[AVR] Fix reads of uninitialized variables from constructor of AVRSubtarget

The initialization order was not correct. These bugs were discovered by
valgrind. They appear to work fine in practice but this patch should
unblock switching the AVR backend on by default as now a standard AVR
llc invocation runs without memory errors.

The AVRISelLowering constructor would run before the subtarget boolean
fields were initialized to false. Now, the initialization order is
correct.

Author: dylanmckay

llvm/lib/Target/AVR/AVRSubtarget.cpp

@@ -29,16 +29,19 @@ namespace llvm {
 
 AVRSubtarget::AVRSubtarget(const Triple &TT, const std::string &CPU,
                            const std::string &FS, const AVRTargetMachine &TM)
-    : AVRGenSubtargetInfo(TT, CPU, FS), ELFArch(0), InstrInfo(), FrameLowering(),
-      TLInfo(TM, initializeSubtargetDependencies(CPU, FS, TM)), TSInfo(),
+    : AVRGenSubtargetInfo(TT, CPU, FS),
+      ELFArch(0),
 
       // Subtarget features
       m_hasSRAM(false), m_hasJMPCALL(false), m_hasIJMPCALL(false),
       m_hasEIJMPCALL(false), m_hasADDSUBIW(false), m_hasSmallStack(false),
       m_hasMOVW(false), m_hasLPM(false), m_hasLPMX(false),  m_hasELPM(false),
       m_hasELPMX(false), m_hasSPM(false), m_hasSPMX(false), m_hasDES(false),
       m_supportsRMW(false), m_supportsMultiplication(false), m_hasBREAK(false),
-      m_hasTinyEncoding(false), m_FeatureSetDummy(false) {
+      m_hasTinyEncoding(false), m_FeatureSetDummy(false),
+
+      InstrInfo(), FrameLowering(),
+      TLInfo(TM, initializeSubtargetDependencies(CPU, FS, TM)), TSInfo() {
   // Parse features string.
   ParseSubtargetFeatures(CPU, FS);
 }

llvm/lib/Target/AVR/AVRSubtarget.h

@@ -85,11 +85,6 @@ class AVRSubtarget : public AVRGenSubtargetInfo {
   /// The ELF e_flags architecture.
   unsigned ELFArch;
 
-  AVRInstrInfo InstrInfo;
-  AVRFrameLowering FrameLowering;
-  AVRTargetLowering TLInfo;
-  AVRSelectionDAGInfo TSInfo;
-
   // Subtarget feature settings
   // See AVR.td for details.
   bool m_hasSRAM;
@@ -114,6 +109,11 @@ class AVRSubtarget : public AVRGenSubtargetInfo {
   // Dummy member, used by FeatureSet's. We cannot have a SubtargetFeature with
   // no variable, so we instead bind pseudo features to this variable.
   bool m_FeatureSetDummy;
+
+  AVRInstrInfo InstrInfo;
+  AVRFrameLowering FrameLowering;
+  AVRTargetLowering TLInfo;
+  AVRSelectionDAGInfo TSInfo;
 };
 
 } // end namespace llvm