[Clang] Allow parsing arbitrary order of attributes for declarations #133107

DenisGZM · 2025-03-26T15:30:50Z

Enable parsing alignas attribute after GNU attributes, before ParseDeclaration

This might be useful for cuda code where shared and other specificators may be mixed with align.

I'd be glad to see if there are any better places or other technique to process this attribute without interrupting current flow of parsing.

llvmbot · 2025-03-26T15:31:27Z

@llvm/pr-subscribers-clang

Author: Denis.G (DenisGZM)

Changes

Enable parsing alignas attribute after GNU attributes, before ParseDeclaration

This might be useful for cuda code where shared and other specificators may be mixed with align.

I'd be glad to see if there are any better places or other technique to process this attribute without interrupting current flow of parsing.

Full diff: https://github.com/llvm/llvm-project/pull/133107.diff

2 Files Affected:

(modified) clang/lib/Parse/ParseStmt.cpp (+5)
(added) clang/test/SemaCUDA/cuda-attr-order.cu (+15)

diff --git a/clang/lib/Parse/ParseStmt.cpp b/clang/lib/Parse/ParseStmt.cpp
index 150b2879fc94f..33b9f63bcfa08 100644
--- a/clang/lib/Parse/ParseStmt.cpp
+++ b/clang/lib/Parse/ParseStmt.cpp
@@ -296,6 +296,11 @@ StmtResult Parser::ParseStatementOrDeclarationAfterAttributes(
     goto Retry;
   }
 
+  case tok::kw_alignas: {
+    ParseAlignmentSpecifier(CXX11Attrs);
+    goto Retry;
+  }
+
   case tok::kw_template: {
     SourceLocation DeclEnd;
     ParseTemplateDeclarationOrSpecialization(DeclaratorContext::Block, DeclEnd,
diff --git a/clang/test/SemaCUDA/cuda-attr-order.cu b/clang/test/SemaCUDA/cuda-attr-order.cu
new file mode 100644
index 0000000000000..d3bf5b014d1c6
--- /dev/null
+++ b/clang/test/SemaCUDA/cuda-attr-order.cu
@@ -0,0 +1,15 @@
+// Verify that we can parse a simple CUDA file with different attributes order.
+// RUN: %clang_cc1 "-triple" "nvptx-nvidia-cuda"  -fsyntax-only -verify %s
+// expected-no-diagnostics
+#include "Inputs/cuda.h"
+
+struct alignas(16) float4 {
+    float x, y, z, w;
+};
+
+__attribute__((device)) float func() {
+    __shared__ alignas(alignof(float4)) float As[4][4];  // Both combinations
+    alignas(alignof(float4)) __shared__  float Bs[4][4]; // must be legal
+
+    return As[0][0] + Bs[0][0];
+}

Sirraide

At a glance this does seem like the right place to do this, but this is still missing a release note.

It seems like GCC allows e.g. __attribute__(()) alignas(16) int x in any case, so I don’t see why we shouldn’t allow this too. Can you also add some tests that use __attribute__(()) directly and which aren’t CUDA-specific?

Oh, and can you add solmething like this as a test as well:

struct S { __attribute__((deprecated)) alignas(16) int x; };

Sirraide · 2025-03-27T18:32:37Z

CC @erichkeane in case there’s a specific reason I’m not aware of as to why we currently don’t allow this.

DenisGZM · 2025-03-28T10:17:27Z

At a glance this does seem like the right place to do this, but this is still missing a release note.

It seems like GCC allows e.g. __attribute__(()) alignas(16) int x in any case, so I don’t see why we shouldn’t allow this too. Can you also add some tests that use __attribute__(()) directly and which aren’t CUDA-specific?

Oh, and can you add solmething like this as a test as well:
struct S { __attribute__((deprecated)) alignas(16) int x; };

Actually this test doesn't work with this patch...

In this case all attributes are processed in ParseDeclarationSpecifiers, which in my first view was the right place to fix, but has way more complicated logic and easy to break diagnostics.

