[JITLink] Optimize GOTPCRELX Relocations

[StephenFan]

Summary

This patch optimize the GOTPCRELX Reloations, which is described in X86-64 psabi chapter B.2. And Not all optimization of this chapter is implemented.

1. Convert call and jmp has been implemented
2. Convert mov, but the optimization that when the symbol is defined in the lower 32-bit address space, memory operand in mov can be convertted into immediate operand has not been implemented.
3. Conver Test and Binop has not been implemented.

The new test file named ELF_got_plt_optimizations.s has been added, and I moved some test cases about optimization of got/plt from ELF_x86_64_small_pic_relocations.s to the new test file.

By referencing the lld, so, the optimization Convert call and jmp is not same as what psabi says, and I have explained it in the comment.

[StephenFan]

Change test file name from ELF_got_plt_optimization.s to ELF_x86-64_got_plt_optimizations.s

[lhames]

This is very cool -- thanks Stephen!

Please address the inline comments, otherwise LGTM!

llvm/lib/ExecutionEngine/JITLink/x86_64.cpp

82–84

BlockData should be FixupData.

I know that was my name originally, but now that this function is getting more attention we should fix my old mistakes. ;)

95

The preferred way to express this is JITTargetAddress EdgeAddr = B->getFixupAddress(E).

100–101

I assume the rationale here is "if neither of these is true then there's no point looking at the instruction sequence more closely"? I would add a comment for that to make it clear.

llvm/test/ExecutionEngine/JITLink/X86/ELF_x86-64_small_pic_relocations.s

71–81

Could you add to the comment to note that leal is used here to suppress optimization (in case anybody looks at it and wonders why it isn't a mov).

[StephenFan]

1. Local variable BlockData renamed to FixupData.
2. Get edge address by class block's getFixupAddress method.
3. Add comment.

[StephenFan]

Improve comments.

llvm/include/llvm/ExecutionEngine/JITLink/ELF_x86_64.h

 namespace jitlink {
 
 namespace ELF_x86_64_Edges {
 enum ELFX86RelocationKind : Edge::Kind {
   Branch32 = Edge::FirstRelocation,
   Pointer64,
   PCRel32,
   PCRel32GOTLoad,
-  PCRel32REXGOTLoad,
+  PCRel32GOTLoadRelaxable,
+  PCRel32REXGOTLoadRelaxable,
   PCRel64GOT,
   GOTOFF64,
   GOT64,
   Delta64,
 };
 
