[llvm] e1e3b6e - [M68k] Avoid UB in disassembler

[Ricky Taylor via llvm-commits]

Author: Ricky Taylor
Date: 2021-09-29T22:07:14+01:00
New Revision: e1e3b6ee72977c584052a9da8735c31c2a17385a

URL: https://github.com/llvm/llvm-project/commit/e1e3b6ee72977c584052a9da8735c31c2a17385a
DIFF: https://github.com/llvm/llvm-project/commit/e1e3b6ee72977c584052a9da8735c31c2a17385a.diff

LOG: [M68k] Avoid UB in disassembler

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.

Differential Revision: https://reviews.llvm.org/D110769

Added: 
    

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

Removed: 
    


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diff  --git a/llvm/lib/Target/M68k/Disassembler/M68kDisassembler.cpp b/llvm/lib/Target/M68k/Disassembler/M68kDisassembler.cpp
index a8453c838493d..1e397f0f4641d 100644
--- a/llvm/lib/Target/M68k/Disassembler/M68kDisassembler.cpp
+++ b/llvm/lib/Target/M68k/Disassembler/M68kDisassembler.cpp
@@ -451,7 +451,8 @@ void M68kDisassembler::decodeImm(MCInst &Instr, unsigned Bead,
     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,