[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 created https://github.com/llvm/llvm-project/pull/91689

I noticed this when changing some of my local projects to use my preferred driver mode, GCC, instead of the CL driver mode.

>From looking at the rest of code and from my own understanding the driver mode is supposed to be independent of MSVC compatibility when the target triple is `*-windows-msvc`.
Therefore strict aliasing should be disabled by default when the target triple is `*-windows-msvc` so code assuming MSVC behaves as expected when compiled with Clang.

Let me know if this requires a note in the release notes.
It didn't feel like this change belonged under the "Clang Frontend Potentially Breaking Changes" section or the "Windows Support" section.

>From b43a6302009b9a11ef672f3bd01e10342102de44 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] [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 0a2ea96de7382..8286c7ae51276 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -5651,11 +5651,10 @@ 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