[clang][PPC] Supporting -mcpu=405

[qiongsiwu1]

Summary

The ClangBuiltLinux project relies on -mcpu=405. Before https://reviews.llvm.org/D139720, clang treated -mcpu=405 implicitly in the same way as -mcpu=generic, because 405 was an unknown value and clang did not validate unknown input values. https://reviews.llvm.org/D139720 added the validation of -mcpu input value, and clang now generates an error with -mcpu=405. For further details of the problem, see https://github.com/ClangBuiltLinux/linux/issues/1771.

This patch adds support of -mcpu=405 explicitly, and treats it as an equivalent of -mcpu=generic.

[nemanjai]

I don't think the comments require another round of review. Thanks for fixing this.

clang/lib/Driver/ToolChains/Arch/PPC.cpp

23–29

While I understand the motivation here, I think that this is overkill. We in fact hope not to have to grow the set of CPU ID's that we treat as generic.

42

I think we should change this to check for generic or 405 and add a comment similar to the following:

// Clang/LLVM does not actually support code generation
// for the 405 CPU. However, there are uses of this CPU ID
// in projects that previously used GCC and rely on Clang
// accepting it. Clang has always ignored it and passed the
// generic CPU ID to the back end.

[qiongsiwu1]

Addressing review comments. Thanks for the review @nemanjai !!

[qiongsiwu1]

The pre-commit CI crashes do not seem to relate to this patch. I will land it soon.

clang/lib/Driver/ToolChains/Arch/PPC.cpp

 #include "llvm/Option/ArgList.h"
 #include "llvm/Support/Host.h"
 
 using namespace clang::driver;
 using namespace clang::driver::tools;
 using namespace clang;
 using namespace llvm::opt;
 
 static std::string getPPCGenericTargetCPU(const llvm::Triple &T) {
   // LLVM may default to generating code for the native CPU,
   // but, like gcc, we default to a more generic option for
   // each architecture. (except on AIX)
   if (T.isOSAIX())
     return "pwr7";
   else if (T.getArch() == llvm::Triple::ppc64le)
     return "ppc64le";
   else if (T.getArch() == llvm::Triple::ppc64)
     return "ppc64";
   else
     return "ppc";
 }
 
 /// getPPCTargetCPU - Get the (LLVM) name of the PowerPC cpu we are targeting.
 std::string ppc::getPPCTargetCPU(const ArgList &Args, const llvm::Triple &T) {
   if (Arg *A = Args.getLastArg(clang::driver::options::OPT_mcpu_EQ)) {
     StringRef CPUName = A->getValue();
 
-    if (CPUName == "generic")
+    // Clang/LLVM does not actually support code generation
+    // for the 405 CPU. However, there are uses of this CPU ID
+    // in projects that previously used GCC and rely on Clang
+    // accepting it. Clang has always ignored it and passed the
+    // generic CPU ID to the back end.
+    if (CPUName == "generic" || CPUName == "405")
       return getPPCGenericTargetCPU(T);
 
     if (CPUName == "native") {
       std::string CPU = std::string(llvm::sys::getHostCPUName());
       if (!CPU.empty() && CPU != "generic")
         return CPU;
       else
         return getPPCGenericTargetCPU(T);

clang/test/Driver/ppc-cpus.c

 // RUN: %clang -### -c -target powerpc64 %s -mcpu=pwr6 2>&1 | FileCheck %s --check-prefix=NO_PPC64
 // RUN: %clang -### -c -target powerpc64 %s -mcpu=pwr7 2>&1 | FileCheck %s --check-prefix=NO_PPC64
 // RUN: %clang -### -c -target powerpc64 %s -mcpu=pwr8 2>&1 | FileCheck %s --check-prefix=NO_PPC64
 
 // NO_PPC64-NOT: "-target-cpu" "ppc64"
 
 // RUN: %clang -### -c --target=powerpc64 %s -mcpu=generic -mtune=pwr9 2>&1 | FileCheck %s --check-prefix=TUNE
 // TUNE: "-target-cpu" "ppc64" "-tune-cpu" "pwr9"
+
+/// Test mcpu options that are equivalent to "generic"
+// RUN: %clang -### -c -target powerpc64 %s -mcpu=generic 2>&1 | FileCheck %s --check-prefix=GENERIC
+// RUN: %clang -### -c -target powerpc64 %s -mcpu=405     2>&1 | FileCheck %s --check-prefix=GENERIC
+//
+// GENERIC: "-target-cpu" "ppc64"