In ParseDeclarationSpecifiers we parse kw__attributes and other CXX11 Attributes and set bool AttrsLastTime = true to check that last parsed piece was attr. Later this block prohibit attributes with AttrsLastTime = false,

ParseDecl.cpp

    DoneWithDeclSpec:
      if (!AttrsLastTime)
        ProhibitAttributes(attrs);

And AttrsLastTime is always false in declarations of the form: <attributes> <type> <identifier> , because last token we parse is type

Another approach i tried is to add processing alignas-cxx11 just like it is done for C: kw__Alignas and kw_alignas (c23).
Well, it do the parsing but later it skips CXX11 attributes when correcting declaration type (assumed that attributes must have been processed before)

Sirraide · 2025-03-28T14:06:19Z

Hmm, @erichkeane probably knows where this needs to be parsed then; I might take another look at this myself later (because I’m not sure either off the top of my head), but I’m rather busy today unfortunately...

…MemberDeclaration

DenisGZM · 2025-03-28T15:36:13Z

I added parsing all attributes in ParseCXXClassMemberDeclaration before calling ParseDeclarationSpecifiers and it seems to solve problem, but it also changes annotation ranges for struct and class members

Sirraide

Seems reasonable to me, but I’d still like @erichkeane to take a look at this as the attributes code owner

erichkeane

Parsing of attributes is admittedly the part I'm least comfortable with here. I would love tests for how this interacts with our __declspec spelling attributes though, and to help determine why we wouldn't parse all 3 together here.

As a followup/future direction for some one, there is perhaps value of a MaybeParseAnyAttributes that does all 3 in a loop.

clang/lib/Parse/ParseDeclCXX.cpp

erichkeane

I think this looks reasonable? I would like @AaronBallman to stop by though, he might think of some reason why this isn't right per-grammar.

AaronBallman

Thanks for the fix! The changes should come with a release note in clang/docs/ReleaseNotes.rst so users know about the fix.

clang/lib/Parse/ParseStmt.cpp

clang/lib/Parse/ParseDeclCXX.cpp

clang/test/Parser/cxx0x-attributes.cpp

DenisGZM · 2025-04-08T18:40:21Z

Reworked approach for parsing.

We needed to support arbitrary attribute parsing rather than just alignas for CXX. So please check new commit

github-actions · 2025-04-08T19:36:28Z

✅ With the latest revision this PR passed the C/C++ code formatter.

clang/include/clang/Parse/Parser.h

clang/test/Parser/c2x-alignas.c

AaronBallman · 2025-04-15T16:19:51Z

clang/test/Index/annotate-attribute.cpp

@@ -24,7 +24,7 @@ int templateFunction(T value) __attribute__((annotate("works")));

 // CHECK: ClassDecl=Test:3:7 (Definition) Extent=[3:1 - 17:2]
 // CHECK-NEXT: CXXAccessSpecifier=:4:1 (Definition) Extent=[4:1 - 4:8]
-// CHECK-NEXT: CXXMethod=aMethod:5:51 Extent=[5:3 - 5:60]
+// CHECK-NEXT: CXXMethod=aMethod:5:51 Extent=[5:46 - 5:60]


This means we went from pointing to the start of __attribute__ to pointing to the start of void which is a bit unfortunate.

Do you think we should avoid it somehow? Or just accept it as is?

I'm on the fence. It's not the worst regression in behavior, but it does make the diagnostic slightly harder for users to reason about. WDYT @erichkeane ?

its really quite unfortunate... I think it is at least worth seeing how much work needs to be done to get this 'right', and see if it is worth the effort.

The main problem here is to determine 'right' :)
Now I set annotated declaration range at the begining of the first parsed attribute and it might be not only DeclSpecAttr.
Earlier we parsed only CXX attrs before ParseDeclarationSpecifiers and then annotated range could only contain DeclSpecAttrs in it.

Examples:

class Test { public: __attribute__((annotate("spiffy_method"))) [[deprecated]] void aMethod(); // Error before, now: Extent=[5:3 - 5:75] }; class Test { public: [[deprecated]] __attribute__((annotate("spiffy_method"))) void aMethod(); // Before: Extent=[5:18 - 5:75], now: Extent=[5:3 - 5:75] };

Is it what expected to be done?

I think the underlying issue is that we're using in-band information about source ranges that's no longer true. We used to be able to rely on the source range because the order was more strict, but as we've relaxed it, you can now mix declaration and decl specifier attributes in more exotic ways.

