[MIPS][ELF] Use PC-relative relocations in .eh_frame when possible

[arichardson]

Summary

When compiling position-independent executables, we now use
DW_EH_PE_pcrel | DW_EH_PE_sdata4. However, the MIPS ABI does not define a
64-bit PC-relative ELF relocation so we cannot use sdata8 for the large
code model case. When using the large code model, we fall back to the
previous behaviour of generating absolute relocations.

With this change clang-generated .o files can be linked by LLD without
having to pass -Wl,-z,notext (which creates text relocations).
This is simpler than the approach used by ld.bfd, which rewrites the
.eh_frame section to convert absolute relocations into relative references.

I saw in D13104 that apparently ld.bfd did not accept pc-relative relocations
for MIPS ouput at some point. However, I also checked that recent ld.bfd
can process the clang-generated .o files so this no longer seems true.

[merge_guards_bot]

Unit tests: pass. 61249 tests passed, 0 failed and 736 were skipped.

clang-tidy: pass.

clang-format: pass.

Build artifacts: diff.json, clang-tidy.txt, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

[joerg]

What's the fixup behavior of the linker here, e.g. does it make sense to just use absolute encoding here and let the linker fix it up?

[arichardson]

In D72228#1804852, @joerg wrote:

What's the fixup behavior of the linker here, e.g. does it make sense to just use absolute encoding here and let the linker fix it up?

Currently LLD will not convert the encoding. This change allows me to build and link MIPS64 FreeBSD using clang+lld without having to fix up things in the linker.
I think this is a much simpler approach than making LLD reassemble the eh_frame with relative values.

ld.bfd also accepts these clang generated input files and generates the same eh_frame output as lld.

We could still add the eh_frame rewriting support in the future.

[arichardson]

ping? I believe this is the last blocker before FreeBSD can move to using Clang+LLD for MIPS64 by default. It would be really nice to get this in for LLVM 10.

[MaskRay]

llvm/test/MC/Mips/eh-frame.s

21

llvm-readobj -r may have a nicer output than llvm-objdump -r

[dsanders]

I see you've added me as a reviewer but I haven't worked on MIPS for a few years so I'm not really familiar with the current state of their linkers and binaries. AFAIK @atanasyan is still actively working on MIPS so hopefully he can take a look or bring in a colleague who can.
FWIW, this sounds like a good change and the code looks good to me but I think you should get another LGTM from someone more up to date on MIPS.

[atanasyan]

I'm sorry for the delay. The patch is LGTM.
Thanks.

