointers in asm const operand #138618

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086e52e
Collect constants within `global_asm!` in collector
nbdd0121 Mar 12, 2025
8337efc
Add FnDef/Closure -> FnPtr coercion for inline asm const operand
nbdd0121 Dec 5, 2025
e7467b6
Give global_asm symbol names
nbdd0121 Dec 15, 2025
b60bb73
Delay stringification of const to backend
nbdd0121 Dec 18, 2025
b5bfc59
Unify handling of asm const and sym fn using CTFE
nbdd0121 Dec 18, 2025
9822024
Unify handling of `GlobalAlloc` inside backend
nbdd0121 Dec 19, 2025
5916d6e
Handle pointers with offset for asm const
nbdd0121 Dec 18, 2025
d6b2782
Support codegen of asm const pointers without provenance
nbdd0121 Dec 18, 2025
0846b11
Generate unique symbol names if const pointers refer to promoted static
nbdd0121 Dec 19, 2025
f4011e8
Implement asm_const_ptr feature
nbdd0121 Oct 24, 2024
7c877c9
Add test if asm const are MIR inlined and monomorphized twice
nbdd0121 Apr 22, 2026
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68 changes: 48 additions & 20 deletions compiler/rustc_codegen_cranelift/src/global_asm.rs
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Original file line number Diff line number Diff line change
@@ -1,13 +1,15 @@
//! The AOT driver uses [`cranelift_object`] to write object files suitable for linking into a
//! standalone executable.

use std::fmt::Write as _;
use std::io::Write;
use std::path::PathBuf;
use std::process::{Command, Stdio};
use std::sync::Arc;

use rustc_ast::{InlineAsmOptions, InlineAsmTemplatePiece};
use rustc_codegen_ssa::traits::{AsmCodegenMethods, GlobalAsmOperandRef};
use rustc_middle::mir::interpret::{GlobalAlloc, PointerArithmetic, Scalar as ConstScalar};
use rustc_middle::ty::TyCtxt;
use rustc_middle::ty::layout::{
FnAbiError, FnAbiOfHelpers, FnAbiRequest, HasTyCtxt, HasTypingEnv, LayoutError, LayoutOfHelpers,
Expand Down Expand Up @@ -108,27 +110,54 @@ fn codegen_global_asm_inner<'tcx>(
InlineAsmTemplatePiece::Placeholder { operand_idx, modifier: _, span } => {
use rustc_codegen_ssa::back::symbol_export::escape_symbol_name;
match operands[operand_idx] {
GlobalAsmOperandRef::Const { ref string } => {
global_asm.push_str(string);
}
GlobalAsmOperandRef::SymFn { instance } => {
if cfg!(not(feature = "inline_asm_sym")) {
tcx.dcx().span_err(
span,
"asm! and global_asm! sym operands are not yet supported",
);
}
GlobalAsmOperandRef::Const { value, ty, instance: _ } => {
match value {
ConstScalar::Int(int) => {
let string = rustc_codegen_ssa::common::asm_const_to_str(
tcx,
int,
FullyMonomorphizedLayoutCx(tcx).layout_of(ty),
);
global_asm.push_str(&string);
}

let symbol = tcx.symbol_name(instance);
let symbol_name = if tcx.sess.target.is_like_darwin {
format!("_{}", symbol.name)
} else {
symbol.name.to_owned()
};
ConstScalar::Ptr(ptr, _) => {
let (prov, offset) = ptr.prov_and_relative_offset();
let global_alloc = tcx.global_alloc(prov.alloc_id());
let symbol = match global_alloc {
GlobalAlloc::Function { instance } => {
if cfg!(not(feature = "inline_asm_sym")) {
tcx.dcx().span_err(
span,
"asm! and global_asm! sym operands are not yet supported",
);
}
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@bjorn3 bjorn3 Apr 30, 2026 •
edited by rustbot
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This error should apply to all constants that reference a symbol.

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// FIXME handle the case where the function was made private to the
// current codegen unit
global_asm.push_str(&escape_symbol_name(tcx, &symbol_name, span));
// FIXME handle the case where the function was made private to the
// current codegen unit
tcx.symbol_name(instance)
}
GlobalAlloc::Static(def_id) => {
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@bjorn3 bjorn3 Apr 30, 2026 •
edited by rustbot
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What is the difference between GlobalAsmOperandRef::Const with a static and GlobalAsmOperandRef::SymStatic?

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@nbdd0121 nbdd0121 Apr 30, 2026

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See #138618 (comment)

#[thread_local] works with sym STATIC today, but it cannot be referenced in CTFE. I would actually be in favour of removing that feature.

