[PATCH] D91993: [ELF] Don't relax R_X86_64_GOTPCRELX if addend != -4

[Fangrui Song via Phabricator via llvm-commits]

MaskRay updated this revision to Diff 308085.
MaskRay marked 2 inline comments as done.
MaskRay added a comment.

comments


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D91993/new/

https://reviews.llvm.org/D91993

Files:
  lld/ELF/Arch/X86_64.cpp
  lld/test/ELF/x86-64-gotpc-offset.s


Index: lld/test/ELF/x86-64-gotpc-offset.s
===================================================================
--- /dev/null
+++ lld/test/ELF/x86-64-gotpc-offset.s
@@ -0,0 +1,30 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
+# RUN: ld.lld %t.o -o %t
+# RUN: llvm-objdump -s -d %t | FileCheck %s
+
+# CHECK:      Contents of section .got:
+# CHECK-NEXT: {{^}} [[#%x,ADDR:]] {{.*}} 00000000
+
+# CHECK:      leal {{.*}}(%rip), %eax  # {{.*}} <foo>
+# CHECK-NEXT: movl {{.*}}(%rip), %eax  # [[#ADDR+4]]
+# CHECK-NEXT: movq {{.*}}(%rip), %rax  # [[#ADDR+1]]
+
+## movl foo at GOTPCREL(%rip), %eax
+  movl 0(%rip), %eax
+  .reloc .-4, R_X86_64_GOTPCRELX, foo-4
+
+## The instruction has an offset (addend!=-4). It is incorrect to relax movl to leal.
+## movl foo at GOTPCREL+4(%rip), %eax
+  movl 0(%rip), %eax
+  .reloc .-4, R_X86_64_GOTPCRELX, foo
+
+## This does not make sense because it loads one byte past the GOT entry.
+## It is just to demonstrate the behavior.
+## movq foo at GOTPCREL+1(%rip), %rax
+  movq 0(%rip), %rax
+  .reloc .-4, R_X86_64_REX_GOTPCRELX, foo-3
+
+.globl foo
+foo:
+  nop
Index: lld/ELF/Arch/X86_64.cpp
===================================================================
--- lld/ELF/Arch/X86_64.cpp
+++ lld/ELF/Arch/X86_64.cpp
@@ -730,7 +730,12 @@
 
 RelExpr X86_64::adjustGotPcExpr(RelType type, int64_t addend,
                                 const uint8_t *loc) const {
-  if (type != R_X86_64_GOTPCRELX && type != R_X86_64_REX_GOTPCRELX)
+  // Only R_X86_64_[REX_]GOTPCRELX can be relaxed. GNU as may emit GOTPCRELX
+  // with addend != -4. Such an instruction does not load the full GOT entry, so
+  // we cannot relax the relocation. E.g. movl x at GOTPCREL+4(%rip), %rax
+  // (addend=0) loads the high 32 bits of the GOT entry.
+  if ((type != R_X86_64_GOTPCRELX && type != R_X86_64_REX_GOTPCRELX) ||
+      addend != -4)
     return R_GOT_PC;
   const uint8_t op = loc[-2];
   const uint8_t modRm = loc[-1];


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