lld/test/ELF/mips-eh_frame-pic.s

+# REQUIRES: mips
+## Check that we can link a shared library containing an eh_frame section without
+## -z notext. This was not possible LLVM started emitting values using the
+## DW_EH_PE_pcrel | DW_EH_PE_sdata4 encoding.
+
+## It should not be possible to link code compiled without -fPIC:
+# RUN: llvm-mc -filetype=obj -triple=mips64-unknown-linux %s -o %t-nopic.o
+# RUN: llvm-dwarfdump --eh-frame %t-nopic.o | FileCheck %s --check-prefix=ABS64-EH-FRAME
+# RUN: llvm-readobj -r %t-nopic.o | FileCheck %s --check-prefixes=RELOCS,ABS64-RELOCS
+# RUN: not ld.lld -shared %t-nopic.o -o /dev/null 2>&1 | FileCheck %s --check-prefix=NOPIC-ERR
+## Note: ld.bfd can link this file because it rewrites the .eh_frame section to use
+## relative addressing.
+# NOPIC-ERR: ld.lld: error: can't create dynamic relocation R_MIPS_64 against local symbol in readonly segment
+
+## For -fPIC, .eh_frame should contain DW_EH_PE_pcrel | DW_EH_PE_sdata4 values:
+# RUN: llvm-mc -filetype=obj -triple=mips64-unknown-linux --position-independent %s -o %t-pic.o
+# RUN: llvm-readobj -r %t-pic.o | FileCheck %s --check-prefixes=RELOCS,PIC64-RELOCS
+# RUN: ld.lld -shared %t-pic.o -o %t-pic.so
+# RUN: llvm-dwarfdump --eh-frame %t-pic.so | FileCheck %s --check-prefix=PIC-EH-FRAME
+
+## Also check MIPS32:
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux %s -o %t-nopic32.o
+# RUN: llvm-dwarfdump --eh-frame %t-nopic32.o | FileCheck %s --check-prefix=ABS32-EH-FRAME
+# RUN: llvm-readobj -r %t-nopic32.o | FileCheck %s --check-prefixes=RELOCS,ABS32-RELOCS
+# RUN: not ld.lld -shared %t-nopic32.o -o /dev/null 2>&1 | FileCheck %s --check-prefix=NOPIC32-ERR
+## Note: ld.bfd can link this file because it rewrites the .eh_frame section to use
+## relative addressing.
+# NOPIC32-ERR: ld.lld: error: can't create dynamic relocation R_MIPS_32 against local symbol in readonly segment
+
+## For -fPIC, .eh_frame should contain DW_EH_PE_pcrel | DW_EH_PE_sdata4 values:
+# RUN: llvm-mc -filetype=obj -triple=mips-unknown-linux --position-independent %s -o %t-pic32.o
+# RUN: llvm-readobj -r %t-pic32.o | FileCheck %s --check-prefixes=RELOCS,PIC32-RELOCS
+# RUN: ld.lld -shared %t-pic32.o -o %t-pic32.so
+# RUN: llvm-dwarfdump --eh-frame %t-pic32.so | FileCheck %s --check-prefix=PIC-EH-FRAME
+
+# RELOCS:            .rel{{a?}}.eh_frame {
+# ABS32-RELOCS-NEXT:   0x1C R_MIPS_32 .text 0x0
+# ABS64-RELOCS-NEXT:   0x1C R_MIPS_64/R_MIPS_NONE/R_MIPS_NONE .text 0x0
+# PIC64-RELOCS-NEXT:   0x1C R_MIPS_PC32/R_MIPS_NONE/R_MIPS_NONE - 0x0
+# PIC32-RELOCS-NEXT:   0x1C R_MIPS_PC32 - 0x0
+# RELOCS-NEXT:       }
+
+# ABS64-EH-FRAME: Augmentation data: 0C
+##                                   ^^ fde pointer encoding: DW_EH_PE_sdata8
+# ABS32-EH-FRAME: Augmentation data: 0B
+##                                   ^^ fde pointer encoding: DW_EH_PE_sdata4
+# PIC-EH-FRAME: Augmentation data: 1B
+##                                 ^^ fde pointer encoding: DW_EH_PE_pcrel | DW_EH_PE_sdata4
+## Note: ld.bfd converts the R_MIPS_64 relocs to DW_EH_PE_pcrel | DW_EH_PE_sdata8
+## for N64 ABI (and DW_EH_PE_pcrel | DW_EH_PE_sdata4 for MIPS32)
+
+.ent func
+.global func
+func:
+  .cfi_startproc
+  nop
+  .cfi_endproc
+.end func

llvm/lib/MC/MCObjectFileInfo.cpp

 }
 
 void MCObjectFileInfo::initELFMCObjectFileInfo(const Triple &T, bool Large) {
   switch (T.getArch()) {
   case Triple::mips:
   case Triple::mipsel:
   case Triple::mips64:
   case Triple::mips64el:
+    // We cannot use DW_EH_PE_sdata8 for the large PositionIndependent case
+    // since there is no R_MIPS_PC64 relocation (only a 32-bit version).
+    if (PositionIndependent && !Large)
+      FDECFIEncoding = dwarf::DW_EH_PE_pcrel | dwarf::DW_EH_PE_sdata4;
+    else
       FDECFIEncoding = Ctx->getAsmInfo()->getCodePointerSize() == 4
                            ? dwarf::DW_EH_PE_sdata4
                            : dwarf::DW_EH_PE_sdata8;
     break;
   case Triple::ppc64:
   case Triple::ppc64le:
   case Triple::x86_64:
     FDECFIEncoding = dwarf::DW_EH_PE_pcrel |
                      (Large ? dwarf::DW_EH_PE_sdata8 : dwarf::DW_EH_PE_sdata4);
     break;
   case Triple::bpfel:

llvm/lib/Object/RelocationResolver.cpp

static uint64_t resolveBPF(RelocationRef R, uint64_t S, uint64_t A) {
   }
 }
 
 static bool supportsMips64(uint64_t Type) {
   switch (Type) {
   case ELF::R_MIPS_32:
   case ELF::R_MIPS_64:
   case ELF::R_MIPS_TLS_DTPREL64:
+  case ELF::R_MIPS_PC32:
     return true;
   default:
     return false;
   }
 }
 
