[mips][mc][clang] Use pc-relative relocations in .eh_frame

[atanasyan]

Summary

Use pc-relative relocations in .eh_frame. R_MIPS_PC32 relocation is generated in 32-bit case. R_MIPS_PC32 | R_MIPS_64 relocation chain is generated in 64-bit case for all code model.

With this change object files can be linked by LLD without having to pass -Wl,-z,notext options.

For compatibility with tools unsupported 64-bit pc-relative relocations the patch introduces new command line options in Clang: mmips-pc64-rel and mno-mips-pc64-rel. These options passed to LLVM by Clang driver as -mmips-pc64-rel={true|false}.

[atanasyan]

I cannot select default behaviour for the LLVM in case of generating .eh_frame sections for MIPS:

1. Unconditionally use pc-relative relocations.
2. Use pc-relative relocations by default, but provide options for switching to absolute relocations.
3. Use absolute relocations by default, but provide options for switching to pc-relative relocations.

I prefer the second variant. Any comments?

[joewholden]

Hiya,

I'd probably also go for 2 also as the default, as long as its configurable then just a sensible default is good

Thanks

[joewholden]

FYI, I backported this and the other diff to llvm 10 and its all working nicely, now building all packages with lld without issue

Thanks!

[MaskRay]

LGTM.

[joewholden]

Hi guys,

any chance of this making it into 11.1.0 or 12?

[atanasyan]

Hi,

In D80392#2537794, @joewholden wrote:

Hi guys,

any chance of this making it into 11.1.0 or 12?

This task was out of my scope for a while. As far as I remember there were some problems with test cases. One more thing - I couldn't figure out how to enable/disable this functionality by clang command line options. MCObjectFileInfo and TargetLoweringObjectFileELF classes are far away from any target related context/settings. I do not think that this change (especially without any "disable" functionality) is a good candidate for 11.1.0 or 12 releases. But I will try to push it to the main branch soon. I'm sorry for the delay.

[joewholden]

No worries - whenever you have time!

[atanasyan]

For compatibility with tools unsupported 64-bit pc-relative relocations the patch introduces new command line options in Clang: mmips-pc64-rel and mno-mips-pc64-rel. These options passed to LLVM by Clang driver as -mmips-pc64-rel={true|false}. I could not find any better way to pass the option into MC layer.

[MaskRay]

MCTargetOptionsCommandFlags.cpp may be a better place for the internal cl::opt option.

llvm/lib/MC/MCObjectFileInfo.cpp

27

-m prefix is for assembler and dropver options.

Internal codegen options don't need the prefix.

[MaskRay]

If you want to do this proper, you may look at how -fbinutils-version= was implemented.

[wzssyqa]

I don't think that we need that option: what we need to do is to update the old tools.

[wzssyqa]

In D80392#3876782, @wzssyqa wrote:

I don't think that we need that option: what we need to do is to update the old tools.

the ld.bfd support R_MIPS_PC32 here.
I think that we can just switch the behavior without any condtions.

And I am working to fix GCC/GAS for it.

[wzssyqa]

When link with ld.bfd, we get:

ld/ld-new: .eh_frame_hdr entry overflow
ld/ld-new: final link failed: bad value

It seems due to the initial_loc in the object from LLVM is quite big than the gcc/gas one.

initial_loc: 0x100000cd0, vma: 0xe78

[MaskRay]

In D80392#4028011, @wzssyqa wrote:

When link with ld.bfd, we get:

ld/ld-new: .eh_frame_hdr entry overflow
ld/ld-new: final link failed: bad value

It seems due to the initial_loc in the object from LLVM is quite big than the gcc/gas one.

initial_loc: 0x100000cd0, vma: 0xe78

I have some notes on https://maskray.me/blog/2020-11-08-stack-unwinding#dwarf-call-frame-information
(See https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic.pdf)
Different targets may use different encoding for the initial_location field.

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:
+## For -fPIC, .eh_frame should contain DW_EH_PE_pcrel | DW_EH_PE_sdata8 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
+# RUN: llvm-dwarfdump --eh-frame %t-pic.so | FileCheck %s --check-prefix=PIC64-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
+# RUN: llvm-dwarfdump --eh-frame %t-pic32.so | FileCheck %s --check-prefix=PIC32-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
+# PIC64-RELOCS-NEXT:   0x1C R_MIPS_PC32/R_MIPS_64/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)
+# PIC32-EH-FRAME: Augmentation data: 1B
+##                                   ^^ fde pointer encoding: DW_EH_PE_pcrel | DW_EH_PE_sdata4
+# PIC64-EH-FRAME: Augmentation data: 1C
+##                                   ^^ fde pointer encoding: DW_EH_PE_pcrel | DW_EH_PE_sdata8
 
 .ent func
 .global func
 func:
   .cfi_startproc
   nop
   .cfi_endproc
 .end func

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

case Triple::mips64el:
     TTypeEncoding = dwarf::DW_EH_PE_indirect | dwarf::DW_EH_PE_pcrel |
                     dwarf::DW_EH_PE_sdata4;
     // We don't support PC-relative LSDA references in GAS so we use the default
     // DW_EH_PE_absptr for those.
 
