Fix uninitialized value in ABIArgInfo

[serge-sans-paille]

Summary

GCC-9 takes advantage of this uninitialized values to optimize stuff, which ends up in failing validation when compiling clang.

This fixes https://bugs.llvm.org/show_bug.cgi?id=40547

[hans]

I've been staring at this, trying to figure out if the code somehow ends up using the uninitialized values, but I can't find it. So either it's hard to find, or GCC is doing something wrong.

But anyway, I think committing your change makes sense, but I think don't think we can say "Fix uninitialized value in ABIArgInfo", because it's not clear what needs fixing. Instead, I'd say "Always initialize the members in ABIArgInfo". That kind of makes sense in itself as it makes the code safer and possibly simpler, and of course it's also good that it fixes this problem.

What do you think?

[serge-sans-paille]

@hans agreed; Thanks for taking the time to try to reproduce the original issue o/

cfe/trunk/include/clang/CodeGen/CGFunctionInfo.h

private:
   }
 
   void setUnpaddedCoerceToType(llvm::Type *T) {
     assert(isCoerceAndExpand());
     UnpaddedCoerceAndExpandType = T;
   }
 
   ABIArgInfo(Kind K)
-      : TheKind(K), PaddingInReg(false), InReg(false), SuppressSRet(false) {
-  }
+      : TypeData(nullptr), PaddingType(nullptr), DirectOffset(0), TheKind(K),
+        PaddingInReg(false), InAllocaSRet(false), IndirectByVal(false),
+        IndirectRealign(false), SRetAfterThis(false), InReg(false),
+        CanBeFlattened(false), SignExt(false), SuppressSRet(false) {}
 
-public:
-  ABIArgInfo()
-      : TypeData(nullptr), PaddingType(nullptr), DirectOffset(0),
-        TheKind(Direct), PaddingInReg(false), InReg(false),
-        SuppressSRet(false) {}
+public : ABIArgInfo()
+    : ABIArgInfo(Direct) {
+}
 
   static ABIArgInfo getDirect(llvm::Type *T = nullptr, unsigned Offset = 0,
                               llvm::Type *Padding = nullptr,
                               bool CanBeFlattened = true) {
     auto AI = ABIArgInfo(Direct);
     AI.setCoerceToType(T);
     AI.setPaddingType(Padding);
     AI.setDirectOffset(Offset);