 static uint64_t resolveMips64(RelocationRef R, uint64_t S, uint64_t A) {
   switch (R.getType()) {
   case ELF::R_MIPS_32:
     return (S + getELFAddend(R)) & 0xFFFFFFFF;
   case ELF::R_MIPS_64:
     return S + getELFAddend(R);
   case ELF::R_MIPS_TLS_DTPREL64:
     return S + getELFAddend(R) - 0x8000;
+  case ELF::R_MIPS_PC32:
+    return S + getELFAddend(R) - R.getOffset();
   default:
     llvm_unreachable("Invalid relocation type");
   }
 }
 
 static bool supportsPPC64(uint64_t Type) {
   switch (Type) {
   case ELF::R_PPC64_ADDR32:

llvm/test/DebugInfo/Mips/eh_frame.ll

 ; RUN: llc -mtriple mips-unknown-linux-gnu -mattr=+micromips -relocation-model=static -O3 -filetype=obj -o - %s | \
-; RUN:     llvm-readelf -r | FileCheck %s --check-prefix=CHECK-READELF
+; RUN:     llvm-readelf -r | FileCheck %s --check-prefixes=CHECK-READELF,CHECK-READELF-STATIC
 ; RUN: llc -mtriple mips-unknown-linux-gnu -mattr=+micromips -relocation-model=pic -O3 -filetype=obj -o - %s | \
-; RUN:     llvm-readelf -r | FileCheck %s --check-prefix=CHECK-READELF
+; RUN:     llvm-readelf -r | FileCheck %s --check-prefixes=CHECK-READELF,CHECK-READELF-PIC
 ; RUN: llc -mtriple mips-unknown-linux-gnu -mattr=+micromips -relocation-model=static -O3 -filetype=obj -o - %s | \
 ; RUN:     llvm-objdump -s -j .gcc_except_table - | FileCheck %s --check-prefix=CHECK-EXCEPT-TABLE-STATIC
 ; RUN: llc -mtriple mips-unknown-linux-gnu -mattr=+micromips -relocation-model=pic -O3 -filetype=obj -o - %s | \
 ; RUN:     llvm-objdump -s -j .gcc_except_table - | FileCheck %s --check-prefix=CHECK-EXCEPT-TABLE-PIC
 
 ; CHECK-READELF: .rel.eh_frame
 ; CHECK-READELF: DW.ref.__gxx_personality_v0
-; CHECK-READELF-NEXT: .text
+; CHECK-READELF-STATIC-NEXT: R_MIPS_32 00000000 .text
+; CHECK-READELF-PIC-NEXT: R_MIPS_PC32
 ; CHECK-READELF-NEXT: .gcc_except_table
 
 ; CHECK-EXCEPT-TABLE-STATIC: 0000 ff9b1501 0c011500 00150e23 01231e00  ...........#.#..
 ; CHECK-EXCEPT-TABLE-STATIC: 0010 00010000 00000000
 ; CHECK-EXCEPT-TABLE-PIC:    0000 ff9b1501 0c012d00 002d133f 013f2a00 ......-..-.?.?*.
 ; CHECK-EXCEPT-TABLE-PIC:    0010 00010000 00000000                    ........
 
 @_ZTIi = external constant i8*

llvm/test/MC/Mips/eh-frame.s

 // Test the bits of .eh_frame on mips that are already implemented correctly.
 
 // RUN: llvm-mc -filetype=obj %s -o %t.o -triple mips-unknown-linux-gnu
-// RUN: llvm-objdump -r -section=.rel.eh_frame %t.o | FileCheck --check-prefix=REL32 %s
-// RUN: llvm-dwarfdump -eh-frame %t.o | FileCheck --check-prefix=DWARF32 %s
+// RUN: llvm-readobj -r %t.o | FileCheck --check-prefixes=RELOCS,ABS32 %s
+// RUN: llvm-dwarfdump -eh-frame %t.o | FileCheck --check-prefixes=DWARF32,DWARF32_ABS %s
 
 // RUN: llvm-mc -filetype=obj %s -o %t.o -triple mipsel-unknown-linux-gnu
-// RUN: llvm-objdump -r -section=.rel.eh_frame %t.o | FileCheck --check-prefix=REL32 %s
-// RUN: llvm-dwarfdump -eh-frame %t.o | FileCheck --check-prefix=DWARF32 %s
+// RUN: llvm-readobj -r %t.o | FileCheck --check-prefixes=RELOCS,ABS32 %s
+// RUN: llvm-dwarfdump -eh-frame %t.o | FileCheck --check-prefixes=DWARF32,DWARF32_ABS %s
 
