[X86][CET] Add Diags for targets pre to i686 for `-fcf-protection`

[pengfei]

Summary

Intel Control-flow Enforcement Technology (CET) provides new instructions endbr32/64 for the indirect branch control. They are NOPs on i686 and new targets. We need to check for that in case it crashes on older targets.

Fixes #58737

[pengfei]

Fix lit fail.

[nickdesaulniers]

Seems fine, though looking at llvm::X86::CPUKind, why do you choose llvm::X86::CK_PentiumPro as the lower bounds? Surely llvm::X86::CK_Pentium2 doesn't support CET?

Does the backend already have an equivalent CPU target check that could be reused by clang?

[craig.topper]

In D137317#3905643, @nickdesaulniers wrote:

Seems fine, though looking at llvm::X86::CPUKind, why do you choose llvm::X86::CK_PentiumPro as the lower bounds? Surely llvm::X86::CK_Pentium2 doesn't support CET?

Does the backend already have an equivalent CPU target check that could be reused by clang?

My understanding was that the CET encodings reuse some encodings that have been equivalent to multibyte NOPs since PentiumPro. I think the intent was that a CET mitigated binary would still run on a CPU that didn't support CET.

[nickdesaulniers]

In D137317#3905654, @craig.topper wrote:

In D137317#3905643, @nickdesaulniers wrote:

Seems fine, though looking at llvm::X86::CPUKind, why do you choose llvm::X86::CK_PentiumPro as the lower bounds? Surely llvm::X86::CK_Pentium2 doesn't support CET?

Does the backend already have an equivalent CPU target check that could be reused by clang?

My understanding was that the CET encodings reuse some encodings that have been equivalent to multibyte NOPs since PentiumPro. I think the intent was that a CET mitigated binary would still run on a CPU that didn't support CET.

Makes sense; consider adding that additional information as context to the commit message @pengfei .

[pengfei]

Thanks @nickdesaulniers and @craig.topper!

clang/lib/Basic/Targets/X86.h

public:
   bool validateOutputSize(const llvm::StringMap<bool> &FeatureMap,
                           StringRef Constraint, unsigned Size) const override;
 
   bool validateInputSize(const llvm::StringMap<bool> &FeatureMap,
                          StringRef Constraint, unsigned Size) const override;
 
   bool
   checkCFProtectionReturnSupported(DiagnosticsEngine &Diags) const override {
+    if (CPU == llvm::X86::CK_None || CPU >= llvm::X86::CK_PentiumPro)
       return true;
+    return TargetInfo::checkCFProtectionReturnSupported(Diags);
   };
 
   bool
   checkCFProtectionBranchSupported(DiagnosticsEngine &Diags) const override {
+    if (CPU == llvm::X86::CK_None || CPU >= llvm::X86::CK_PentiumPro)
       return true;
+    return TargetInfo::checkCFProtectionBranchSupported(Diags);
   };
 
   virtual bool validateOperandSize(const llvm::StringMap<bool> &FeatureMap,
                                    StringRef Constraint, unsigned Size) const;
 
   std::string convertConstraint(const char *&Constraint) const override;
   const char *getClobbers() const override {
     return "~{dirflag},~{fpsr},~{flags}";