     // FreeBSD must be explicit about the data size and using pcrel since it's
     // assembler/linker won't do the automatic conversion that the Linux tools
     // do.
-    if (TgtM.getTargetTriple().isOSFreeBSD()) {
+    if (isPositionIndependent() || TgtM.getTargetTriple().isOSFreeBSD()) {
       PersonalityEncoding |= dwarf::DW_EH_PE_pcrel | dwarf::DW_EH_PE_sdata4;
       LSDAEncoding = dwarf::DW_EH_PE_pcrel | dwarf::DW_EH_PE_sdata4;
     }
     break;
   case Triple::ppc64:
   case Triple::ppc64le:
     PersonalityEncoding = dwarf::DW_EH_PE_indirect | dwarf::DW_EH_PE_pcrel |
       dwarf::DW_EH_PE_udata8;

llvm/lib/MC/MCObjectFileInfo.cpp

 #include "llvm/MC/MCSectionMachO.h"
 #include "llvm/MC/MCSectionWasm.h"
 #include "llvm/MC/MCSectionXCOFF.h"
 
 using namespace llvm;
 
 static bool useCompactUnwind(const Triple &T) {
   // Only on darwin.
   if (!T.isOSDarwin())
     return false;
 
   // aarch64 always has it.
   if (T.getArch() == Triple::aarch64 || T.getArch() == Triple::aarch64_32)
     return true;
 
   // armv7k always has it.
   if (T.isWatchABI())

 }
 
 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;
+    if (PositionIndependent)
+      FDECFIEncoding |= dwarf::DW_EH_PE_pcrel;
     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 resolveLanai(RelocationRef R, uint64_t S, uint64_t A) {
   if (R.getType() == ELF::R_LANAI_32)
     return (S + getELFAddend(R)) & 0xFFFFFFFF;
   llvm_unreachable("Invalid relocation type");
 }
 
 static bool supportsMips32(uint64_t Type) {
   switch (Type) {
   case ELF::R_MIPS_32:
+  case ELF::R_MIPS_PC32:
   case ELF::R_MIPS_TLS_DTPREL32:
     return true;
   default:
     return false;
   }
 }
 
 static uint64_t resolveMips32(RelocationRef R, uint64_t S, uint64_t A) {
   // FIXME: Take in account implicit addends to get correct results.
   uint32_t Rel = R.getType();
   if (Rel == ELF::R_MIPS_32)
     return (S + A) & 0xFFFFFFFF;
+  if (Rel == ELF::R_MIPS_PC32)
+    return (S + getELFAddend(R) - R.getOffset()) & 0xFFFFFFFF;
   if (Rel == ELF::R_MIPS_TLS_DTPREL32)
     return (S + A) & 0xFFFFFFFF;
   llvm_unreachable("Invalid relocation type");
 }
 
 static bool supportsSparc32(uint64_t Type) {
   switch (Type) {
   case ELF::R_SPARC_32:

llvm/test/CodeGen/Mips/ehframe-indirect.ll

 ; RUN: llc -mtriple=mipsel-linux-gnu < %s -asm-verbose -relocation-model=pic | \
-; RUN:     FileCheck -check-prefixes=ALL,LINUX,LINUX-O32,O32 %s
+; RUN:     FileCheck -check-prefixes=ALL,O32 %s
 ; RUN: llc -mtriple=mipsel-linux-android < %s -asm-verbose -relocation-model=pic | \
-; RUN:     FileCheck -check-prefixes=ALL,LINUX,LINUX-O32,O32 %s
+; RUN:     FileCheck -check-prefixes=ALL,O32 %s
 ; RUN: llc -mtriple=mips64el-linux-gnu -target-abi=n32 < %s -asm-verbose -relocation-model=pic | \
-; RUN:     FileCheck -check-prefixes=ALL,LINUX,LINUX-N32,N32 %s
+; RUN:     FileCheck -check-prefixes=ALL,N32 %s
 ; RUN: llc -mtriple=mips64el-linux-gnu < %s -asm-verbose -relocation-model=pic | \
-; RUN:     FileCheck -check-prefixes=ALL,LINUX,LINUX-N64,N64 %s
+; RUN:     FileCheck -check-prefixes=ALL,N64 %s
 ; RUN: llc -mtriple=mips64el-linux-android < %s -asm-verbose -relocation-model=pic | \
-; RUN:     FileCheck -check-prefixes=ALL,LINUX,LINUX-N64,N64 %s
+; RUN:     FileCheck -check-prefixes=ALL,N64 %s
 ; RUN: llc -mtriple=mips64el-linux-gnu < %s -asm-verbose -relocation-model=pic | \
-; RUN:     FileCheck -check-prefixes=ALL,LINUX,LINUX-N64,N64 %s
+; RUN:     FileCheck -check-prefixes=ALL,N64 %s
 ; RUN: llc -mtriple=mips64-unknown-freebsd11.0 < %s -asm-verbose -relocation-model=pic | \
-; RUN:     FileCheck -check-prefixes=ALL,FREEBSD,FREEBSD-N64,N64 %s
+; RUN:     FileCheck -check-prefixes=ALL,N64 %s
 
