[scan-build] fix warning emitted on LLVM Bitcode code base

[apelete]

Summary

Fix a logic error warning of the type "assigned value is garbage or
undefined" emitted by Clang Static Analyzer on the following file:

• lib/Bitcode/Writer/BitcodeWriter.cpp.

Signed-off-by: Apelete Seketeli <[e-mail]>

[tejohnson]

Fix a logic error warning of the type "called C++ object pointer is null" emitted by Clang Static Analyzer on the following file

What does this error mean? It seems like a case where we would get a maybe uninitialized warning, if -Wmaybe-uninitialized was enabled (but its my understanding that there is a conscious decision not to enable this warning for llvm). In this case, we have assertion checks (noted below) that ensure we never use these without initializing them.

If we need to fix this case to silence an analyzer, I'd rather initialize those abbrevs to 0 and add an assert down near the uses that they are non-zero (as noted below the default added in this patch doesn't make sense for this bitcode record type).

lib/Bitcode/Writer/BitcodeWriter.cpp

2512

These are not correct abbrev ids for this code.

2576

The above 2 asserts will ensure that we never reach here without initializing FnEntry*BitAbbrev.

[apelete]

lib/Bitcode/Writer/BitcodeWriter.cpp

2512

These were needed to suppress a warning in case branch at line 2513 evaluates to 'false'.
From the analyzer point of view, assigned value at line 2585 would then be garbage or undefined (commit message was mistaken in the wording of the warning, will fix).

I was not sure about which abbrev ids to assign variables FnEntry8BitAbbrev, FnEntry7BitAbbrev and FnEntry6BitAbbrev.
Will fix by assigning 0 and assert their value before use if that's OK for you.

[apelete]

[scan-build] fix warning emitted on LLVM Bitcode code base

Changes since last revision:

• initialize FnEntry8BitAbbrev, FnEntry7BitAbbrev and FnEntry6BitAbbrev variables to 0 and assert they are non-zero prior being read.

lib/Bitcode/Writer/BitcodeWriter.cpp

if (IsModuleLevel && hasVSTOffsetPlaceholder()) {
     assert((VSTOffset & 31) == 0 && "VST block not 32-bit aligned");
     Stream.BackpatchWord(VSTOffsetPlaceholder, VSTOffset / 32);
   }
 
   Stream.EnterSubblock(bitc::VALUE_SYMTAB_BLOCK_ID, 4);
 
   // For the module-level VST, add abbrev Ids for the VST_CODE_FNENTRY
   // records, which are not used in the per-function VSTs.
-  unsigned FnEntry8BitAbbrev;
-  unsigned FnEntry7BitAbbrev;
-  unsigned FnEntry6BitAbbrev;
+  unsigned FnEntry8BitAbbrev = 0;
+  unsigned FnEntry7BitAbbrev = 0;
+  unsigned FnEntry6BitAbbrev = 0;
   if (IsModuleLevel && hasVSTOffsetPlaceholder()) {
     // 8-bit fixed-width VST_CODE_FNENTRY function strings.
     BitCodeAbbrev *Abbv = new BitCodeAbbrev();
     Abbv->Add(BitCodeAbbrevOp(bitc::VST_CODE_FNENTRY));
     Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8)); // value id
     Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8)); // funcoffset
     Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Array));
     Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 8));
if (isa<BasicBlock>(Name.getValue())) {
       Code = bitc::VST_CODE_BBENTRY;
       if (Bits == SE_Char6)
         AbbrevToUse = VST_BBENTRY_6_ABBREV;
     } else if (F && !F->isDeclaration()) {
       // Must be the module-level VST, where we pass in the Index and
       // have a VSTOffsetPlaceholder. The function-level VST should not
       // contain any Function symbols.
       assert(FunctionToBitcodeIndex);
       assert(hasVSTOffsetPlaceholder());
 
       // Save the word offset of the function (from the start of the
       // actual bitcode written to the stream).
       uint64_t BitcodeIndex = (*FunctionToBitcodeIndex)[F] - bitcodeStartBit();
       assert((BitcodeIndex & 31) == 0 && "function block not 32-bit aligned");
       NameVals.push_back(BitcodeIndex / 32);
 
       Code = bitc::VST_CODE_FNENTRY;
+      assert(FnEntry8BitAbbrev && "abbrev was not computed");
       AbbrevToUse = FnEntry8BitAbbrev;
-      if (Bits == SE_Char6)
+      if (Bits == SE_Char6) {
+        assert(FnEntry6BitAbbrev && "abbrev was not computed");
         AbbrevToUse = FnEntry6BitAbbrev;
-      else if (Bits == SE_Fixed7)
+      } else if (Bits == SE_Fixed7) {
+        assert(FnEntry7BitAbbrev && "abbrev was not computed");
         AbbrevToUse = FnEntry7BitAbbrev;
+      }
     } else {
       Code = bitc::VST_CODE_ENTRY;
       if (Bits == SE_Char6)
         AbbrevToUse = VST_ENTRY_6_ABBREV;
       else if (Bits == SE_Fixed7)
         AbbrevToUse = VST_ENTRY_7_ABBREV;
     }