From: Dimitry Andric <dimitry@andric.com>
Date: Mon, 3 May 2021 18:08:49 +0000 (+0200)
Subject: [PATCH] [MC][ELF] Work around R_MIPS_LO16 relocation handling problem
X-Git-Tag: archive/raspbian/1%12.0.1-19+rpi1^2^2~32
X-Git-Url: https://dgit.raspbian.org/?a=commitdiff_plain;h=33c7753f8863177a9925110a59629edd10c5656b;p=llvm-toolchain-12.git

[PATCH] [MC][ELF] Work around R_MIPS_LO16 relocation handling problem

This fixes PR49821, and avoids "ld.lld: error: test.o:(.rodata.str1.1):
offset is outside the section" errors when linking MIPS objects with
negative R_MIPS_LO16 implicit addends.

ld.lld handles R_MIPS_HI16/R_MIPS_LO16 separately, not as a whole, so it
doesn't know that an R_MIPS_HI16 with implicit addend 1 and an
R_MIPS_LO16 with implicit addend -32768 represents 32768, which is in
range of a MergeInputSection. We could introduce a new RelExpr member
(like R_RISCV_PC_INDIRECT for R_RISCV_PCREL_HI20 / R_RISCV_PCREL_LO12)
but the complexity is unnecessary given that GNU as keeps the original
symbol for this case as well.

Reviewed By: atanasyan, MaskRay

Differential Revision: https://reviews.llvm.org/D101773

Gbp-Pq: Topic mips
Gbp-Pq: Name mips-D101773-reloc.patch
---

diff --git a/llvm/lib/MC/ELFObjectWriter.cpp b/llvm/lib/MC/ELFObjectWriter.cpp
index 2d810ffd35..6d16adae16 100644
--- a/llvm/lib/MC/ELFObjectWriter.cpp
+++ b/llvm/lib/MC/ELFObjectWriter.cpp
@@ -1408,6 +1408,17 @@ bool ELFObjectWriter::shouldRelocateWithSymbol(const MCAssembler &Asm,
       if (TargetObjectWriter->getEMachine() == ELF::EM_MIPS &&
           !hasRelocationAddend())
         return true;
+
+      // ld.lld handles R_MIPS_HI16/R_MIPS_LO16 separately, not as a whole, so
+      // it doesn't know that an R_MIPS_HI16 with implicit addend 1 and an
+      // R_MIPS_LO16 with implicit addend -32768 represents 32768, which is in
+      // range of a MergeInputSection. We could introduce a new RelExpr member
+      // (like R_RISCV_PC_INDIRECT for R_RISCV_PCREL_HI20 / R_RISCV_PCREL_LO12)
+      // but the complexity is unnecessary given that GNU as keeps the original
+      // symbol for this case as well.
+      if (TargetObjectWriter->getEMachine() == ELF::EM_MIPS &&
+          !hasRelocationAddend())
+        return true;
     }
 
     // Most TLS relocations use a got, so they need the symbol. Even those that