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

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.

Author: MaxEW707

clang/docs/ReleaseNotes.rst

@@ -878,6 +878,10 @@ 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
 ^^^^^^^^^^^^^^^^^
 

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))

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

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