[AMDGPU] Improve disassembler error handling

Summary:
llvm-objdump now disassembles unrecognised opcodes as data, using
the .long directive. We treat unrecognised opcodes as being 32 bit
values, so move along 4 bytes rather than the single byte which
previously resulted in a cascade of bogus disassembly following an
unrecognised opcode.

While no solution can always disassemble code that contains
embedded data correctly this provides a significant improvement.

The disassembler will now cope with an arbitrary length section
as it no longer truncates it to a multiple of 4 bytes, and will
use the .byte directive for trailing bytes.

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D44685

llvm-svn: 328553

1 files changed, 4 insertions, 1 deletions

diff --git a/llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp b/llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp
index 75356e9a9d3..9732d11adee 100644
--- a/llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp
+++ b/llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp
@@ -246,7 +246,10 @@ DecodeStatus AMDGPUDisassembler::getInstruction(MCInst &MI, uint64_t &Size,
   if (Res && IsSDWA)
     Res = convertSDWAInst(MI);
 
-  Size = Res ? (MaxInstBytesNum - Bytes.size()) : 0;
+  // if the opcode was not recognized we'll assume a Size of 4 bytes
+  // (unless there are fewer bytes left)
+  Size = Res ? (MaxInstBytesNum - Bytes.size())
+             : std::min((size_t)4, Bytes_.size());
   return Res;
 }