[X86] Initialize TargetOptions::StackProtectorGuardOffset member to its default value

[LemonBoy]

Summary

D88631 introduced a set of knobs to tweak how the stack protector is codegen'd for x86 targets, including the offset from the base register where the stack cookie is located. The StackProtectorGuardOffset field in TargetOptions was left uninitialized instead of being reset to its neutral value -1, making it possible to emit nonsensical code if the frontend doesn't change the field value at all before feeding the TargetOptions to the target machine initializer.

[nickdesaulniers]

llvm/include/llvm/Target/TargetOptions.h

335

Is the cast necessary?

[LemonBoy]

llvm/include/llvm/Target/TargetOptions.h

335

Not really, unless -Wsign-conversion or equivalent options are passed.
I'm partial to keeping the explicit cast, if there are strong reasons to remove it I'll get rid of it.

[craig.topper]

llvm/include/llvm/Target/TargetOptions.h

335

Use -1U?

[LemonBoy]

Use suffix instead of an explicit cast.

[LemonBoy]

llvm/include/llvm/Target/TargetOptions.h

335

Good call, thanks.

[xiangzhangllvm]

llvm/include/llvm/Target/TargetOptions.h

335

Does it duplicated with
clang/include/clang/Basic/CodeGenOptions.def:369:VALUE_CODEGENOPT(StackProtectorGuardOffset, 32, (unsigned)-1) ?

llvm/include/llvm/Target/TargetOptions.h

public:
 
     /// Emit DWARF debug frame section.
     unsigned ForceDwarfFrameSection : 1;
 
     /// Emit XRay Function Index section
     unsigned XRayOmitFunctionIndex : 1;
 
     /// Stack protector guard offset to use.
-    unsigned StackProtectorGuardOffset : 32;
+    unsigned StackProtectorGuardOffset = -1U;
 
     /// Stack protector guard mode to use, e.g. tls, global.
     StackProtectorGuards StackProtectorGuard =
                                          StackProtectorGuards::None;
 
     /// Stack protector guard reg to use, e.g. usually fs or gs in X86.
     std::string StackProtectorGuardReg = "None";