[sanitizers] Add DFSan support for AArch64 42-bit VMA

[zatrazz]

Summary

This patch adds support for dfsan on aarch64-linux with 42-bit VMA
(current default config for 64K pagesize kernels). The support is
enabled by defining the SANITIZER_AARCH64_VMA to 42 at build time
for both clang/llvm and compiler-rt. The default VMA is 39 bits.

[rengolin]

LGTM. Thanks!

The 42VMA bot is up again, let's see how it copes.

lib/Transforms/Instrumentation/DataFlowSanitizer.cpp

 #include "llvm/Transforms/Utils/Local.h"
 #include <algorithm>
 #include <iterator>
 #include <set>
 #include <utility>
 
 using namespace llvm;
 
+// VMA size definition for architecture that support multiple sizes.
+// AArch64 has 3 VMA sizes: 39, 42 and 48.
+#ifndef SANITIZER_AARCH64_VMA
+# define SANITIZER_AARCH64_VMA 39
+#else
+# if SANITIZER_AARCH64_VMA != 39 && SANITIZER_AARCH64_VMA != 42
+#  error "invalid SANITIZER_AARCH64_VMA size"
+# endif
+#endif
+
 // The -dfsan-preserve-alignment flag controls whether this pass assumes that
 // alignment requirements provided by the input IR are correct.  For example,
 // if the input IR contains a load with alignment 8, this flag will cause
 // the shadow load to have alignment 16.  This flag is disabled by default as
 // we have unfortunately encountered too much code (including Clang itself;
 // see PR14291) which performs misaligned access.
 static cl::opt<bool> ClPreserveAlignment(
     "dfsan-preserve-alignment",
bool DataFlowSanitizer::doInitialization(Module &M) {
   IntptrTy = DL.getIntPtrType(*Ctx);
   ZeroShadow = ConstantInt::getSigned(ShadowTy, 0);
   ShadowPtrMul = ConstantInt::getSigned(IntptrTy, ShadowWidth / 8);
   if (IsX86_64)
     ShadowPtrMask = ConstantInt::getSigned(IntptrTy, ~0x700000000000LL);
   else if (IsMIPS64)
     ShadowPtrMask = ConstantInt::getSigned(IntptrTy, ~0xF000000000LL);
   else if (IsAArch64)
+#if SANITIZER_AARCH64_VMA == 39
     ShadowPtrMask = ConstantInt::getSigned(IntptrTy, ~0x7800000000LL);
+#else
+    ShadowPtrMask = ConstantInt::getSigned(IntptrTy, ~0x3c000000000LL);
+#endif
   else
     report_fatal_error("unsupported triple");
 
   Type *DFSanUnionArgs[2] = { ShadowTy, ShadowTy };
   DFSanUnionFnTy =
       FunctionType::get(ShadowTy, DFSanUnionArgs, /*isVarArg=*/ false);
   Type *DFSanUnionLoadArgs[2] = { ShadowPtrTy, IntptrTy };
   DFSanUnionLoadFnTy =