[PATCH] D47396: [LLD] Place .nv_fatbin section at the beginning of the executable.

[Artem Belevich via Phabricator via llvm-commits]

tra created this revision.
tra added a reviewer: ruiu.
Herald added subscribers: bixia, arichardson, jlebar, emaste, sanjoy.
Herald added a reviewer: espindola.

This avoids relocation overflows in large CUDA binaries.


https://reviews.llvm.org/D47396

Files:
  lld/ELF/Writer.cpp
  lld/test/ELF/nv_fatbin-at-beginning.s


Index: lld/test/ELF/nv_fatbin-at-beginning.s
===================================================================
--- /dev/null
+++ lld/test/ELF/nv_fatbin-at-beginning.s
@@ -0,0 +1,19 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t
+
+# RUN: ld.lld --hash-style=gnu -o %t1  %t -shared
+# RUN: llvm-readobj -elf-output-style=GNU -s %t1 | FileCheck %s
+
+# .nv_fatbin has the same priority as .dynsym/.dynstr and is expected to
+# located before .rodata and .text
+# CHECK-DAG: .nv_fatbin      {{.*}}   A
+# CHECK-DAG: .dynsym  {{.*}}   A
+# CHECK-DAG: .dynstr  {{.*}}   A
+# CHECK: .rodata    {{.*}}   A
+# CHECK: .text    {{.*}}   AX
+
+.section .rodata, "a"
+.byte 1
+
+.section .nv_fatbin, "a"
+.byte 0
Index: lld/ELF/Writer.cpp
===================================================================
--- lld/ELF/Writer.cpp
+++ lld/ELF/Writer.cpp
@@ -744,6 +744,13 @@
   if (Sec->Type == SHT_DYNSYM || Sec->Type == SHT_STRTAB)
     return Rank | RF_ALLOC_FIRST;
 
+  // GPU binaries may grow quite large which may lead to relocation overflows if
+  // the section is placed between .text and regular data segments. Placing the
+  // section at the beginning of SHF_ALLOC, similarly to .dynstr/.dynsym above
+  // mitigates the problem.
+  if (Sec->Name == ".nv_fatbin")
+    return Rank |= RF_ALLOC_FIRST;
+
   // Sort sections based on their access permission in the following
   // order: R, RX, RWX, RW.  This order is based on the following
   // considerations:


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