[clang][MinGW] Explicitly ignore `-fPIC` & friends

[zero9178]

Summary

GCC on Windows ignores this flag completely [0] which some build systems sadly rely on when compiling for Windows using MinGW. The current behaviour of clang however is to error out as -fPIC & friends has no effect on Windows.

This patch instead changes the behaviour for MinGW to ignore the option for the sake of compatibility

Fixes https://github.com/llvm/llvm-project/issues/52947

[0] https://gcc.gnu.org/legacy-ml/gcc-patches/2015-08/msg00836.html

[mstorsjo]

LGTM, thanks. The fact that GCC used to emit a non-fatal warning for this option, but no longer does, probably is the key deciding factor here.

clang/lib/Driver/ToolChains/CommonArgs.cpp

tools::ParsePICArgs(const ToolChain &ToolChain, const ArgList &Args) {
   // The last argument relating to either PIC or PIE wins, and no
   // other argument is used. If the last argument is any flavor of the
   // '-fno-...' arguments, both PIC and PIE are disabled. Any PIE
   // option implicitly enables PIC at the same level.
   Arg *LastPICArg = Args.getLastArg(options::OPT_fPIC, options::OPT_fno_PIC,
                                     options::OPT_fpic, options::OPT_fno_pic,
                                     options::OPT_fPIE, options::OPT_fno_PIE,
                                     options::OPT_fpie, options::OPT_fno_pie);
-  if (Triple.isOSWindows() && LastPICArg &&
-      LastPICArg ==
-          Args.getLastArg(options::OPT_fPIC, options::OPT_fpic,
+  if (Triple.isOSWindows() && !Triple.isOSCygMing() && LastPICArg &&
+      LastPICArg == Args.getLastArg(options::OPT_fPIC, options::OPT_fpic,
                                     options::OPT_fPIE, options::OPT_fpie)) {
     ToolChain.getDriver().Diag(diag::err_drv_unsupported_opt_for_target)
         << LastPICArg->getSpelling() << Triple.str();
     if (Triple.getArch() == llvm::Triple::x86_64)
       return std::make_tuple(llvm::Reloc::PIC_, 2U, false);
     return std::make_tuple(llvm::Reloc::Static, 0U, false);
   }
 
   // Check whether the tool chain trumps the PIC-ness decision. If the PIC-ness

clang/lib/Driver/ToolChains/MinGW.cpp

bool toolchains::MinGW::isPICDefault() const {
   return getArch() == llvm::Triple::x86_64 ||
          getArch() == llvm::Triple::aarch64;
 }
 
 bool toolchains::MinGW::isPIEDefault(const llvm::opt::ArgList &Args) const {
   return false;
 }
 
-bool toolchains::MinGW::isPICDefaultForced() const {
-  return getArch() == llvm::Triple::x86_64 ||
-         getArch() == llvm::Triple::aarch64;
-}
+bool toolchains::MinGW::isPICDefaultForced() const { return true; }
 
 llvm::ExceptionHandling
 toolchains::MinGW::GetExceptionModel(const ArgList &Args) const {
   if (getArch() == llvm::Triple::x86_64 || getArch() == llvm::Triple::aarch64)
     return llvm::ExceptionHandling::WinEH;
   return llvm::ExceptionHandling::DwarfCFI;
 }
 

clang/test/Driver/pic.c

 // RUN: %clang -c %s -target x86_64-pc-windows-msvc18.0.0 -### 2>&1 \
 // RUN:   | FileCheck %s --check-prefix=CHECK-PIC2
 // RUN: %clang -c %s -target x86_64-pc-windows-gnu -### 2>&1 \
 // RUN:   | FileCheck %s --check-prefix=CHECK-PIC2
 // RUN: %clang -c %s -target aarch64-windows-msvc -### 2>&1 \
 // RUN:   | FileCheck %s --check-prefix=CHECK-PIC2
 // RUN: %clang -c %s -target aarch64-windows-gnu -### 2>&1 \
 // RUN:   | FileCheck %s --check-prefix=CHECK-PIC2
+//
+// On MinGW, allow specifying -fPIC & friends but ignore them
+// RUN: %clang -fno-PIC -c %s -target x86_64-pc-windows-gnu -### 2>&1 \
+// RUN:   | FileCheck %s --check-prefix=CHECK-PIC2
+// RUN: %clang -fPIC -c %s -target i686-pc-windows-gnu -### 2>&1 \
+// RUN:   | FileCheck %s --check-prefix=CHECK-NO-PIC
+// RUN: %clang -fno-PIC -c %s -target aarch64-pc-windows-gnu -### 2>&1 \
+// RUN:   | FileCheck %s --check-prefix=CHECK-PIC2
+// RUN: %clang -fPIC -c %s -target armv7-pc-windows-gnu -### 2>&1 \
+// RUN:   | FileCheck %s --check-prefix=CHECK-NO-PIC

clang/test/Driver/windows-pic.cpp

 // RUN: %clang -### -target i686-windows-itanium -fpic %s 2>&1 | FileCheck -check-prefix CHECK-pic-ERROR %s
 // RUN: %clang -### -target i686-windows-itanium -fPIE %s 2>&1 | FileCheck -check-prefix CHECK-PIE-ERROR %s
 // RUN: %clang -### -target i686-windows-itanium -fpie %s 2>&1 | FileCheck -check-prefix CHECK-pie-ERROR %s
 // RUN: %clang -### -target i686-windows-itanium -fPIC -fno-pic %s
 // RUN: %clang -### -target i686-windows-itanium -Fpic -fno-pic %s
 // RUN: %clang -### -target i686-windows-itanium -fPIE -fno-pie %s
 // RUN: %clang -### -target i686-windows-itanium -fpie -fno-pie %s
 
