[PATCH] D110769: [M68k] Avoid UB in disassembler

[Ricky Taylor via Phabricator via llvm-commits]

ricky26 created this revision.
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When reading 32 bits a 32-bit shift would be executed.

This is undefined behaviour, but in this case we can just replace the
entire scratch value to avoid it.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D110769

Files:
  llvm/lib/Target/M68k/Disassembler/M68kDisassembler.cpp


Index: llvm/lib/Target/M68k/Disassembler/M68kDisassembler.cpp
===================================================================
--- llvm/lib/Target/M68k/Disassembler/M68kDisassembler.cpp
+++ llvm/lib/Target/M68k/Disassembler/M68kDisassembler.cpp
@@ -451,7 +451,8 @@
     llvm_unreachable("invalid imm");
   }
 
-  Scratch = (Scratch << NumToRead) | Reader.readBits(NumToRead);
+  Scratch = (NumToRead < 32) ? (Scratch << NumToRead) : 0;
+  Scratch |= Reader.readBits(NumToRead);
 }
 
 DecodeStatus M68kDisassembler::getInstruction(MCInst &Instr, uint64_t &Size,


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