 } // end namespace ELF_x86_64_Edges
 

llvm/lib/ExecutionEngine/JITLink/ELF_x86_64.cpp

getRelocationKind(const uint32_t Type) {
     case ELF::R_X86_64_PC32:
       return ELF_x86_64_Edges::ELFX86RelocationKind::PCRel32;
     case ELF::R_X86_64_PC64:
     case ELF::R_X86_64_GOTPC64:
       return ELF_x86_64_Edges::ELFX86RelocationKind::Delta64;
     case ELF::R_X86_64_64:
       return ELF_x86_64_Edges::ELFX86RelocationKind::Pointer64;
     case ELF::R_X86_64_GOTPCREL:
-    case ELF::R_X86_64_GOTPCRELX:
       return ELF_x86_64_Edges::ELFX86RelocationKind::PCRel32GOTLoad;
+    case ELF::R_X86_64_GOTPCRELX:
+      return ELF_x86_64_Edges::ELFX86RelocationKind::PCRel32GOTLoadRelaxable;
     case ELF::R_X86_64_REX_GOTPCRELX:
-      return ELF_x86_64_Edges::ELFX86RelocationKind::PCRel32REXGOTLoad;
+      return ELF_x86_64_Edges::ELFX86RelocationKind::PCRel32REXGOTLoadRelaxable;
     case ELF::R_X86_64_GOTPCREL64:
       return ELF_x86_64_Edges::ELFX86RelocationKind::PCRel64GOT;
     case ELF::R_X86_64_GOT64:
       return ELF_x86_64_Edges::ELFX86RelocationKind::GOT64;
     case ELF::R_X86_64_GOTOFF64:
       return ELF_x86_64_Edges::ELFX86RelocationKind::GOTOFF64;
     case ELF::R_X86_64_PLT32:
       return ELF_x86_64_Edges::ELFX86RelocationKind::Branch32;
for (auto &SecRef : Sections) {
           break;
         case Delta64:
           Kind = x86_64::Delta64;
           break;
         case Pointer64:
           Kind = x86_64::Pointer64;
           break;
         case PCRel32GOTLoad: {
-          Kind = x86_64::RequestGOTAndTransformToPCRel32GOTLoadRelaxable;
-          Addend = 0;
+          Kind = x86_64::RequestGOTAndTransformToDelta32;
           break;
         }
-        case PCRel32REXGOTLoad: {
+        case PCRel32REXGOTLoadRelaxable: {
           Kind = x86_64::RequestGOTAndTransformToPCRel32GOTLoadREXRelaxable;
           Addend = 0;
           break;
         }
+        case PCRel32GOTLoadRelaxable: {
+          Kind = x86_64::RequestGOTAndTransformToPCRel32GOTLoadRelaxable;
+          Addend = 0;
+          break;
+        }
         case PCRel64GOT: {
           Kind = x86_64::RequestGOTAndTransformToDelta64;
           break;
         }
         case GOT64: {
           Kind = x86_64::RequestGOTAndTransformToDelta64FromGOT;
           break;
         }
const char *getELFX86RelocationKindName(Edge::Kind R) {
   case Branch32:
     return "Branch32";
   case Pointer64:
     return "Pointer64";
   case PCRel32:
     return "PCRel32";
   case PCRel32GOTLoad:
     return "PCRel32GOTLoad";
-  case PCRel32REXGOTLoad:
+  case PCRel32GOTLoadRelaxable:
+    return "PCRel32GOTLoadRelaxable";
+  case PCRel32REXGOTLoadRelaxable:
     return "PCRel32REXGOTLoad";
   case PCRel64GOT:
     return "PCRel64GOT";
   case Delta64:
     return "Delta64";
   case GOT64:
     return "GOT64";
   case GOTOFF64:
     return "GOTOFF64";
   }
   return getGenericEdgeKindName(static_cast<Edge::Kind>(R));
 }
 } // end namespace jitlink
 } // end namespace llvm

llvm/lib/ExecutionEngine/JITLink/x86_64.cpp

 
 const char PointerJumpStubContent[6] = {
     static_cast<char>(0xFFu), 0x25, 0x00, 0x00, 0x00, 0x00};
 
 Error optimize_x86_64_GOTAndStubs(LinkGraph &G) {
   LLVM_DEBUG(dbgs() << "Optimizing GOT entries and stubs:\n");
 
   for (auto *B : G.blocks())
-    for (auto &E : B->edges())
-      if (E.getKind() == x86_64::PCRel32GOTLoadREXRelaxable) {
-        // Replace GOT load with LEA only for MOVQ instructions.
-        assert(E.getOffset() >= 3 && "GOT edge occurs too early in block");
-
-        constexpr uint8_t MOVQRIPRel[] = {0x48, 0x8b};
-        if (strncmp(B->getContent().data() + E.getOffset() - 3,
-                    reinterpret_cast<const char *>(MOVQRIPRel), 2) != 0)
-          continue;
+    for (auto &E : B->edges()) {
+      if (E.getKind() == x86_64::PCRel32GOTLoadRelaxable ||
+          E.getKind() == x86_64::PCRel32GOTLoadREXRelaxable) {
+        bool REXPrefix = E.getKind() == x86_64::PCRel32GOTLoadREXRelaxable;
+        assert(E.getOffset() >= (REXPrefix ? 3 : 2) &&
+               "GOT edge occurs too early in block");
+        auto *BlockData = reinterpret_cast<uint8_t *>(
+                              const_cast<char *>(B->getContent().data())) +
+                          E.getOffset();
+        const uint8_t Op = BlockData[-2];
+        const uint8_t ModRM = BlockData[-1];
 
-        auto &GOTBlock = E.getTarget().getBlock();
-        assert(GOTBlock.getSize() == G.getPointerSize() &&
+        auto &GOTEntryBlock = E.getTarget().getBlock();
+        assert(GOTEntryBlock.getSize() == G.getPointerSize() &&
                "GOT entry block should be pointer sized");
-        assert(GOTBlock.edges_size() == 1 &&
+        assert(GOTEntryBlock.edges_size() == 1 &&
                "GOT entry should only have one outgoing edge");
-
-        auto &GOTTarget = GOTBlock.edges().begin()->getTarget();
-        JITTargetAddress EdgeAddr = B->getAddress() + E.getOffset();
+        auto &GOTTarget = GOTEntryBlock.edges().begin()->getTarget();
         JITTargetAddress TargetAddr = GOTTarget.getAddress();
-
+        JITTargetAddress EdgeAddr = B->getAddress() + E.getOffset();
         int64_t Displacement = TargetAddr - EdgeAddr + 4;
-        if (isInRangeForImmS32(Displacement)) {
-          // Change the edge kind as we don't go through GOT anymore. This is
-          // for formal correctness only. Technically, the two relocation kinds
-          // are resolved the same way.
+        bool TargetInRangeForImmU32 = isInRangeForImmU32(TargetAddr);
+        bool DisplacementInRangeForImmS32 = isInRangeForImmS32(Displacement);
+
+        if (!(TargetInRangeForImmU32 || DisplacementInRangeForImmS32))
+          continue;
+
+        // Transform "mov foo@GOTPCREL(%rip),%reg" to "lea foo(%rip),%reg".
+        if (Op == 0x8b && DisplacementInRangeForImmS32) {
+          BlockData[-2] = 0x8d;
           E.setKind(x86_64::Delta32);
           E.setTarget(GOTTarget);
           E.setAddend(E.getAddend() - 4);
-          auto *BlockData = reinterpret_cast<uint8_t *>(
-              const_cast<char *>(B->getContent().data()));
-          BlockData[E.getOffset() - 2] = 0x8d;
           LLVM_DEBUG({
             dbgs() << "  Replaced GOT load wih LEA:\n    ";
             printEdge(dbgs(), *B, E, getEdgeKindName(E.getKind()));
             dbgs() << "\n";
           });
+          continue;
+        }
+
+        // Transform call/jmp instructions
+        if (Op == 0xff && TargetInRangeForImmU32) {
+          if (ModRM == 0x15) {
+            // ABI says we can convert "call *foo@GOTPCREL(%rip)" to "nop; call
+            // foo" But lld convert it to "addr32 call foo, because that makes
+            // result expression to be a single instruction.
+            BlockData[-2] = 0x67;
+            BlockData[-1] = 0xe8;
+            LLVM_DEBUG({
+              dbgs() << "  replaced call instruction's memory operand wih imm "
+                        "operand:\n    ";
+              printEdge(dbgs(), *B, E, getEdgeKindName(E.getKind()));
+              dbgs() << "\n";
+            });
+          } else {
+            // Transform "jmp *foo@GOTPCREL(%rip)" to "jmp foo; nop"
+            assert(ModRM == 0x25 && "Invalid ModRm for call/jmp instructions");
+            BlockData[-2] = 0xe9;
+            BlockData[3] = 0x90;
+            E.setOffset(E.getOffset() - 1);
+            LLVM_DEBUG({
+              dbgs() << "  replaced jmp instruction's memory operand wih imm "
+                        "operand:\n    ";
+              printEdge(dbgs(), *B, E, getEdgeKindName(E.getKind()));
+              dbgs() << "\n";
+            });
+          }
+          E.setKind(x86_64::Pointer32);
+          E.setTarget(GOTTarget);
+          continue;
         }
       } else if (E.getKind() == x86_64::BranchPCRel32ToPtrJumpStubBypassable) {
         auto &StubBlock = E.getTarget().getBlock();
         assert(StubBlock.getSize() == sizeof(PointerJumpStubContent) &&
                "Stub block should be stub sized");
         assert(StubBlock.edges_size() == 1 &&
                "Stub block should only have one outgoing edge");
 