-// RUN: %clang -### -target i686-windows-gnu -fPIC %s 2>&1 | FileCheck -check-prefix CHECK-PIC-ERROR %s
-// RUN: %clang -### -target i686-windows-gnu -fpic %s 2>&1 | FileCheck -check-prefix CHECK-pic-ERROR %s
-// RUN: %clang -### -target i686-windows-gnu -fPIE %s 2>&1 | FileCheck -check-prefix CHECK-PIE-ERROR %s
-// RUN: %clang -### -target i686-windows-gnu -fpie %s 2>&1 | FileCheck -check-prefix CHECK-pie-ERROR %s
-// RUN: %clang -### -target i686-windows-gnu -fPIC -fno-pic %s
-// RUN: %clang -### -target i686-windows-gnu -Fpic -fno-pic %s
-// RUN: %clang -### -target i686-windows-gnu -fPIE -fno-pie %s
-// RUN: %clang -### -target i686-windows-gnu -fpie -fno-pie %s
-
 // RUN: %clang -### -target x86_64-windows -fPIC %s 2>&1 | FileCheck -check-prefix CHECK-PIC-ERROR %s
 // RUN: %clang -### -target x86_64-windows -fpic %s 2>&1 | FileCheck -check-prefix CHECK-pic-ERROR %s
 // RUN: %clang -### -target x86_64-windows -fPIE %s 2>&1 | FileCheck -check-prefix CHECK-PIE-ERROR %s
 // RUN: %clang -### -target x86_64-windows -fpie %s 2>&1 | FileCheck -check-prefix CHECK-pie-ERROR %s
 // RUN: %clang -### -target x86_64-windows -fPIC -fno-pic %s
 // RUN: %clang -### -target x86_64-windows -Fpic -fno-pic %s
 // RUN: %clang -### -target x86_64-windows -fPIE -fno-pie %s
 // RUN: %clang -### -target x86_64-windows -fpie -fno-pie %s
 
 // RUN: %clang -### -target x86_64-windows-itanium -fPIC %s 2>&1 | FileCheck -check-prefix CHECK-PIC-ERROR %s
 // RUN: %clang -### -target x86_64-windows-itanium -fpic %s 2>&1 | FileCheck -check-prefix CHECK-pic-ERROR %s
 // RUN: %clang -### -target x86_64-windows-itanium -fPIE %s 2>&1 | FileCheck -check-prefix CHECK-PIE-ERROR %s
 // RUN: %clang -### -target x86_64-windows-itanium -fpie %s 2>&1 | FileCheck -check-prefix CHECK-pie-ERROR %s
 // RUN: %clang -### -target x86_64-windows-itanium -fPIC -fno-pic %s
 // RUN: %clang -### -target x86_64-windows-itanium -Fpic -fno-pic %s
 // RUN: %clang -### -target x86_64-windows-itanium -fPIE -fno-pie %s
 // RUN: %clang -### -target x86_64-windows-itanium -fpie -fno-pie %s
 
-// RUN: %clang -### -target x86_64-windows-gnu -fPIC %s 2>&1 | FileCheck -check-prefix CHECK-PIC-ERROR %s
-// RUN: %clang -### -target x86_64-windows-gnu -fpic %s 2>&1 | FileCheck -check-prefix CHECK-pic-ERROR %s
-// RUN: %clang -### -target x86_64-windows-gnu -fPIE %s 2>&1 | FileCheck -check-prefix CHECK-PIE-ERROR %s
-// RUN: %clang -### -target x86_64-windows-gnu -fpie %s 2>&1 | FileCheck -check-prefix CHECK-pie-ERROR %s
-// RUN: %clang -### -target x86_64-windows-gnu -fPIC -fno-pic %s
-// RUN: %clang -### -target x86_64-windows-gnu -Fpic -fno-pic %s
-// RUN: %clang -### -target x86_64-windows-gnu -fPIE -fno-pie %s
-// RUN: %clang -### -target x86_64-windows-gnu -fpie -fno-pie %s
-
 // CHECK-PIC-ERROR: unsupported option '-fPIC' for target '{{.*}}
 // CHECK-pic-ERROR: unsupported option '-fpic' for target '{{.*}}
 // CHECK-PIE-ERROR: unsupported option '-fPIE' for target '{{.*}}
 // CHECK-pie-ERROR: unsupported option '-fpie' for target '{{.*}}