linux-kernel - [PATCH v2 2/5] objtool: Handle special cases of discard.reachable

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Message-ID: <20241105123906.26072-3-yangtiezhu@loongson.cn>
Date: Tue,  5 Nov 2024 20:39:03 +0800
From: Tiezhu Yang <yangtiezhu@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Cc: loongarch@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2 2/5] objtool: Handle special cases of discard.reachable

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@...ngson.cn>
---
 tools/objtool/check.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 9601235e908d..6607cd56459b 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -711,6 +711,18 @@ static int add_dead_ends(struct objtool_file *file)
 		}

 		insn->dead_end = false;
+
+		/* Handle the special cases compiled with Clang on LoongArch */
+		if (file->elf->ehdr.e_machine == EM_LOONGARCH &&
+		    reloc->sym->type == STT_SECTION) {
+			while (insn && insn_func(insn)) {
+				insn = prev_insn_same_sym(file, insn);
+				if (insn && insn->dead_end) {
+					insn->dead_end = false;
+					break;
+				}
+			}
+		}
 	}

 	return 0;
-- 
2.42.0