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Relax ordering rules for asm! operands #1323

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Mar 26, 2023
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Relax ordering rules for asm! operands #1323

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11 changes: 5 additions & 6 deletions src/inline-assembly.md
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Original file line number Diff line number Diff line change
Expand Up @@ -43,13 +43,13 @@ format_string := STRING_LITERAL / RAW_STRING_LITERAL
dir_spec := "in" / "out" / "lateout" / "inout" / "inlateout"
reg_spec := <register class> / "\"" <explicit register> "\""
operand_expr := expr / "_" / expr "=>" expr / expr "=>" "_"
reg_operand := dir_spec "(" reg_spec ")" operand_expr
operand := reg_operand
reg_operand := [ident "="] dir_spec "(" reg_spec ")" operand_expr
clobber_abi := "clobber_abi(" <abi> *("," <abi>) [","] ")"
option := "pure" / "nomem" / "readonly" / "preserves_flags" / "noreturn" / "nostack" / "att_syntax" / "raw"
options := "options(" option *("," option) [","] ")"
asm := "asm!(" format_string *("," format_string) *("," [ident "="] operand) *("," clobber_abi) *("," options) [","] ")"
global_asm := "global_asm!(" format_string *("," format_string) *("," [ident "="] operand) *("," options) [","] ")"
operand := reg_operand / clobber_abi / options
asm := "asm!(" format_string *("," format_string) *("," operand) [","] ")"
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This grammar doesn't seem to capture the order requirements for how it is parsed. Am I missing something for how this is defined? That is, why is it not something closer to:

asm := "asm!(" format_string *("," format_string) *("," operand) *("," ident "=" operand) *("," options) [","] ")"

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The new rules allow options and operands to be freely mixed, so there are no order requirements anymore (except that the template strings must come first).

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I think what I'm getting after is to capture the rule that positional operands must come before named operands. This grammar seems to allow:

let x: i64;
asm!("add {}, {foo}, {}", out(reg) x, foo=in(reg) 2, in(reg) 1);

But that generates an error "positional arguments cannot follow named arguments or explicit register arguments".

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@Amanieu Amanieu Mar 19, 2023 •
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Right, but then again options and clobber_abi can appear in any position between named & positional arguments. It's a bit difficult to express all the details in the grammar.

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If I understand how it parses correctly, the following doesn't seem too unwieldy:

asm := "asm!(" format_string *("," format_string) operands ")"
operands := *("," reg_operand / clobber_abi / options) *("," ident "=" reg_operand) *("," clobber_abi / options)

Does that seem like it would be OK to have?

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I gave this a try but I think it just makes the grammar harder to read overall. I prefer leaving it as it currently is and describing the operand ordering constraints in the text.

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The example you gave isn't actually correct since it doesn't cover explicit register operands which don't have names and can be mixed with named operands.

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That's fair. Thanks for taking a look!

global_asm := "global_asm!(" format_string *("," format_string) *("," operand) [","] ")"
```


Expand All @@ -74,8 +74,7 @@ An `asm!` invocation may have one or more template string arguments; an `asm!` w
The expected usage is for each template string argument to correspond to a line of assembly code.
All template string arguments must appear before any other arguments.

As with format strings, named arguments must appear after positional arguments.
Explicit [register operands](#register-operands) must appear at the end of the operand list, after named arguments if any.
As with format strings, positional arguments must appear before named arguments and explicit [register operands](#register-operands).

Explicit register operands cannot be used by placeholders in the template string.
All other named and positional operands must appear at least once in the template string, otherwise a compiler error is generated.
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