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Fix discarded in-out constraint in inline asm (#110)

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Fixes #109
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antoyo 2021-12-15 22:06:16 -05:00 • committed by GitHub
parent ebe6f6785c
commit ddb015a09e
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2 changed files with 73 additions and 65 deletions
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111
src/asm.rs
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@ -18,30 +18,30 @@ use crate::type_of::LayoutGccExt;
// Rust asm! and GCC Extended Asm semantics differ substantially.
//
// 1. Rust asm operands go along as one list of operands. Operands themselves indicate
// if they're "in" or "out". "In" and "out" operands can interleave. One operand can be
// 1. Rust asm operands go along as one list of operands. Operands themselves indicate
// if they're "in" or "out". "In" and "out" operands can interleave. One operand can be
// both "in" and "out" (`inout(reg)`).
//
// GCC asm has two different lists for "in" and "out" operands. In terms of gccjit,
// this means that all "out" operands must go before "in" operands. "In" and "out" operands
// GCC asm has two different lists for "in" and "out" operands. In terms of gccjit,
// this means that all "out" operands must go before "in" operands. "In" and "out" operands
// cannot interleave.
//
// 2. Operand lists in both Rust and GCC are indexed. Index starts from 0. Indexes are important
// 2. Operand lists in both Rust and GCC are indexed. Index starts from 0. Indexes are important
// because the asm template refers to operands by index.
//
// Mapping from Rust to GCC index would be 1-1 if it wasn't for...
//
// 3. Clobbers. GCC has a separate list of clobbers, and clobbers don't have indexes.
// Contrary, Rust expresses clobbers through "out" operands that aren't tied to
// 3. Clobbers. GCC has a separate list of clobbers, and clobbers don't have indexes.
// Contrary, Rust expresses clobbers through "out" operands that aren't tied to
// a variable (`_`), and such "clobbers" do have index.
//
// 4. Furthermore, GCC Extended Asm does not support explicit register constraints
// (like `out("eax")`) directly, offering so-called "local register variables"
// as a workaround. These variables need to be declared and initialized *before*
// the Extended Asm block but *after* normal local variables
// 4. Furthermore, GCC Extended Asm does not support explicit register constraints
// (like `out("eax")`) directly, offering so-called "local register variables"
// as a workaround. These variables need to be declared and initialized *before*
// the Extended Asm block but *after* normal local variables
// (see comment in `codegen_inline_asm` for explanation).
//
// With that in mind, let's see how we translate Rust syntax to GCC
// With that in mind, let's see how we translate Rust syntax to GCC
// (from now on, `CC` stands for "constraint code"):
//
// * `out(reg_class) var` -> translated to output operand: `"=CC"(var)`
@ -52,18 +52,17 @@ use crate::type_of::LayoutGccExt;
//
// * `out("explicit register") _` -> not translated to any operands, register is simply added to clobbers list
//
// * `inout(reg_class) in_var => out_var` -> translated to two operands:
// * `inout(reg_class) in_var => out_var` -> translated to two operands:
// output: `"=CC"(in_var)`
// input: `"num"(out_var)` where num is the GCC index
// input: `"num"(out_var)` where num is the GCC index
// of the corresponding output operand
//
// * `inout(reg_class) in_var => _` -> same as `inout(reg_class) in_var => tmp`,
// * `inout(reg_class) in_var => _` -> same as `inout(reg_class) in_var => tmp`,
// where "tmp" is a temporary unused variable
//
// * `out/in/inout("explicit register") var` -> translated to one or two operands as described above
// with `"r"(var)` constraint,
// * `out/in/inout("explicit register") var` -> translated to one or two operands as described above
// with `"r"(var)` constraint,
// and one register variable assigned to the desired register.
//
const ATT_SYNTAX_INS: &str = ".att_syntax noprefix\n\t";
const INTEL_SYNTAX_INS: &str = "\n\t.intel_syntax noprefix";
@ -124,7 +123,7 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
let att_dialect = is_x86 && options.contains(InlineAsmOptions::ATT_SYNTAX);
let intel_dialect = is_x86 && !options.contains(InlineAsmOptions::ATT_SYNTAX);
// GCC index of an output operand equals its position in the array
// GCC index of an output operand equals its position in the array
let mut outputs = vec![];
// GCC index of an input operand equals its position in the array
@ -138,9 +137,9 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
let mut constants_len = 0;
// There are rules we must adhere to if we want GCC to do the right thing:
//
//
// * Every local variable that the asm block uses as an output must be declared *before*
// the asm block.
// the asm block.
// * There must be no instructions whatsoever between the register variables and the asm.
//
// Therefore, the backend must generate the instructions strictly in this order:
@ -152,7 +151,7 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
// We also must make sure that no input operands are emitted before output operands.
//
// This is why we work in passes, first emitting local vars, then local register vars.
// Also, we don't emit any asm operands immediately; we save them to
// Also, we don't emit any asm operands immediately; we save them to
// the one of the buffers to be emitted later.
// 1. Normal variables (and saving operands to buffers).
@ -165,7 +164,7 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
(Constraint(constraint), Some(place)) => (constraint, place.layout.gcc_type(self.cx, false)),
// When `reg` is a class and not an explicit register but the out place is not specified,
// we need to create an unused output variable to assign the output to. This var
// needs to be of a type that's "compatible" with the register class, but specific type
// needs to be of a type that's "compatible" with the register class, but specific type
// doesn't matter.
(Constraint(constraint), None) => (constraint, dummy_output_type(self.cx, reg.reg_class())),
(Register(_), Some(_)) => {
@ -193,7 +192,7 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
let tmp_var = self.current_func().new_local(None, ty, "output_register");
outputs.push(AsmOutOperand {
constraint,
constraint,
rust_idx,
late,
readwrite: false,
@ -204,12 +203,12 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
InlineAsmOperandRef::In { reg, value } => {
if let ConstraintOrRegister::Constraint(constraint) = reg_to_gcc(reg) {
inputs.push(AsmInOperand {
constraint: Cow::Borrowed(constraint),
rust_idx,
inputs.push(AsmInOperand {
constraint: Cow::Borrowed(constraint),
rust_idx,
val: value.immediate()
});
}
}
else {
// left for the next pass
continue
@ -219,7 +218,7 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
InlineAsmOperandRef::InOut { reg, late, in_value, out_place } => {
let constraint = if let ConstraintOrRegister::Constraint(constraint) = reg_to_gcc(reg) {
constraint
}
}
else {
// left for the next pass
continue
@ -228,22 +227,22 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
// Rustc frontend guarantees that input and output types are "compatible",
// so we can just use input var's type for the output variable.
//
// This decision is also backed by the fact that LLVM needs in and out
// values to be of *exactly the same type*, not just "compatible".
// This decision is also backed by the fact that LLVM needs in and out
// values to be of *exactly the same type*, not just "compatible".
// I'm not sure if GCC is so picky too, but better safe than sorry.
let ty = in_value.layout.gcc_type(self.cx, false);
let tmp_var = self.current_func().new_local(None, ty, "output_register");
// If the out_place is None (i.e `inout(reg) _` syntax was used), we translate
// it to one "readwrite (+) output variable", otherwise we translate it to two
// it to one "readwrite (+) output variable", otherwise we translate it to two
// "out and tied in" vars as described above.
let readwrite = out_place.is_none();
outputs.push(AsmOutOperand {
constraint,
constraint,
rust_idx,
late,
readwrite,
tmp_var,
tmp_var,
out_place,
});
@ -252,8 +251,8 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
let constraint = Cow::Owned(out_gcc_idx.to_string());
inputs.push(AsmInOperand {
constraint,
rust_idx,
constraint,
rust_idx,
val: in_value.immediate()
});
}
@ -280,7 +279,7 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
if let ConstraintOrRegister::Register(reg_name) = reg_to_gcc(reg) {
let out_place = if let Some(place) = place {
place
}
}
else {
// processed in the previous pass
continue
@ -291,7 +290,7 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
tmp_var.set_register_name(reg_name);
outputs.push(AsmOutOperand {
constraint: "r".into(),
constraint: "r".into(),
rust_idx,
late,
readwrite: false,
@ -311,9 +310,9 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
reg_var.set_register_name(reg_name);
self.llbb().add_assignment(None, reg_var, value.immediate());
inputs.push(AsmInOperand {
constraint: "r".into(),
rust_idx,
inputs.push(AsmInOperand {
constraint: "r".into(),
rust_idx,
val: reg_var.to_rvalue()
});
}
@ -324,31 +323,23 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
// `inout("explicit register") in_var => out_var`
InlineAsmOperandRef::InOut { reg, late, in_value, out_place } => {
if let ConstraintOrRegister::Register(reg_name) = reg_to_gcc(reg) {
let out_place = if let Some(place) = out_place {
place
}
else {
// processed in the previous pass
continue
};
// See explanation in the first pass.
