[PATCH] D101773: [MC][ELF] Work around R_MIPS_LO16 relocation handling problem

[Dimitry Andric via Phabricator via llvm-commits]

dim updated this revision to Diff 342499.
dim added a comment.
Herald added a subscriber: jrtc27.

Add test case, and update comment to explicitly mention ld.lld.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D101773/new/

https://reviews.llvm.org/D101773

Files:
  llvm/lib/MC/ELFObjectWriter.cpp
  llvm/test/MC/Mips/mips_lo16.s


Index: llvm/test/MC/Mips/mips_lo16.s
===================================================================
--- /dev/null
+++ llvm/test/MC/Mips/mips_lo16.s
@@ -0,0 +1,18 @@
+# PR49821: Check that R_MIPS_LO16 relocs do not wrap around with large addends.
+
+# RUN: llvm-mc %s -triple mips-unknown-unknown -filetype=obj | \
+# RUN:	 llvm-objdump -d -r - | FileCheck %s
+
+	.text
+foo:
+	lui	$2, %hi(bar)
+# CHECK: 0: 3c 02 00 00   lui     $2, 0 <foo>
+# CHECK:                  00000000:  R_MIPS_HI16  bar
+	addiu	$2, $2, %lo(bar)
+# CHECK: 4: 24 42 00 00   addiu   $2, $2, 0 <foo>
+# CHECK:                  00000004:  R_MIPS_LO16  bar
+
+	.section	.rodata.str1.1,"aMS", at progbits,1
+	.zero 0x8000
+bar:
+	.asciz	"hello"
Index: llvm/lib/MC/ELFObjectWriter.cpp
===================================================================
--- llvm/lib/MC/ELFObjectWriter.cpp
+++ llvm/lib/MC/ELFObjectWriter.cpp
@@ -1381,6 +1381,16 @@
       if (TargetObjectWriter->getEMachine() == ELF::EM_386 &&
           Type == ELF::R_386_GOTOFF)
         return true;
+
+      // Work around an issue in MIPS relocation handling: both R_MIPS_HI16 and
+      // R_MIPS_LO16 are considered as absolute relocations (the ld.lld type is
+      // R_ABS). When we calculate R_MIPS_HI16's addend we find a paired
+      // R_MIPS_LO16, generate a combined addend and return the correct symbol
+      // value from the InputSectionBase::getRelocTargetVA. For R_MIPS_LO16 we
+      // do not do that and use its addend as is.
+      if (TargetObjectWriter->getEMachine() == ELF::EM_MIPS &&
+          !hasRelocationAddend())
+        return true;
     }
 
     // Most TLS relocations use a got, so they need the symbol. Even those that


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