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;
+}
+
+
----------------
hoy wrote:
> dblaikie wrote:
> > aaron.ballman wrote:
> > > 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).
> > Thanks so much, @aaron.ballman that does sound exactly like it summarizes
> > the situation and the suggestion sounds like it could be the right thing.
> >
> > @hoy does that all make sense to you/could you try @aaron.ballman's
> > suggested change/fix?
> Thanks for doing the experiment @aaron.ballman . Actually for the test in
> this diff, we would like the function `bar` to have a unique linkage name.
> The suggested change doesn't seem to fix that.
>
> ```
> // bar should not be given a uniquefied name under
> -funique-internal-linkage-names,
> // since it doesn't come with valid prototype.
> static int bar(a) int a;
> {
> return glob + a;
> }
>
> ```
I assume that comment was more written to describe existing practice than some
intentional approach to dealing with a function like this, yeah?
The overloadable attribute seems to be able to mangle this function correctly -
so I think that was my whole concern - that unique internal linkage names
should, ideally, treat things the same as overloadable unless there's a reason
these things are really different - which I don't know of any reason that they
are.
So this looks like it fixes that gap - by enabling unique internal linkage
names to mangle this case the same way overloadable does?
Repository:
rG LLVM Github Monorepo
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