[clang] [clang][Driver] Fix enabling strict alising by default when the environment is MSVC (PR #91689)

[Max Winkler via cfe-commits]

https://github.com/MaxEW707 updated https://github.com/llvm/llvm-project/pull/91689

>From 3b0967e0a8f564d761ea10858854776ede396bdf Mon Sep 17 00:00:00 2001
From: MaxEW707 <max.enrico.winkler at gmail.com>
Date: Thu, 9 May 2024 21:21:22 -0400
Subject: [PATCH 1/2] [clang][Driver] Fix enabling strict alising by default
 when the environment is MSVC

---
 clang/lib/Driver/ToolChains/Clang.cpp | 5 ++---
 clang/test/Driver/Ofast.c             | 7 +++++--
 clang/test/Driver/clang_f_opts.c      | 6 ++++++
 3 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index c3e6d563f3bd2..4ca13f20b095d 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -5681,11 +5681,10 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   // enabled.  This alias option is being used to simplify the hasFlag logic.
   OptSpecifier StrictAliasingAliasOption =
       OFastEnabled ? options::OPT_Ofast : options::OPT_fstrict_aliasing;
-  // We turn strict aliasing off by default if we're in CL mode, since MSVC
+  // We turn strict aliasing off by default if we're Windows MSVC since MSVC
   // doesn't do any TBAA.
-  bool TBAAOnByDefault = !D.IsCLMode();
   if (!Args.hasFlag(options::OPT_fstrict_aliasing, StrictAliasingAliasOption,
-                    options::OPT_fno_strict_aliasing, TBAAOnByDefault))
+                    options::OPT_fno_strict_aliasing, !IsWindowsMSVC))
     CmdArgs.push_back("-relaxed-aliasing");
   if (!Args.hasFlag(options::OPT_fstruct_path_tbaa,
                     options::OPT_fno_struct_path_tbaa, true))
diff --git a/clang/test/Driver/Ofast.c b/clang/test/Driver/Ofast.c
index 1f9fc78ec1ef8..8b7f2217eca2f 100644
--- a/clang/test/Driver/Ofast.c
+++ b/clang/test/Driver/Ofast.c
@@ -3,7 +3,9 @@
 // RUN: %clang -fno-fast-math -Ofast -### %s 2>&1 | FileCheck -check-prefix=CHECK-OFAST %s
 // RUN: %clang -fno-strict-aliasing -Ofast -### %s 2>&1 | FileCheck -check-prefix=CHECK-OFAST %s
 // RUN: %clang -fno-vectorize -Ofast -### %s 2>&1 | FileCheck -check-prefix=CHECK-OFAST %s
-// RUN: %clang -Ofast -O2 -### %s 2>&1 | FileCheck -check-prefix=CHECK-OFAST-O2 %s
+// RUN: %clang -Ofast -O2 -### %s 2>&1 | FileCheck -check-prefix=CHECK-OFAST-O2 \
+// RUN:  %if target={{.*-windows-msvc.*}} %{ --check-prefix=CHECK-OFAST-O2-ALIASING-MSVC %} \
+// RUN:  %else %{ --check-prefix=CHECK-OFAST-O2-ALIASING %} %s
 // RUN: %clang -Ofast -fno-fast-math -### %s 2>&1 | FileCheck -check-prefix=CHECK-OFAST-NO-FAST-MATH %s
 // RUN: %clang -Ofast -fno-strict-aliasing -### %s 2>&1 | FileCheck -check-prefix=CHECK-OFAST-NO-STRICT-ALIASING %s
 // RUN: %clang -Ofast -fno-vectorize -### %s 2>&1 | FileCheck -check-prefix=CHECK-OFAST-NO-VECTORIZE %s
@@ -15,7 +17,8 @@
 // CHECK-OFAST: -vectorize-loops
 
 // CHECK-OFAST-O2: -cc1
-// CHECK-OFAST-O2-NOT: -relaxed-aliasing
+// CHECK-OFAST-O2-ALIASING-NOT: -relaxed-aliasing
+// CHECK-OFAST-O2-ALIASING-MSVC: -relaxed-aliasing
 // CHECK-OFAST-O2-NOT: -ffast-math
 // CHECK-OFAST-O2-NOT: -Ofast
 // CHECK-OFAST-O2: -vectorize-loops
diff --git a/clang/test/Driver/clang_f_opts.c b/clang/test/Driver/clang_f_opts.c
index 472d0725a7934..d69cd199ac61d 100644
--- a/clang/test/Driver/clang_f_opts.c
+++ b/clang/test/Driver/clang_f_opts.c
@@ -623,3 +623,9 @@
 // RUN: %clang -### --target=aarch64-windows-msvc -fno-ms-volatile %s 2>&1 | FileCheck -check-prefix=CHECK-NO-MS-VOLATILE %s
 // CHECK-MS-VOLATILE: -fms-volatile
 // CHECK-NO-MS-VOLATILE-NOT: -fms-volatile
+
+// RUN: %clang -### --target=x86_64-pc-windows-msvc %s 2>&1 | FileCheck -check-prefix=CHECK-NO-STRICT-ALIASING %s
+// RUN: %clang -### --target=x86_64-pc-windows-msvc -fstrict-aliasing %s 2>&1 | FileCheck -check-prefix=CHECK-STRICT-ALIASING %s
+// RUN: %clang -### --target=x86_64-pc-windows-msvc -fno-strict-aliasing %s 2>&1 | FileCheck -check-prefix=CHECK-NO-STRICT-ALIASING %s
+// CHECK-STRICT-ALIASING-NOT: -relaxed-aliasing
+// CHECK-NO-STRICT-ALIASING: -relaxed-aliasing

>From 6fbe00582c46b86020322eab04bdca3e57825d04 Mon Sep 17 00:00:00 2001
From: MaxEW707 <max.enrico.winkler at gmail.com>
Date: Thu, 23 May 2024 19:24:21 -0400
Subject: [PATCH 2/2] Add a release note

---
 clang/docs/ReleaseNotes.rst | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 65d191b6161a4..ca513ebd61947 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -810,6 +810,10 @@ Windows Support
   including STL headers will no longer slow down compile times since ``intrin.h``
   is not included from MSVC STL.
 
+- When the target triple is `*-windows-msvc` strict aliasing is now disabled by default
+  to ensure compatibility with msvc. Previously strict aliasing was only disabled if the
+  driver mode was cl.
+
 LoongArch Support
 ^^^^^^^^^^^^^^^^^