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Inline assembly: Clarify references to quoted regs #1191

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May 20, 2022
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Inline assembly: Clarify references to quoted regs #1191

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Inline assembly: Clarify references to quoted regs
fw-immunant Apr 12, 2022
f63b37b
Fix confusion around clobbers and register classes
fw-immunant May 20, 2022
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6 changes: 3 additions & 3 deletions src/inline-assembly.md
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Original file line number Diff line number Diff line change
Expand Up @@ -41,7 +41,7 @@ The following ABNF specifies the general syntax:
```text
format_string := STRING_LITERAL / RAW_STRING_LITERAL
dir_spec := "in" / "out" / "lateout" / "inout" / "inlateout"
reg_spec := <register class> / "<explicit register>"
reg_spec := <register class> / "\"" <explicit register> "\""
operand_expr := expr / "_" / expr "=>" expr / expr "=>" "_"
reg_operand := dir_spec "(" reg_spec ")" operand_expr
operand := reg_operand
Expand Down Expand Up @@ -184,7 +184,7 @@ Here is the list of currently supported register classes:
>
> - On x86-64 the high byte registers (e.g. `ah`) are not available in the `reg_byte` register class.
>
> - Some register classes are marked as "Only clobbers" which means that they cannot be used for inputs or outputs, only clobbers of the form `out("reg") _` or `lateout("reg") _`.
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This change is incorrect: clobber-only registers can only be used with explicit registers, not register classes.

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I was misled by the usage of 'reg' here, because 'reg' itself is the name of a register class, so I took this to be naming one register class in a place where any register class was valid. If this is an explicit register name, we should say:

`out(<explicit register>) _` or `lateout(<explicit register>) _`, (for example, `lateout("rax") _`).

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Committed a change that hopefully clarifies this.

> - Some register classes are marked as "Only clobbers" which means that registers in these classes cannot be used for inputs or outputs, only clobbers of the form `out(<explicit register>) _` or `lateout(<explicit register>) _`.

Each register class has constraints on which value types they can be used with.
This is necessary because the way a value is loaded into a register depends on its type.
Expand Down Expand Up @@ -356,7 +356,7 @@ If all references to an operand already have modifiers then the warning is suppr
## ABI clobbers

The `clobber_abi` keyword can be used to apply a default set of clobbers to an `asm!` block.
This will automatically insert the necessary clobber constraints as needed for calling a function with a particular calling convention: if the calling convention does not fully preserve the value of a register across a call then a `lateout("reg") _` is implicitly added to the operands list.
This will automatically insert the necessary clobber constraints as needed for calling a function with a particular calling convention: if the calling convention does not fully preserve the value of a register across a call then `lateout("...") _` is implicitly added to the operands list (where the `...` is replaced by the register's name).

`clobber_abi` may be specified any number of times. It will insert a clobber for all unique registers in the union of all specified calling conventions.

Expand Down