[clang] [Sema] Warn unused functions for FMV based on the target attribute (PR #81302)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Fangrui Song (MaskRay)

<details>
<summary>Changes</summary>

The spurious -Wunused-function warning issue for `target_version` #<!-- -->80227
also applied to `__attribute__((target(...)))` based FMV. #<!-- -->81167 removed
warnings for all `target`-based FMV. This patch restores the warnings
for `__attribute__((target("default")))`.


---
Full diff: https://github.com/llvm/llvm-project/pull/81302.diff


2 Files Affected:

- (modified) clang/lib/AST/Decl.cpp (+5-1) 
- (added) clang/test/SemaCXX/attr-target-mv-warn-unused.cpp (+16) 


``````````diff
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index e281f2d8874a7..5d6bb72a208a1 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -3538,7 +3538,11 @@ bool FunctionDecl::isTargetMultiVersion() const {
 }
 
 bool FunctionDecl::isTargetMultiVersionDefault() const {
-  return isMultiVersion() && hasAttr<TargetVersionAttr>() &&
+  if (!isMultiVersion())
+    return false;
+  if (hasAttr<TargetAttr>())
+    return getAttr<TargetAttr>()->isDefaultVersion();
+  return hasAttr<TargetVersionAttr>() &&
          getAttr<TargetVersionAttr>()->isDefaultVersion();
 }
 
diff --git a/clang/test/SemaCXX/attr-target-mv-warn-unused.cpp b/clang/test/SemaCXX/attr-target-mv-warn-unused.cpp
new file mode 100644
index 0000000000000..1901589ef732f
--- /dev/null
+++ b/clang/test/SemaCXX/attr-target-mv-warn-unused.cpp
@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -fsyntax-only -verify -Wunused %s
+
+__attribute__((target("sse3")))
+static int not_used_fmv() { return 1; }
+__attribute__((target("avx2")))
+static int not_used_fmv() { return 2; }
+__attribute__((target("default")))
+static int not_used_fmv() { return 0; } // expected-warning {{unused function 'not_used_fmv'}}
+
+__attribute__((target("sse3")))
+static int definitely_used_fmv() { return 1; }
+__attribute__((target("avx2")))
+static int definitely_used_fmv() { return 2; }
+__attribute__((target("default")))
+static int definitely_used_fmv() { return 0; }
+int definite_user() { return definitely_used_fmv(); }

``````````

</details>


https://github.com/llvm/llvm-project/pull/81302