 @_ZTISt9exception = external constant i8*
 
 define i32 @main() personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*) {
 ; ALL: .cfi_startproc
 
-; Linux must rely on the assembler/linker converting the encodings.
-; LINUX: .cfi_personality 128, DW.ref.__gxx_personality_v0
-; LINUX-O32: .cfi_lsda 0, $exception0
-; LINUX-NEW: .cfi_lsda 0, .Lexception0
-
-; FreeBSD can (and must) be more direct about the encodings it wants.
-; FREEBSD: .cfi_personality 155, DW.ref.__gxx_personality_v0
-; FREEBSD-O32: .cfi_lsda 27, $exception0
-; FREEBSD-NEW: .cfi_lsda 27, .Lexception0
+; ALL: .cfi_personality 155, DW.ref.__gxx_personality_v0
+; O32: .cfi_lsda 27, $exception0
+; N32: .cfi_lsda 27, .Lexception0
+; N64: .cfi_lsda 27, .Lexception0
 
 entry:
   invoke void @foo() to label %cont unwind label %lpad
 ; ALL: foo
 ; ALL: jalr
 
 lpad:
   %0 = landingpad { i8*, i32 }

llvm/test/DebugInfo/Mips/eh_frame.ll

 ; RUN:     llvm-objdump -s -j .gcc_except_table - | FileCheck %s --check-prefix=EXCEPT-TABLE-PIC
 
 ; STATIC-LABEL: Relocation section '.rel.eh_frame'
 ; STATIC-DAG: R_MIPS_32 00000000 DW.ref.__gxx_personality_v0
 ; STATIC-DAG: R_MIPS_32 00000000 .text
 ; STATIC-DAG: R_MIPS_32 00000000 .gcc_except_table
 
 ; PIC-LABEL: Relocation section '.rel.eh_frame'
-; PIC-DAG: R_MIPS_32   00000000 DW.ref.__gxx_personality_v0
+; PIC-DAG: R_MIPS_PC32 00000000 DW.ref.__gxx_personality_v0
+; PIC-DAG: R_MIPS_PC32
 ; PIC-DAG: R_MIPS_PC32
-; PIC-DAG: R_MIPS_32   00000000 .gcc_except_table
 
 ; CHECK-READELF: DW.ref.__gxx_personality_v0
 ; CHECK-READELF-STATIC-NEXT: R_MIPS_32 00000000 .text
 ; CHECK-READELF-PIC-NEXT: R_MIPS_PC32
 ; CHECK-READELF-NEXT: .gcc_except_table
 
 ; EXCEPT-TABLE-STATIC: 0000 ff9b1501 0c011500 00150e23 01231e00  ...........#.#..
 ; EXCEPT-TABLE-STATIC: 0010 00010000 00000000

llvm/test/MC/Mips/eh-frame.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-readobj -r %t.o | FileCheck --check-prefixes=RELOCS,PIC64-NEXT %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  --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-readobj -r %t.o | FileCheck --check-prefixes=RELOCS,PIC64-NEXT %s
+// RUN: llvm-dwarfdump -eh-frame %t.o | FileCheck --check-prefixes=DWARF64,DWARF64_PIC %s
 
 func:
   .cfi_startproc
   .cfi_endproc
 
 // 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
+// PIC64-NEXT:      R_MIPS_PC32/R_MIPS_64/R_MIPS_NONE
 // RELOCS-NEXT:   }
 
 // DWARF32: 00000000 00000010 00000000 CIE
 // 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

 // 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_PIC:      Augmentation data: 1C
+//                                      ^^ fde pointer encoding: DW_EH_PE_pcrel | DW_EH_PE_sdata8
 // DWARF64-EMPTY:
 // DWARF64-NEXT:     DW_CFA_def_cfa_register: reg29
 // DWARF64_PIC-NEXT: DW_CFA_nop:
 //
 // DWARF64_ABS:      00000014 00000018 00000018 FDE cie=00000000 pc=00000000...00000000
-// DWARF64_PIC:      00000014 00000010 00000018 FDE cie=00000000 pc=00000000...00000000
+// DWARF64_PIC:      00000014 00000018 00000018 FDE cie=00000000 pc=00000000...00000000
 // DWARF64-NEXT:     DW_CFA_nop:
 // DWARF64-NEXT:     DW_CFA_nop:
 // DWARF64-NEXT:     DW_CFA_nop: