For a simple program like below:
-bash-4.4$ cat t.c
int test() {
asm volatile("r0 = r0" ::);
return 0;
}
compiled with
clang -target bpf -O2 -c t.c
the following llvm-objdump command will segfault.
llvm-objdump -d t.o
0: bf 00 00 00 00 00 00 00 nop
llvm-objdump: ../include/llvm/ADT/SmallVector.h:180
...
Assertion `idx < size()' failed
...
abort
...
llvm::BPFInstPrinter::printOperand
llvm::BPFInstPrinter::printInstruction
...
The reason is both NOP and MOV_rr (r0 = r0) having the same encoding.
The disassembly getInstruction() decodes to be a NOP instruciton but
during printInstruction() the same encoding is interpreted as
a MOV_rr instruction. Such a mismatcch caused the segfault.
The fix is to make NOP instruction as CodeGen only so disassembler
will skip NOP insn for disassembling.
Note that instruction "r0 = r0" should not appear in non inline
asm codes since BPF Machine Instruction Peephole optimization will
remove it.
Differential Revision: https://reviews.llvm.org/D80156
Yonghong Song