> On Dec 10, 2015, at 8:31 AM, Joe Groff <jgr...@apple.com> wrote:
>
>>
>> On Dec 9, 2015, at 4:15 PM, Adrian Prantl via swift-dev
>> <swift-dev@swift.org> wrote:
>>
>> In order to write textual SIL -> SIL testcases that exercise the handling of
>> debug information by SIL passes, we need to make a couple of additions to
>> the textual SIL language. In memory, the debug information attached to SIL
>> instructions references information from the AST. If we want to create debug
>> info from parsing a textual .sil file, these bits need to be made explicit.
>>
>> Let me illustrate this with an example. The function
>>
>>> func foo(x : Int) -> Int {
>>> return bar(x)
>>> }
>>
>> is compiled to SIL as
>>
>>> // main.foo (Swift.Int) -> Swift.Int
>>> sil hidden @_TF4main3fooFSiSi : $@convention(thin) (Int) -> Int {
>>> // %0 // users: %1, %2, %4
>>> bb0(%0 : $Int):
>>> debug_value %0 : $Int // let x, argno: 1 // id: %1
>>> line:1:10:in_prologue
>>> return %4 : $Int // id: %5 line:2:3:return
>>> }
>>
>> Note that there is a bunch of information available in comments that will be
>> lost once we parse that textual SIL again. I’d like to add syntax to SIL for
>> the information in the comments. This proposal deals with lifting the debug
>> variable information (the first comment) into actual SIL syntax. A similar
>> proposal for locations will be coming soon.
>> With the proposed syntax, this could like like:
>>
>>> sil hidden @_TF4main3fooFSiSi : $@convention(thin) (Int) -> Int {
>>> bb0(%0 : $Int):
>>> debug_value %0 : $Int, !dbg_var(name: "x", type: "_TTSi", argno: 1)
>>> return %4 : $Int
>>> }
>>
>> More formally, debug variable info may be attached to debug_value,
>> debug_value_addr, alloc_box, and alloc_stack instructions.
>>
>> sil-instruction ::= 'alloc_stack' sil-type dbg-var
>> sil-instruction ::= 'alloc_stack' sil-type dbg-var
>> sil-instruction ::= debug_value sil-operand dbg-var
>> sil-instruction ::= debug_value_addr sil-operand dbg-var
>> dbg-var ::= ‘!dbg_var’ ‘(‘ var-attr (',' var-attr)*) ‘)'
>> var-attr ::= ‘name:’ string-literal
>> var-attr ::= ’type:’ string-literal
>> var-attr ::= ‘argno:’ integer-literal
>>
>> This syntax for `dbg-var` is borrowed straight from LLVM IR and thus invokes
>> a familiar feeling. Since the primary use-case of it will be in test cases,
>> the verbose dictionary-like syntax is really helpful.
>>
>> Syntax alternatives I’ve considered and rejected include:
>> 1. debug_value %0 : $Int, “x”, “_TtSi”, 1
>> Why: Hard to read, potentially ambiguous because some fields are optional.
>>
>> 2. debug_value [name “x”] [type “_TtSi”] [argno 1] %0 : $Int
>> Why: Attributes in square brackets don’t typically have arguments and come
>> before the entity they are modifying.
>>
>> 3. debug_value @var(name: “x”, type: “_TtSi”, argno: 1) %0 : $Int
>> Why: The ‘@‘ sigil is used not just for attributes but also for global
>> symbols and thus creates an ambiguity.
>
> Thanks for working on this, Adrian! My thoughts:
>
> - I don't see a reason to mangle the type name at SIL time. You should
> reference the formal AST type directly in the instruction, and print and
> parse it using the normal (Swift) type parser.
Good point. Is it safe to assume that all types representable in textual SIL?
I’m slightly worried that a SIL pass might introduce a type (e.g., a generic
function specialization) for which we don’t have any Swift syntax. Thinking
about it though — even if there is no syntax, we would probably want to create
one, so the type can be used in the debugger and REPL.
> - Since these are core parts of the instruction, I would say they need no
> decoration at all; they should just be part of the instruction syntax:
>
> debug_value %0 : $Int, name "x", type $Int, argno 1
>
sil-instruction ::= 'alloc_stack' sil-type (‘,' debugvar-attr)*
sil-instruction ::= 'alloc_stack' sil-type (‘,’ debugvar-attr)*
sil-instruction ::= debug_value sil-operand (‘,' debugvar-attr)*
sil-instruction ::= debug_value_addr sil-operand (‘,' debugvar-attr)*
debugvar-attr ::= ‘name’ string-literal
debugvar-attr ::= ’type’ type // only needed where AST type ≠ SIL
type
debugvar-attr ::= ‘argno’ integer-literal
Works for me. Thanks for the feedback,
Adrian
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