However, addressing that may be quite involved. So I think we should probably accept this as-is; pointing to the start of the list is better than pointing to the type.

erichkeane

What do we think @AaronBallman ? I think the diagnostic column is as best effort as we are going to get, so I'm OK with this as-is. WE could perhaps improve that, but I don't think doing that here is worth the effort.

I'll approve, but I want to make sure Aaron has a chance to say otherwise before merging.

AaronBallman · 2025-05-07T17:50:17Z

What do we think @AaronBallman ? I think the diagnostic column is as best effort as we are going to get, so I'm OK with this as-is. WE could perhaps improve that, but I don't think doing that here is worth the effort.

I'll approve, but I want to make sure Aaron has a chance to say otherwise before merging.

Yeah, I think I can live with this. I think not supporting the arbitrary order is more annoying to users than a slight degradation in source location reporting.

AaronBallman · 2025-05-09T10:59:48Z

Do you need us to land the changes on your behalf, btw?

DenisGZM · 2025-05-09T16:41:49Z

Do you need us to land the changes on your behalf, btw?

Yeah, why not

[CLANG] Enable alignas after GNU attributes

c46eda6

llvmbot added clang Clang issues not falling into any other category clang:frontend Language frontend issues, e.g. anything involving "Sema" labels Mar 26, 2025

Sirraide reviewed Mar 27, 2025

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Sirraide requested review from AaronBallman and erichkeane March 27, 2025 18:31

[CLANG] Enable cxx11 attributes after gnu attributes in ParseCXXClass…

517ba9e

…MemberDeclaration

Sirraide reviewed Mar 31, 2025

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erichkeane reviewed Apr 1, 2025

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clang/lib/Parse/ParseDeclCXX.cpp Outdated Show resolved Hide resolved

[CLANG] Move microsoft attrs into loop

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erichkeane approved these changes Apr 4, 2025

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AaronBallman reviewed Apr 4, 2025

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clang/lib/Parse/ParseStmt.cpp Outdated Show resolved Hide resolved

clang/lib/Parse/ParseDeclCXX.cpp Show resolved Hide resolved

clang/lib/Parse/ParseDeclCXX.cpp Show resolved Hide resolved

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[CLANG] Arbitrary attribute order

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DenisGZM requested a review from erichkeane April 8, 2025 18:40

Merge branch 'main' into alignas-order

7388c93

Denis Gerasimov added 2 commits April 8, 2025 23:37

Style fix

4d0a4f0

[CLANG] Added more tests, removed range assert

e517ff6

DenisGZM requested review from AaronBallman and Sirraide April 10, 2025 10:01

Merge branch 'main' into alignas-order

ebd962d

AaronBallman reviewed Apr 15, 2025

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Denis Gerasimov added 2 commits April 16, 2025 10:47

review fixes

453f5b2

Merge remote-tracking branch 'upstream/main' into alignas-order

c5bc577

Denis Gerasimov added 2 commits April 26, 2025 14:03

[CLANG] Set DeclStart at start of parsed attributes

5c8ec3c

Merge branch 'main' into alignas-order

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DenisGZM changed the title ~~[CLANG] Enable alignas after GNU attributes~~ [CLANG] Allow parsing arbitrary order of attributes for declarations Apr 26, 2025

Merge branch 'main' into alignas-order

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erichkeane approved these changes May 6, 2025

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Merge branch 'main' into alignas-order

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Merge branch 'main' into alignas-order

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cor3ntin changed the title ~~[CLANG] Allow parsing arbitrary order of attributes for declarations~~ [Clang] Allow parsing arbitrary order of attributes for declarations May 9, 2025

cor3ntin mentioned this pull request May 9, 2025

Clang 17 and later rejects __attribute__ after static specifier in C++23 lambda expressions #114026

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Merge branch 'main' into alignas-order

4922d9e

AaronBallman merged commit b3a6d43 into llvm:main May 9, 2025
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[Clang] Allow parsing arbitrary order of attributes for declarations #133107

[Clang] Allow parsing arbitrary order of attributes for declarations #133107

DenisGZM commented Mar 26, 2025

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[Clang] Allow parsing arbitrary order of attributes for declarations #133107

[Clang] Allow parsing arbitrary order of attributes for declarations #133107

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