[PATCH] D63003: [ELF] Don't emit dynamic relocations with weak undef in writable sections

[Fangrui Song via Phabricator via llvm-commits]

MaskRay updated this revision to Diff 203529.
MaskRay edited the summary of this revision.
MaskRay added a comment.

Emphasize the benefits


Repository:
  rLLD LLVM Linker

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D63003/new/

https://reviews.llvm.org/D63003

Files:
  ELF/Relocations.cpp
  test/ELF/weak-undef-rw.s


Index: test/ELF/weak-undef-rw.s
===================================================================
--- test/ELF/weak-undef-rw.s
+++ test/ELF/weak-undef-rw.s
@@ -1,9 +1,13 @@
 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
 # RUN: ld.lld %t.o -o %t --export-dynamic
-# RUN: llvm-readobj -r %t | FileCheck %s
+# RUN: llvm-readelf -r %t | FileCheck %s
 
-# CHECK: R_X86_64_64 foobar 0x0
+## gABI leaves the behavior of weak undefined references implementation defined.
+## We choose to resolve it statically and not create a dynamic relocation for
+## implementation simplicity.
+
+# CHECK: no relocations
 
         .global _start
 _start:
Index: ELF/Relocations.cpp
===================================================================
--- ELF/Relocations.cpp
+++ ELF/Relocations.cpp
@@ -911,10 +911,19 @@
 static void processRelocAux(InputSectionBase &Sec, RelExpr Expr, RelType Type,
                             uint64_t Offset, Symbol &Sym, const RelTy &Rel,
                             int64_t Addend) {
-  if (isStaticLinkTimeConstant(Expr, Type, Sym, Sec, Offset)) {
+  // If the relocation is known to be a link-time constant, we know no dynamic
+  // relocation will be created, pass the control to relocateAlloc or
+  // relocateNonAlloc to resolve it.
+  //
+  // The behavior of an undefined weak reference is implementation defined. If
+  // the relocation is to a weak undef, and we are producing executable, give up
+  // on it and let relocate{,Non}Alloc resolve it.
+  if (isStaticLinkTimeConstant(Expr, Type, Sym, Sec, Offset) ||
+      (!Config->Shared && Sym.isUndefWeak())) {
     Sec.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
     return;
   }
+
   bool CanWrite = (Sec.Flags & SHF_WRITE) || !Config->ZText;
   if (CanWrite) {
     // R_GOT refers to a position in the got, even if the symbol is preemptible.
@@ -947,13 +956,6 @@
     }
   }
 
-  // If the relocation is to a weak undef, and we are producing
-  // executable, give up on it and produce a non preemptible 0.
-  if (!Config->Shared && Sym.isUndefWeak()) {
-    Sec.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
-    return;
-  }
-
   if (!CanWrite && (Config->Pic && !isRelExpr(Expr))) {
     error(
         "can't create dynamic relocation " + toString(Type) + " against " +


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