aaron.ballman 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;
+}
+
+
----------------
dblaikie wrote:
> 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.
To be honest, I wasn't aware this code was even valid (where you mix and match
between identifier lists and parameter type lists), so that's neat.
I might be confused, but in my tests, the behavior of
`collectFunctionDeclProps()` is the same regardless of order, but the calls to
`isUniqueInternalLinkageDecl()` are different. Given an invocation of: `-cc1
-triple x86_64-unknown-linux -debug-info-kind=limited -dwarf-version=4
-funique-internal-linkage-names -emit-llvm -o - test.c` with test.c as:
```
static int go(int);
static int go(a) int a;
{
return 1 + a;
}
void baz() {
go(2);
}
```
I see `isUniqueInternalLinkageDecl()` gets called:
* once for `go` with no prototype
* once for `baz` with no prototype
and `collectFunctionDeclProps()` gets called:
* once for `baz` with no prototype
* once for `go` with a prototype
The end result is that I see `!13 = distinct !DISubprogram(name: "go", scope:
!8, file: !8, line: 3, type: !14, scopeLine: 4, flags: DIFlagPrototyped,
spFlags: DISPFlagLocalToUnit | DISPFlagDefinition, unit: !0, retainedNodes:
!2)` in the output.
However, with test.c as:
```
static int go(int);
void baz() {
go(2);
}
static int go(a) int a;
{
return 1 + a;
}
```
I see `isUniqueInternalLinkageDecl()` gets called:
* once for `baz` with no prototype
* once for `go` with a prototype
* another one for `go` with a prototype
and `collectFunctionDeclProps()` gets called:
* once for `baz` with no prototype
* once for `go` with a prototype
The end result is that I see `!13 = distinct !DISubprogram(name: "go",
linkageName: "_ZL2goi.__uniq.39558841650144213141281977295187289852", scope:
!8, file: !8, line: 7, type: !14, scopeLine: 8, flags: DIFlagPrototyped,
spFlags: DISPFlagLocalToUnit | DISPFlagDefinition, unit: !0, retainedNodes: !2)`
When I change `isUniqueInternalLinkageDecl()` to use
`!FD->getCanonicalDecl()->hasPrototype()`, I get the same behavior with either
ordering. When I run the full test suite with that change, I get no test
failures, so that may be a reasonable fix worth investigating (I'm not super
familiar with the ins and outs of name mangling and whether this change would
be correct or not).
Repository:
rG LLVM Github Monorepo
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https://reviews.llvm.org/D98799/new/
https://reviews.llvm.org/D98799
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