 // RUN: llvm-mc -filetype=obj %s -o %t.o -triple mips64-unknown-linux-gnu
-// RUN: llvm-objdump -r -section=.rela.eh_frame %t.o | FileCheck --check-prefix=REL64 %s
-// RUN: llvm-dwarfdump -eh-frame %t.o | FileCheck --check-prefix=DWARF64 %s
+// RUN: llvm-readobj -r %t.o | FileCheck --check-prefixes=RELOCS,ABS64 %s
+// RUN: llvm-dwarfdump -eh-frame %t.o | FileCheck --check-prefixes=DWARF64,DWARF64_ABS %s
 
 // RUN: llvm-mc -filetype=obj %s -o %t.o -triple mips64el-unknown-linux-gnu
-// RUN: llvm-objdump -r -section=.rela.eh_frame %t.o | FileCheck --check-prefix=REL64 %s
-// RUN: llvm-dwarfdump -eh-frame %t.o | FileCheck --check-prefix=DWARF64 %s
+// RUN: llvm-readobj -r %t.o | FileCheck --check-prefixes=RELOCS,ABS64 %s
+// RUN: llvm-dwarfdump -eh-frame %t.o | FileCheck --check-prefixes=DWARF64,DWARF64_ABS %s
+
+/// Check that position-indenpendent code use PC-relative relocations:
+// RUN: llvm-mc -filetype=obj %s -o %t.o -triple mips-unknown-linux-gnu --position-independent
+// RUN: llvm-readobj -r %t.o | FileCheck --check-prefixes=RELOCS,PIC32 %s
+// RUN: llvm-dwarfdump -eh-frame %t.o | FileCheck --check-prefixes=DWARF32,DWARF32_PIC %s
+
+// RUN: llvm-mc -filetype=obj %s -o %t.o -triple mipsel-unknown-linux-gnu --position-independent
+// RUN: llvm-readobj -r %t.o | FileCheck --check-prefixes=RELOCS,PIC32 %s
+// RUN: llvm-dwarfdump -eh-frame %t.o | FileCheck --check-prefixes=DWARF32,DWARF32_PIC %s
+
+// RUN: llvm-mc -filetype=obj %s -o %t.o -triple mips64-unknown-linux-gnu --position-independent
+// RUN: llvm-readobj -r %t.o | FileCheck --check-prefixes=RELOCS,PIC64 %s
+// RUN: llvm-dwarfdump -eh-frame %t.o | FileCheck --check-prefixes=DWARF64,DWARF64_PIC %s
+
+// RUN: llvm-mc -filetype=obj %s -o %t.o -triple mips64el-unknown-linux-gnu --position-independent
+// RUN: llvm-readobj -r %t.o | FileCheck --check-prefixes=RELOCS,PIC64 %s
+// RUN: llvm-dwarfdump -eh-frame %t.o | FileCheck --check-prefixes=DWARF64,DWARF64_PIC %s
+
+/// However using the large code model forces R_MIPS_64 since there is no R_MIPS_PC64 relocation:
+// RUN: llvm-mc -filetype=obj %s -o %t.o -triple mips64-unknown-linux-gnu --position-independent --large-code-model
+// RUN: llvm-readobj -r %t.o | FileCheck --check-prefixes=RELOCS,ABS64 %s
+// RUN: llvm-dwarfdump -eh-frame %t.o | FileCheck --check-prefixes=DWARF64,DWARF64_ABS %s
+
+// RUN: llvm-mc -filetype=obj %s -o %t.o -triple mips64el-unknown-linux-gnu --position-independent  --large-code-model
+// RUN: llvm-readobj -r %t.o | FileCheck --check-prefixes=RELOCS,ABS64 %s
+// RUN: llvm-dwarfdump -eh-frame %t.o | FileCheck --check-prefixes=DWARF64,DWARF64_ABS %s
 
 func:
   .cfi_startproc
   .cfi_endproc
 
-// REL32: R_MIPS_32
-// REL64: R_MIPS_64/R_MIPS_NONE/R_MIPS_NONE
+// RELOCS:      Relocations [
+// RELOCS:        Section ({{.+}}) .rel{{a?}}.eh_frame {
+// ABS32-NEXT:      R_MIPS_32
+// ABS64-NEXT:      R_MIPS_64/R_MIPS_NONE/R_MIPS_NONE
+// PIC32-NEXT:      R_MIPS_PC32
+// PIC64-NEXT:      R_MIPS_PC32/R_MIPS_NONE/R_MIPS_NONE
+// RELOCS-NEXT:   }
 
