[ADT] APFloatBase: Prevent collapsing semPPCDoubleDouble and semBogus

Provide a distinct contents for semBogus and semPPCDoubleDouble in order
to prevent compilers from collapsing them to a single memory address,
while we heavily rely on every semantic having distinct address.

This happens if insecure optimization collapsing identical values is
enabled. As a result, APFloats of semBogus are indistinguishable from
semPPCDoubleDouble -- and whenever the move constructor is used, the old
value beings being incorrectly recognized as a semPPCDoubleDouble.

Since the values in semPPCDoubleDouble are not used anywhere,
we can easily solve this issue via altering the value of one of the
fields and therefore ensuring that the collapse can not occur.

Differential Revision: https://reviews.llvm.org/D28112

llvm-svn: 290896

llvm/lib/Support/APFloat.cpp

@@ -76,8 +76,12 @@ namespace llvm {
      compile-time arithmetic on PPC double-double numbers, it is not able
      to represent all possible values held by a PPC double-double number,
      for example: (long double) 1.0 + (long double) 0x1p-106
-     Should this be replaced by a full emulation of PPC double-double?  */
-  static const fltSemantics semPPCDoubleDouble = {0, 0, 0, 0};
+     Should this be replaced by a full emulation of PPC double-double?
+
+     Note: we need to make the value different from semBogus as otherwise
+     an unsafe optimization may collapse both values to a single address,
+     and we heavily rely on them having distinct addresses.             */
+  static const fltSemantics semPPCDoubleDouble = {-1, 0, 0, 0};
 
   /* There are temporary semantics for the real PPCDoubleDouble implementation.
      Currently, APFloat of PPCDoubleDouble holds one PPCDoubleDoubleImpl as the