[lld] 669c755 - [NFC][ELF][AArch64][MTE] Replace addend hack with less-confusing code

[via llvm-commits]

Author: Jessica Clarke
Date: 2025-12-22T16:19:50Z
New Revision: 669c755d1fb7b79d7978ce49640356c9f431939d

URL: https://github.com/llvm/llvm-project/commit/669c755d1fb7b79d7978ce49640356c9f431939d
DIFF: https://github.com/llvm/llvm-project/commit/669c755d1fb7b79d7978ce49640356c9f431939d.diff

LOG: [NFC][ELF][AArch64][MTE] Replace addend hack with less-confusing code

The current implementation in addRelativeReloc makes it look like we're
writing the symbol's VA + addend to the section, because that's what the
given relocation will evaluate to, but we're supposed to be writing the
negated original addend (since the relative relocation's addend will be
the sum of the symbol's VA and the original addend). This only works
because deep down in AArch64::relocate we throw away the computed value
and peek back inside the relocation to extract the addend and negate it.

Do this properly by having a relocation that evaluates to the right
value instead.

Reviewers: kovdan01, MaskRay

Reviewed By: MaskRay

Pull Request: https://github.com/llvm/llvm-project/pull/171182

Added: 
    

Modified: 
    lld/ELF/Arch/AArch64.cpp
    lld/ELF/InputSection.cpp
    lld/ELF/Relocations.cpp
    lld/ELF/Relocations.h

Removed: 
    


################################################################################
diff  --git a/lld/ELF/Arch/AArch64.cpp b/lld/ELF/Arch/AArch64.cpp
index 4613539342f57..f68403b69419f 100644
--- a/lld/ELF/Arch/AArch64.cpp
+++ b/lld/ELF/Arch/AArch64.cpp
@@ -527,19 +527,7 @@ void AArch64::relocate(uint8_t *loc, const Relocation &rel,
     write32(ctx, loc, val);
     break;
   case R_AARCH64_ABS64:
-    // AArch64 relocations to tagged symbols have extended semantics, as
-    // described here:
-    // https://github.com/ARM-software/abi-aa/blob/main/memtagabielf64/memtagabielf64.rst#841extended-semantics-of-r_aarch64_relative.
-    // tl;dr: encode the symbol's special addend in the place, which is an
-    // offset to the point where the logical tag is derived from. Quick hack, if
-    // the addend is within the symbol's bounds, no need to encode the tag
-    // derivation offset.
-    if (rel.sym && rel.sym->isTagged() &&
-        (rel.addend < 0 ||
-         rel.addend >= static_cast<int64_t>(rel.sym->getSize())))
-      write64(ctx, loc, -rel.addend);
-    else
-      write64(ctx, loc, val);
+    write64(ctx, loc, val);
     break;
   case R_AARCH64_PREL64:
     write64(ctx, loc, val);

diff  --git a/lld/ELF/InputSection.cpp b/lld/ELF/InputSection.cpp
index ff7ef2dce5c79..ca8a9c0d27f29 100644
--- a/lld/ELF/InputSection.cpp
+++ b/lld/ELF/InputSection.cpp
@@ -784,6 +784,8 @@ uint64_t InputSectionBase::getRelocTargetVA(Ctx &ctx, const Relocation &r,
     return r.sym->getVA(ctx, a);
   case R_ADDEND:
     return a;
+  case R_ADDEND_NEG:
+    return -static_cast<uint64_t>(a);
   case R_RELAX_HINT:
     return 0;
   case RE_ARM_SBREL:

diff  --git a/lld/ELF/Relocations.cpp b/lld/ELF/Relocations.cpp
index d6c63ed919910..5c23c76c80f9e 100644
--- a/lld/ELF/Relocations.cpp
+++ b/lld/ELF/Relocations.cpp
@@ -717,7 +717,7 @@ static void addRelativeReloc(Ctx &ctx, InputSectionBase &isec,
     // field. This is described in further detail in:
     // https://github.com/ARM-software/abi-aa/blob/main/memtagabielf64/memtagabielf64.rst#841extended-semantics-of-r_aarch64_relative
     if (addend < 0 || static_cast<uint64_t>(addend) >= sym.getSize())
-      isec.relocations.push_back({expr, type, offsetInSec, addend, &sym});
+      isec.relocations.push_back({R_ADDEND_NEG, type, offsetInSec, addend, &sym});
     return;
   }
 

diff  --git a/lld/ELF/Relocations.h b/lld/ELF/Relocations.h
index 86ca298cd7a56..4cb09f329953a 100644
--- a/lld/ELF/Relocations.h
+++ b/lld/ELF/Relocations.h
@@ -42,6 +42,7 @@ using JumpModType = uint32_t;
 enum RelExpr {
   R_ABS,
   R_ADDEND,
+  R_ADDEND_NEG,
   R_DTPREL,
   R_GOT,
   R_GOT_OFF,