[clang] Note that optnone and target attributes do not apply to nested functions

[rnk]

This behavior is true for all attributes, but this behavior can be surprising for attributes which have function-wide effects, such as optnone and target. Most other function attributes affect the prototype or semantics, but do not affect code generation in the function body. I believe it is worth calling this out in the documentation of these function-wide attributes. There may be more, these were the two that came to mind.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Reid Kleckner (rnk)

Changes

This behavior is true for all attributes, but this behavior can be surprising for attributes which have function-wide effects, such as optnone and target. Most other function attributes affect the prototype or semantics, but do not affect code generation in the function body. I believe it is worth calling this out in the documentation of these function-wide attributes. There may be more, these were the two that came to mind.

---

Full diff: https://github.com/llvm/llvm-project/pull/82815.diff

1 Files Affected:

• (modified) clang/include/clang/Basic/AttrDocs.td (+9)

diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
index b96fbddd51154c..3c58256c77201f 100644
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -2486,6 +2486,9 @@ Example "subtarget features" from the x86 backend include: "mmx", "sse", "sse4.2
 "avx", "xop" and largely correspond to the machine specific options handled by
 the front end.
 
+Note that this attribute does not apply transitively to nested functions such
+as blocks or C++ lambdas.
+
 Additionally, this attribute supports function multiversioning for ELF based
 x86/x86-64 targets, which can be used to create multiple implementations of the
 same function that will be resolved at runtime based on the priority of their
@@ -3714,6 +3717,12 @@ for that function.
 
 This attribute is incompatible with the ``always_inline`` and ``minsize``
 attributes.
+
+Note that this attribute does not apply recursively to nested functions such as
+lambdas or blocks when using declaration-specific attribute syntaxes such as double
+square brackets (``[[]]``) or ``__attribute__``. The ``#pragma`` syntax can be
+used to apply the attribute to all functions, including nested functions, in a
+range of source code.
   }];
 }
 

[sam-mccall]

Makes sense to me. If this ends up applying to lots of attributes then maybe we should hoist the text somewhere common, but agree that it doesn't seem to.

[cor3ntin]

@rnk @sam-mccall Should we still merge this?

[rnk]

Thanks for the reminder, I missed the update.