 // DWARF32: 00000000 00000010 ffffffff CIE
-// DWARF32:   Version:               1
-// DWARF32:   Augmentation:          "zR"
-// DWARF32:   Code alignment factor: 1
-// DWARF32:   Data alignment factor: -4
-// DWARF32:   Return address column: 31
-// DWARF32:   Augmentation data:     0B
-//                                   ^^ fde pointer encoding: DW_EH_PE_sdata4
-// DWARF32:   DW_CFA_def_cfa_register: reg29
+// DWARF32-NEXT:     Version:               1
+// DWARF32-NEXT:     Augmentation:          "zR"
+// DWARF32-NEXT:     Code alignment factor: 1
+// DWARF32-NEXT:     Data alignment factor: -4
+// DWARF32-NEXT:     Return address column: 31
+// DWARF32_ABS-NEXT: Augmentation data: 0B
+//                                      ^^ fde pointer encoding: DW_EH_PE_sdata4
+// DWARF32_PIC-NEXT: Augmentation data: 1B
+//                                      ^^ fde pointer encoding: DW_EH_PE_pcrel | DW_EH_PE_sdata4
+// DWARF32-EMPTY:
+// DWARF32-NEXT:     DW_CFA_def_cfa_register: reg29
 //
 // DWARF32: 00000014 00000010 00000018 FDE cie=00000018 pc=00000000...00000000
-// DWARF32:   DW_CFA_nop:
-// DWARF32:   DW_CFA_nop:
-// DWARF32:   DW_CFA_nop:
+// DWARF32-NEXT:     DW_CFA_nop:
+// DWARF32-NEXT:     DW_CFA_nop:
+// DWARF32-NEXT:     DW_CFA_nop:
+
 
 // DWARF64: 00000000 00000010 ffffffff CIE
-// DWARF64:   Version:               1
-// DWARF64:   Augmentation:          "zR"
-// DWARF64:   Code alignment factor: 1
-// DWARF64:   Data alignment factor: -8
-//                                   ^^ GAS uses -4. Should be ok as long as
-//                                      all offsets we need are a multiple of 8.
-// DWARF64:   Return address column: 31
-// DWARF64:   Augmentation data:     0C
-//                                   ^^ fde pointer encoding: DW_EH_PE_sdata8
-// DWARF64:   DW_CFA_def_cfa_register: reg29
+// DWARF64-NEXT:     Version:               1
+// DWARF64-NEXT:     Augmentation:          "zR"
+// DWARF64-NEXT:     Code alignment factor: 1
+// DWARF64-NEXT:     Data alignment factor: -8
+//                                          ^^ GAS uses -4. Should be ok as long as
+//                                             all offsets we need are a multiple of 8.
+// DWARF64-NEXT:     Return address column: 31
+// DWARF64_ABS-NEXT: Augmentation data: 0C
+//                                      ^^ fde pointer encoding: DW_EH_PE_sdata8
+// DWARF64_PIC:      Augmentation data: 1B
+//                                      ^^ fde pointer encoding: DW_EH_PE_pcrel | DW_EH_PE_sdata4
+// DWARF64-EMPTY:
+// DWARF64-NEXT:     DW_CFA_def_cfa_register: reg29
+// DWARF64_PIC-NEXT: DW_CFA_nop:
 //
-// DWARF64: 00000014 00000018 00000018 FDE cie=00000018 pc=00000000...00000000
-// DWARF64:   DW_CFA_nop:
-// DWARF64:   DW_CFA_nop:
-// DWARF64:   DW_CFA_nop:
+// DWARF64_ABS:      00000014 00000018 00000018 FDE cie=00000018 pc=00000000...00000000
+// DWARF64_PIC:      00000014 00000010 00000018 FDE cie=00000018 pc=00000000...00000000
+// DWARF64-NEXT:     DW_CFA_nop:
+// DWARF64-NEXT:     DW_CFA_nop:
+// DWARF64-NEXT:     DW_CFA_nop: