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

arichardson · arichardson · commit 894f742acb97 · 2020-01-13T14:14:03.000Z
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. Reviewed By: atanasyan Differential Revision: https://reviews.llvm.org/D72228
diff --git a/lld/test/ELF/mips-eh_frame-pic.s b/lld/test/ELF/mips-eh_frame-pic.s
@@ -0,0 +1,58 @@
+# 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
diff --git a/llvm/lib/MC/MCObjectFileInfo.cpp b/llvm/lib/MC/MCObjectFileInfo.cpp
@@ -303,9 +303,14 @@ void MCObjectFileInfo::initELFMCObjectFileInfo(const Triple &T, bool Large) {
   case Triple::mipsel:
   case Triple::mips64:
   case Triple::mips64el:
-    FDECFIEncoding = Ctx->getAsmInfo()->getCodePointerSize() == 4
-                         ? dwarf::DW_EH_PE_sdata4
-                         : dwarf::DW_EH_PE_sdata8;
+    // 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:
diff --git a/llvm/lib/Object/RelocationResolver.cpp b/llvm/lib/Object/RelocationResolver.cpp
@@ -105,6 +105,7 @@ static bool supportsMips64(uint64_t 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;
@@ -119,6 +120,8 @@ static uint64_t resolveMips64(RelocationRef R, uint64_t S, uint64_t A) {
     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");
   }
diff --git a/llvm/test/DebugInfo/Mips/eh_frame.ll b/llvm/test/DebugInfo/Mips/eh_frame.ll
@@ -1,15 +1,16 @@
 ; 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  ...........#.#..
diff --git a/llvm/test/MC/Mips/eh-frame.s b/llvm/test/MC/Mips/eh-frame.s
@@ -1,56 +1,96 @@
 // 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:
