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

[Fangrui Song via cfe-commits]

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

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")))`.


>From 77fac857f70202a5da2cab24b1159dea3911ec6c Mon Sep 17 00:00:00 2001
From: Fangrui Song <i at maskray.me>
Date: Fri, 9 Feb 2024 11:10:39 -0800
Subject: [PATCH] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20initia?=
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Created using spr 1.3.4
---
 clang/lib/AST/Decl.cpp                           |  6 +++++-
 .../test/SemaCXX/attr-target-mv-warn-unused.cpp  | 16 ++++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/SemaCXX/attr-target-mv-warn-unused.cpp

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(); }