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I would tend to agree with removing #[thread_local] support for sym STATIC. On some platforms (cough Windows cough) #[thread_local] accesses go through a function shim to ensure cross-dylib accesses work, so sym STATIC would probably already be broken for those platforms.

let instance = Instance::mono(tcx, def_id);
tcx.symbol_name(instance)
}
GlobalAlloc::Memory(_)
| GlobalAlloc::VTable(..)
| GlobalAlloc::TypeId { .. } => unreachable!(),
};
let symbol_name = if tcx.sess.target.is_like_darwin {
format!("_{}", symbol.name)
} else {
symbol.name.to_owned()
};
global_asm.push_str(&escape_symbol_name(tcx, &symbol_name, span));

if offset != Size::ZERO {
let offset = tcx.sign_extend_to_target_isize(offset.bytes());
write!(global_asm, "{offset:+}").unwrap();
}
}
}
}
GlobalAsmOperandRef::SymStatic { def_id } => {
if cfg!(not(feature = "inline_asm_sym")) {
Expand All @@ -137,7 +166,6 @@ fn codegen_global_asm_inner<'tcx>(
"asm! and global_asm! sym operands are not yet supported",
);
}

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@bjorn3 bjorn3 Apr 30, 2026 •
edited by rustbot
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Please restore this newline.

View changes since the review

let instance = Instance::mono(tcx, def_id);
let symbol = tcx.symbol_name(instance);
let symbol_name = if tcx.sess.target.is_like_darwin {
Expand Down
11 changes: 9 additions & 2 deletions compiler/rustc_codegen_cranelift/src/inline_asm.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -96,10 +96,17 @@ pub(crate) fn codegen_inline_asm_terminator<'tcx>(
}
InlineAsmOperand::Const { ref value } => {
let (const_value, ty) = crate::constant::eval_mir_constant(fx, value);
let mir::ConstValue::Scalar(scalar) = const_value else {
span_bug!(
span,
"expected Scalar for promoted asm const, but got {:#?}",
const_value
)
};

let value = rustc_codegen_ssa::common::asm_const_to_str(
fx.tcx,
span,
const_value,
scalar.assert_scalar_int(),
fx.layout_of(ty),
);
CInlineAsmOperand::Const { value }
Expand Down
146 changes: 96 additions & 50 deletions compiler/rustc_codegen_gcc/src/asm.rs
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Original file line number Diff line number Diff line change
@@ -1,17 +1,21 @@
// cSpell:ignoreRegExp [afkspqvwy]reg

use std::borrow::Cow;
use std::fmt::Write;

use gccjit::{LValue, RValue, ToRValue, Type};
use rustc_abi::Size;
use rustc_ast::ast::{InlineAsmOptions, InlineAsmTemplatePiece};
use rustc_codegen_ssa::mir::operand::OperandValue;
use rustc_codegen_ssa::mir::place::PlaceRef;
use rustc_codegen_ssa::traits::{
AsmBuilderMethods, AsmCodegenMethods, BaseTypeCodegenMethods, BuilderMethods,
GlobalAsmOperandRef, InlineAsmOperandRef,
ConstCodegenMethods, GlobalAsmOperandRef, InlineAsmOperandRef,
};
use rustc_middle::bug;
use rustc_middle::mir::interpret::{GlobalAlloc, PointerArithmetic, Scalar};
use rustc_middle::ty::Instance;
use rustc_middle::ty::layout::LayoutOf;
use rustc_span::Span;
use rustc_target::asm::*;

Expand Down Expand Up @@ -143,6 +147,9 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
// Clobbers collected from `out("explicit register") _` and `inout("explicit_reg") var => _`
let mut clobbers = vec![];

// Symbols name that needs to be inserted to asm const ptr template string.
let mut const_syms = vec![];

// We're trying to preallocate space for the template
let mut constants_len = 0;

Expand Down Expand Up @@ -303,16 +310,11 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
}
}

InlineAsmOperandRef::Const { ref string } => {
constants_len += string.len() + att_dialect as usize;
InlineAsmOperandRef::Const { .. } => {
// We don't know the size at this point, just some estimate.
constants_len += 20;
}

InlineAsmOperandRef::SymFn { instance } => {
// FIXME(@Amanieu): Additional mangling is needed on
// some targets to add a leading underscore (Mach-O)
// or byte count suffixes (x86 Windows).
constants_len += self.tcx.symbol_name(instance).name.len();
}
InlineAsmOperandRef::SymStatic { def_id } => {
// FIXME(@Amanieu): Additional mangling is needed on
// some targets to add a leading underscore (Mach-O).
Expand Down Expand Up @@ -402,24 +404,22 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
// processed in the previous pass
}

InlineAsmOperandRef::SymFn { instance } => {
inputs.push(AsmInOperand {
constraint: "X".into(),
rust_idx,
val: get_fn(self.cx, instance).get_address(None),
});
}
InlineAsmOperandRef::Const { value, ty: _, instance } => match value {
Scalar::Int(_) => (),
Scalar::Ptr(ptr, _) => {
let (prov, _) = ptr.prov_and_relative_offset();
let global_alloc = self.tcx.global_alloc(prov.alloc_id());
let (val, sym) = self.cx.alloc_to_backend(global_alloc, instance).unwrap();
const_syms.push(sym.unwrap());
inputs.push(AsmInOperand { constraint: "X".into(), rust_idx, val });
}
},

