Re: [PATCH v4 02/10] objtool: Handle special cases of dead end insn

From: Tiezhu Yang
Date: Tue Nov 26 2024 - 05:42:26 EST


On 11/26/2024 02:45 PM, Josh Poimboeuf wrote:
On Fri, Nov 22, 2024 at 12:49:57PM +0800, Tiezhu Yang wrote:
There are some "unreachable instruction" objtool warnings when compling
with Clang on LoongArch, this is because the "break" instruction is set
as dead end due to its type is INSN_BUG in decode_instructions() at the
beginning, and it does not set insn->dead_end of the "break" instruction
as false after checking ".rela.discard.reachable" in add_dead_ends(), so
the next instruction of "break" is marked as unreachable.

Actually, it can find the reachable instruction after parsing the section
".rela.discard.reachable", in some cases, the "break" instruction may not
be the first previous instruction with scheduling by Machine Instruction
Scheduler of LLVM, it should find more times and then set insn->dead_end
of the "break" instruction as false.

This is preparation for later patch on LoongArch, there is no effect for
the other archs with this patch.

Signed-off-by: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>

I'm having trouble understanding this commit log, is the problem that
the compiler is sometimes inserting code between 'break' and the
unreachable() inline asm?

If so, this sounds like a problem that was already solved for x86 with:

bfb1a7c91fb7 ("x86/bug: Merge annotate_reachable() into _BUG_FLAGS() asm")

Can you check if that fixes it?

I will try, thank you.