let ty = in_value.layout.gcc_type(self.cx, false);
let tmp_var = self.current_func().new_local(None, ty, "output_register");
tmp_var.set_register_name(reg_name);
outputs.push(AsmOutOperand {
constraint: "r".into(),
constraint: "r".into(),
rust_idx,
late,
readwrite: false,
tmp_var,
out_place: Some(out_place)
out_place,
});
let constraint = Cow::Owned((outputs.len() - 1).to_string());
inputs.push(AsmInOperand {
constraint,
inputs.push(AsmInOperand {
constraint,
rust_idx,
val: in_value.immediate()
});
@ -357,8 +348,8 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
// processed in the previous pass
}
InlineAsmOperandRef::Const { .. }
| InlineAsmOperandRef::SymFn { .. }
InlineAsmOperandRef::Const { .. }
| InlineAsmOperandRef::SymFn { .. }
| InlineAsmOperandRef::SymStatic { .. } => {
// processed in the previous pass
}
@ -453,7 +444,7 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
if !intel_dialect {
template_str.push_str(INTEL_SYNTAX_INS);
}
// 4. Generate Extended Asm block
let block = self.llbb();
@ -472,7 +463,7 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
}
if !options.contains(InlineAsmOptions::PRESERVES_FLAGS) {
// TODO(@Commeownist): I'm not 100% sure this one clobber is sufficient
// TODO(@Commeownist): I'm not 100% sure this one clobber is sufficient
// on all architectures. For instance, what about FP stack?
extended_asm.add_clobber("cc");
}
@ -491,10 +482,10 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
self.call(self.type_void(), builtin_unreachable, &[], None);
}
// Write results to outputs.
// Write results to outputs.
//
// We need to do this because:
// 1. Turning `PlaceRef` into `RValue` is error-prone and has nasty edge cases
// 1. Turning `PlaceRef` into `RValue` is error-prone and has nasty edge cases
// (especially with current `rustc_backend_ssa` API).
// 2. Not every output operand has an `out_place`, and it's required by `add_output_operand`.
//
@ -502,7 +493,7 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
// generates `out_place = tmp_var;` assignments if out_place exists.
for op in &outputs {
if let Some(place) = op.out_place {
OperandValue::Immediate(op.tmp_var.to_rvalue()).store(self, place);
OperandValue::Immediate(op.tmp_var.to_rvalue()).store(self, place);
}
}

27
tests/run/asm.rs
View file

@ -17,6 +17,16 @@ extern "C" {
fn add_asm(a: i64, b: i64) -> i64;
}
pub unsafe fn mem_cpy(dst: *mut u8, src: *const u8, len: usize) {
asm!(
"rep movsb",
inout("rdi") dst => _,
inout("rsi") src => _,
inout("rcx") len => _,
options(preserves_flags, nostack)
);
}
fn main() {
unsafe {
asm!("nop");
@ -62,11 +72,11 @@ fn main() {
}
assert_eq!(x, 43);
// check inout(reg_class) x
// check inout(reg_class) x
let mut x: u64 = 42;
unsafe {
asm!("add {0}, {0}",
inout(reg) x
inout(reg) x
);
}
assert_eq!(x, 84);
@ -75,7 +85,7 @@ fn main() {
let mut x: u64 = 42;
unsafe {
asm!("add r11, r11",
inout("r11") x
inout("r11") x
);
}
assert_eq!(x, 84);
@ -98,12 +108,12 @@ fn main() {
assert_eq!(res, 7);
assert_eq!(rem, 2);
// check const
// check const
let mut x: u64 = 42;
unsafe {
asm!("add {}, {}",
inout(reg) x,
const 1
const 1
);
}
assert_eq!(x, 43);
@ -150,4 +160,11 @@ fn main() {
assert_eq!(x, 42);
assert_eq!(unsafe { add_asm(40, 2) }, 42);
let array1 = [1u8, 2, 3];
let mut array2 = [0u8, 0, 0];
unsafe {
mem_cpy(array2.as_mut_ptr(), array1.as_ptr(), 3);
}
assert_eq!(array1, array2);
}