InlineAsmOperandRef::SymStatic { def_id } => {
inputs.push(AsmInOperand {
constraint: "X".into(),
rust_idx,
val: self.cx.get_static(def_id).get_address(None),
});
}

InlineAsmOperandRef::Const { .. } => {
// processed in the previous pass
// FIXME(@Amanieu): Additional mangling is needed on
// some targets to add a leading underscore (MachO).
constants_len +=
self.tcx.symbol_name(Instance::mono(self.tcx, def_id)).name.len();
}

InlineAsmOperandRef::Label { .. } => {
Expand Down Expand Up @@ -495,12 +495,33 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
push_to_template(modifier, gcc_index);
}

InlineAsmOperandRef::SymFn { instance } => {
// FIXME(@Amanieu): Additional mangling is needed on
// some targets to add a leading underscore (Mach-O)
// or byte count suffixes (x86 Windows).
let name = self.tcx.symbol_name(instance).name;
template_str.push_str(name);
InlineAsmOperandRef::Const { value, ty, instance: _ } => {
match value {
Scalar::Int(int) => {
// Const operands get injected directly into the template
let string = rustc_codegen_ssa::common::asm_const_to_str(
self.tcx,
int,
self.layout_of(ty),
);
template_str.push_str(&string);
}

Scalar::Ptr(ptr, _) => {
let (_, offset) = ptr.prov_and_relative_offset();
let instance = const_syms.remove(0);
// FIXME(@Amanieu): Additional mangling is needed on
// some targets to add a leading underscore (Mach-O)
// or byte count suffixes (x86 Windows).
template_str.push_str(self.tcx.symbol_name(instance).name);

if offset != Size::ZERO {
let offset =
self.sign_extend_to_target_isize(offset.bytes());
write!(template_str, "{offset:+}").unwrap();
}
}
}
}

InlineAsmOperandRef::SymStatic { def_id } => {
Expand All @@ -511,10 +532,6 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
template_str.push_str(name);
}

InlineAsmOperandRef::Const { ref string } => {
template_str.push_str(string);
}

InlineAsmOperandRef::Label { label } => {
let label_gcc_index =
labels.iter().position(|&l| l == label).expect("wrong rust index");
Expand Down Expand Up @@ -907,23 +924,52 @@ impl<'gcc, 'tcx> AsmCodegenMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
}
InlineAsmTemplatePiece::Placeholder { operand_idx, modifier: _, span: _ } => {
match operands[operand_idx] {
GlobalAsmOperandRef::Const { ref string } => {
// Const operands get injected directly into the
// template. Note that we don't need to escape %
// here unlike normal inline assembly.
template_str.push_str(string);
}
GlobalAsmOperandRef::Const { value, ty, instance } => {
match value {
Scalar::Int(int) => {
// Const operands get injected directly into the
// template. Note that we don't need to escape %
// here unlike normal inline assembly.
let string = rustc_codegen_ssa::common::asm_const_to_str(
self.tcx,
int,
self.layout_of(ty),
);
template_str.push_str(&string);
}

GlobalAsmOperandRef::SymFn { instance } => {
let function = get_fn(self, instance);
self.add_used_function(function);
// FIXME(@Amanieu): Additional mangling is needed on
// some targets to add a leading underscore (Mach-O)
// or byte count suffixes (x86 Windows).
let name = self.tcx.symbol_name(instance).name;
template_str.push_str(name);
Scalar::Ptr(ptr, _) => {
let (prov, offset) = ptr.prov_and_relative_offset();
let global_alloc = self.tcx.global_alloc(prov.alloc_id());
let symbol_name = match global_alloc {
GlobalAlloc::Function { instance } => {
let function = get_fn(self, instance);
self.add_used_function(function);
// FIXME(@Amanieu): Additional mangling is needed on
// some targets to add a leading underscore (Mach-O)
// or byte count suffixes (x86 Windows).
self.tcx.symbol_name(instance)
}
_ => {
let (_, syms) = self
.alloc_to_backend(global_alloc, instance)
.unwrap();
// FIXME(antoyo): set the global variable as used.
// FIXME(@Amanieu): Additional mangling is needed on
// some targets to add a leading underscore (Mach-O).
self.tcx.symbol_name(syms.unwrap())
}
};
template_str.push_str(symbol_name.name);

if offset != Size::ZERO {
let offset =
self.sign_extend_to_target_isize(offset.bytes());
write!(template_str, "{offset:+}").unwrap();
}
}
}
}

GlobalAsmOperandRef::SymStatic { def_id } => {
// FIXME(antoyo): set the global variable as used.
// FIXME(@Amanieu): Additional mangling is needed on
Expand Down
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