[clang] [clang] Note that optnone and target attributes do not apply to nested functions (PR #82815)

[Reid Kleckner via cfe-commits]

https://github.com/rnk created https://github.com/llvm/llvm-project/pull/82815

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.

>From 4cbb7fa32fd87e1add520a53d8ecc7443a00a29a Mon Sep 17 00:00:00 2001
From: Reid Kleckner <rnk at google.com>
Date: Thu, 22 Feb 2024 17:55:30 +0000
Subject: [PATCH] [clang] Note that optnone and target attributes do not apply
 to nested functions

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 funciton-wide attributes. There may be more,
these were the two that came to mind.
---
 clang/include/clang/Basic/AttrDocs.td | 9 +++++++++
 1 file changed, 9 insertions(+)

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.
   }];
 }