[LLVMdev] MIPS asm backend emitting weird symbols into object file?

[Daniel Sanders]

Hi,

Could you attach the assembly output produced by adding -save-temps to your command?

I encountered a similar issue over the weekend so I think I might know what the problem is. The current pc symbol '.' is implemented by dropping a private label (currently named $tmp*) and referencing that. Private labels aren't supposed to appear in the object output for the N32/N64. They currently appear because the private label naming convention is ABI dependant and LLVM doesn't know this yet. It turns out that on O32, private labels begin with '$' but on N32/N64 they begin with '.L'. I'll have a patch for this shortly.

> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
> On Behalf Of Sandeep Raju
> Sent: 15 May 2015 23:15
> To: LLVM Developers Mailing List
> Subject: [LLVMdev] MIPS asm backend emitting weird symbols into object
> file?
> 
> I'm cross-compiling for MIPS. The test-case is as simple as it can be:
> 
> void foo() {}
> 
> $clang -target mips64-octeon-linux -c -B
> path/to/cross/compiled/mips/assembler a.c
> 
> And then I look at the object file:
> 
> $ nm a.o
> 0000000000000020 t $tmp0
> 0000000000000000 T foo
> 
> I would like to know what "$tmp0" is. Furthermore, if I pass -g to
> clang, I see a whole bunch of such symbols. Some of them appear to be
> MIPS specific DWARF extensions. But the ones I'm not able to figure
> out are the $tmp* symbols:
> 
> # nm a.o
> 0000000000000000 N $.debug_info_begin0
> 0000000000000000 N $debug_cmdline
> 0000000000000020 t $debug_end0
> 0000000000000000 N $debug_range
> 0000000000000000 t $func_begin0
> 0000000000000020 t $func_end0
> 0000000000000000 N $info_string
> 0000000000000000 N $info_string0
> 00000000000000a7 N $info_string1
> 00000000000000ab N $info_string2
> 00000000000000cf N $info_string3
> 0000000000000000 N $line_table_start0
> 0000000000000004 N $pubNames_begin0
> 000000000000001a N $pubNames_end0
> 0000000000000000 N $section_abbrev
> 0000000000000000 N $section_debug_loc
> 0000000000000000 N $section_info
> 0000000000000000 N $section_line
> 0000000000000004 t $tmp0
> 0000000000000008 t $tmp1
> 000000000000000c t $tmp2
> 0000000000000020 t $tmp3
> 0000000000000020 t $tmp4
> 0000000000000000 T foo
> 
> These symbols are not from the IR. They seem to be inserted by the
> MIPS asm back-end.
> 
> I wouldn't be worried if these are MIPS specific temporary structures
> to hold debug/dwarf info and such, but the reason I started to look
> into this is that some of the symbols are appearing in the debugger
> backtrace (I'm using gdb 7.9) of a mips binary.
> 
> Any suggestions on where to start looking or what they might be?
> 
> Thanks,
> Sandeep
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