dblaikie added inline comments.
================
Comment at: clang/test/CodeGen/unique-internal-linkage-names-dwarf.c:34-39
+static int go(a) int a;
+{
+ return glob + a;
+}
+
+
----------------
aaron.ballman wrote:
> dblaikie wrote:
> > hoy wrote:
> > > dblaikie wrote:
> > > > dblaikie wrote:
> > > > > hoy wrote:
> > > > > > dblaikie wrote:
> > > > > > > hoy wrote:
> > > > > > > > dblaikie wrote:
> > > > > > > > > hoy wrote:
> > > > > > > > > > dblaikie wrote:
> > > > > > > > > > > hoy wrote:
> > > > > > > > > > > > dblaikie wrote:
> > > > > > > > > > > > > hoy wrote:
> > > > > > > > > > > > > > dblaikie wrote:
> > > > > > > > > > > > > > > hoy wrote:
> > > > > > > > > > > > > > > > dblaikie wrote:
> > > > > > > > > > > > > > > > > Does this need to be down here? Or would the
> > > > > > > > > > > > > > > > > code be a well exercised if it was up next to
> > > > > > > > > > > > > > > > > the go declaration above?
> > > > > > > > > > > > > > > > Yes, it needs to be here. Otherwise it will
> > > > > > > > > > > > > > > > just like the function `bar` above that doesn't
> > > > > > > > > > > > > > > > get a uniquefied name. I think moving the
> > > > > > > > > > > > > > > > definition up to right after the declaration
> > > > > > > > > > > > > > > > hides the declaration.
> > > > > > > > > > > > > > > Not sure I follow - do you mean that if the go
> > > > > > > > > > > > > > > declaration and go definition were next to each
> > > > > > > > > > > > > > > other, this test would (mechanically speaking)
> > > > > > > > > > > > > > > not validate what the patch? Or that it would be
> > > > > > > > > > > > > > > less legible, but still mechanically correct?
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > I think it would be (assuming it's still
> > > > > > > > > > > > > > > mechanically correct) more legible to put the
> > > > > > > > > > > > > > > declaration next to the definition - the comment
> > > > > > > > > > > > > > > describes why the declaration is significant/why
> > > > > > > > > > > > > > > the definition is weird, and seeing all that
> > > > > > > > > > > > > > > together would be clearer to me than spreading it
> > > > > > > > > > > > > > > out/having to look further away to see what's
> > > > > > > > > > > > > > > going on.
> > > > > > > > > > > > > > When the `go` declaration and `go` definition were
> > > > > > > > > > > > > > next to each other, the go function won't get a
> > > > > > > > > > > > > > uniqufied name at all. The declaration will be
> > > > > > > > > > > > > > overwritten by the definition. Only when the
> > > > > > > > > > > > > > declaration is seen by others, such the callsite in
> > > > > > > > > > > > > > `baz`, the declaration makes a difference by having
> > > > > > > > > > > > > > the callsite use a uniqufied name.
> > > > > > > > > > > > > >
> > > > > > > > > > > > > >
> > > > > > > > > > > > > Ah! Interesting, good to know.
> > > > > > > > > > > > >
> > > > > > > > > > > > > Is that worth supporting, I wonder? I guess it falls
> > > > > > > > > > > > > out for free/without significant additional
> > > > > > > > > > > > > complexity. I worry about the subtlety of the
> > > > > > > > > > > > > additional declaration changing the behavior here...
> > > > > > > > > > > > > might be a bit surprising/subtle. But maybe no nice
> > > > > > > > > > > > > way to avoid it either.
> > > > > > > > > > > > It would be ideal if user never writes code like that.
> > > > > > > > > > > > Unfortunately it exists with legacy code (such as
> > > > > > > > > > > > mysql). I think it's worth supporting it from AutoFDO
> > > > > > > > > > > > point of view to avoid a silent mismatch between debug
> > > > > > > > > > > > linkage name and real linkage name.
> > > > > > > > > > > Oh, I agree that we shouldn't mismatch debug info and the
> > > > > > > > > > > actual symbol name - what I meant was whether code like
> > > > > > > > > > > this should get mangled or not when using
> > > > > > > > > > > unique-internal-linkage names.
> > > > > > > > > > >
> > > > > > > > > > > I'm now more curious about this:
> > > > > > > > > > >
> > > > > > > > > > > > When the `go` declaration and `go` definition were next
> > > > > > > > > > > > to each other, the go function won't get a uniqufied
> > > > > > > > > > > > name at all.
> > > > > > > > > > >
> > > > > > > > > > > This doesn't seem to happen with the
> > > > > > > > > > > `__attribute__((overloadable))` attribute, for instance -
> > > > > > > > > > > so any idea what's different about uniquification that's
> > > > > > > > > > > working differently than overloadable?
> > > > > > > > > > >
> > > > > > > > > > > ```
> > > > > > > > > > > $ cat test.c
> > > > > > > > > > > __attribute__((overloadable)) static int go(a) int a; {
> > > > > > > > > > > return 3 + a;
> > > > > > > > > > > }
> > > > > > > > > > > void baz() {
> > > > > > > > > > > go(2);
> > > > > > > > > > > }
> > > > > > > > > > > $ clang-tot test.c -emit-llvm -S -o - | grep go
> > > > > > > > > > > %call = call i32 @_ZL2goi(i32 2)
> > > > > > > > > > > define internal i32 @_ZL2goi(i32 %a) #0 {
> > > > > > > > > > > ```
> > > > > > > > > > Good question. I'm not sure what's exactly going on but it
> > > > > > > > > > looks like with the overloadable attribute, the old-style
> > > > > > > > > > definition is treated as having prototype. But if you do
> > > > > > > > > > this:
> > > > > > > > > >
> > > > > > > > > > ```
> > > > > > > > > > __attribute__((overloadable))
> > > > > > > > > > void baz() {}
> > > > > > > > > > ```
> > > > > > > > > > then there's the error:
> > > > > > > > > >
> > > > > > > > > > ```
> > > > > > > > > > error: 'overloadable' function 'baz' must have a prototype
> > > > > > > > > > void baz() {
> > > > > > > > > > ```
> > > > > > > > > >
> > > > > > > > > > `void baz() {` does not come with a prototype. That's for
> > > > > > > > > > sure. Sounds like `int go(a) int a {;` can have a
> > > > > > > > > > prototype when it is loadable. I'm wondering why it's not
> > > > > > > > > > always treated as having prototype, since the parameter
> > > > > > > > > > type is there.
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > Yeah, that seems like that divergence be worth understanding
> > > > > > > > > (& if possible fixing/avoiding/merging). Ensuring these
> > > > > > > > > features don't have subtle divergence I think will be
> > > > > > > > > valuable to having a model that's easy to
> > > > > > > > > explain/understand/modify/etc.
> > > > > > > > I took another look. I think the divergence comes from
> > > > > > > > `getAs<FunctionProtoType>` vs `hasPrototype`. The debug data
> > > > > > > > generation uses `hasPrototype` while `getAs<FunctionProtoType>`
> > > > > > > > is used as overloadable attribute processing as long as unique
> > > > > > > > linkage name processing before this change. More specifically,
> > > > > > > > the following function definition is represented by
> > > > > > > > `FunctionProtoType` while it does not `hasPrototype`.
> > > > > > > >
> > > > > > > > ```
> > > > > > > > static int go(a) int a; {
> > > > > > > > return 3 + a;
> > > > > > > > }
> > > > > > > > ```
> > > > > > > >
> > > > > > > > I was trying to have `CGDebugInfo` to check `FunctionProtoType`
> > > > > > > > instead of `hasPrototype`. While it works for the code pattern
> > > > > > > > in discussion, it also breaks other tests including objectC
> > > > > > > > tests. More investigation is needed to understand what each
> > > > > > > > term really means.
> > > > > > > Are you undertaking that investigation? It'd be good to address
> > > > > > > this divergence if possible.
> > > > > > >
> > > > > > > (@aprantl or @rsmith maybe you know something about this ObjC
> > > > > > > thing? )
> > > > > > Haven't figured out anything useful yet. As far as I can tell, the
> > > > > > debug info generation code is shared between C++ and ObjC. Using
> > > > > > `getAs<FunctionProtoType>` works for C++ but not for ObjectC where
> > > > > > it crashes when computing a mangled name for something like
> > > > > >
> > > > > >
> > > > > > ```
> > > > > > void test() {
> > > > > > __block A a;
> > > > > > ^{ (void)a; };
> > > > > > }
> > > > > >
> > > > > > ```
> > > > > >
> > > > > > Below are the failing tests which are all like that:
> > > > > >
> > > > > > Clang :: CodeGenCXX/cp-blocks-linetables.cpp
> > > > > > Clang :: CodeGenCXX/debug-info-block-invocation-linkage-name.cpp
> > > > > > Clang :: CodeGenCXX/debug-info-blocks.cpp
> > > > > > Clang :: CodeGenObjCXX/nested-ehlocation.mm
> > > > > > Clang :: CodeGenObjCXX/property-objects.mm
> > > > > > Clang :: CodeGenObjCXX/synthesized-property-cleanup.mm
> > > > > >
> > > > > >
> > > > > > cc @bruno
> > > > > Ping on this - anyone got a chance to take a look? It'd be great to
> > > > > avoid this subtle inconsistency.
> > > > Ping again
> > > I tried a different route instead of using `getAs<FunctionProtoType>` in
> > > debug info generation which beaks the blocks function and objectC cases.
> > > Since the problem here is that the old-C function (`bar` in the test
> > > case) is not considered `hasPrototype`, I tried to unify `isKNRPrototype`
> > > and `hasPrototype` so that old-C functions are considered `hasPrototype`.
> > > It works for the name mangler but it breaks other places where
> > > `isKNRPrototype` should be excluded from `hasPrototype`.
> > Hrm - I'd really like to get to the bottom of this, but not sure who to
> > pull in.
> >
> > @aaron.ballman - do you know who might have some idea of how these
> > different old KNR C declarations work, and how this code might be made more
> > robust?
> Ugh, prototypes. They're not particularly well specified in the C standard
> IMHO. In C, a function with a prototype is one that declares the types of its
> parameters (C17 6.2.1p2) which is further clarified to be a function type
> with a parameter type list explicitly (C17 6.2.7p3, 6.9.1p7). However, the
> very end of 6.9.1p7 goes on to say that once you see the definition of the
> function, you know about its parameter type information, but it doesn't
> clarify whether this means the function now has a prototype or not.
>
> The result of this is that:
> ```
> void f();
> void call_it_once(void) { f(1.2f); }
>
> void f(a) float a; {}
> void call_if_twice(void) { f(1.2f); }
> ```
> in `call_it_once`, the argument is promoted to a double, while in
> `call_it_twice`, the argument is not. I suspect we're hitting another
> variation of this confusing behavior here.
>
@aaron.ballman Thanks for taking a look.
Do you know if/how this code could be phrased so that this code:
```
static int go(int);
void baz() {
foo();
bar(1);
go(2);
}
static int go(a) int a;
{
return glob + a;
}
```
Could test some property of `go` at the function call site that would be
consistent whether the definition of `go` came before or after the call site?
It seems currently this code behaves differently depending on that order and I
think that's a bit of a sharp corner it'd be good not to have, if there's a
tidier/more consistent way to phrase the code.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D98799/new/
https://reviews.llvm.org/D98799
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