for (auto &E : B->edges()) {
           E.setTarget(GOTTarget);
           LLVM_DEBUG({
             dbgs() << "  Replaced stub branch with direct branch:\n    ";
             printEdge(dbgs(), *B, E, getEdgeKindName(E.getKind()));
             dbgs() << "\n";
           });
         }
       }
+    }
 
   return Error::success();
 }
 
 } // end namespace x86_64
 } // end namespace jitlink
 } // end namespace llvm

llvm/test/ExecutionEngine/JITLink/X86/ELF_got_plt_optimize.s

+# RUN: rm -rf %t && mkdir -p %t
+# RUN: llvm-mc -triple=x86_64-unknown-linux -position-independent -filetype=obj \
+# RUN:         -o %t/elf_sm_pic_reloc.o %s
+# RUN: llvm-jitlink -noexec -slab-allocate 100Kb -slab-address 0xfff00000 \
+# RUN:              -define-abs extern_in_range32=0xffe00000 \
+# RUN:              -check %s %t/elf_sm_pic_reloc.o
+#
+
+
+        .text
+        .file   "testcase.c"
+
+# Empty main entry point.
+        .globl  main
+        .p2align        4, 0x90
+        .type   main,@function
+main:
+        retq
+
+        .size   main, .-main
+
+# Test optimize of transforming "call *foo@GOTPCREL(%rip)" to "addr call foo"
+# We need check both the target address and the instruction opcodes
+# jitlink-check: decode_operand(test_call_gotpcrelx, 0)[31:0] = extern_in_range32
+# jitlink-check: *{1}test_call_gotpcrelx = 0x67
+# jitlink-check: *{1}test_call_gotpcrelx+1 = 0xe8
+        .globl test_call_gotpcrelx
+        .p2align      4, 0x90
+        .type   test_call_gotpcrelx,@function
+test_call_gotpcrelx:
+  call    *extern_in_range32@GOTPCREL(%rip)
+
+        .size   test_call_gotpcrelx, .-test_call_gotpcrelx
+
+
+# Test optimize of transforming "jmp *foo@GOTPCREL(%rip)" to "jmp foo ; nop"
+# We need check both the target address and the instruction opcodes
+# jitlink-check: decode_operand(test_call_gotpcrelx, 0)[31:0] = extern_in_range32
+# jitlink-check: *{1}test_jmp_gotpcrelx = 0xe9
+# jitlink-check: *{1}test_jmp_gotpcrelx+5 = 0x90
+        .globl test_jmp_gotpcrelx
+        .p2align      4, 0x90
+        .type   test_jmp_gotpcrelx,@function
+test_jmp_gotpcrelx:
+  jmp    *extern_in_range32@GOTPCREL(%rip)
+
+        .size   test_jmp_gotpcrelx, .-test_jmp_gotpcrelx
+
+# Check R_X86_64_PLT32 handling with a call to an external. This produces a
+# Branch32ToStub edge, because externals are not defined locally. During
+# resolution, the target turns out to be in-range from the callsite and so the
+# edge is relaxed in post-allocation optimization.
+#
+# jitlink-check: decode_operand(test_call_extern, 0) = \
+# jitlink-check:     extern_in_range32 - next_pc(test_call_extern)
+        .globl  test_call_extern
+        .p2align       4, 0x90
+        .type   test_call_extern,@function
+test_call_extern:
+        callq   extern_in_range32@plt
+
+        .size   test_call_extern, .-test_call_extern
+

llvm/test/ExecutionEngine/JITLink/X86/ELF_x86-64_common.s

 # RUN: rm -rf %t && mkdir -p %t
-# RUN: llvm-mc -triple=x86_64-unknown-linux -position-independent -filetype=obj -o %t/elf_common.o %s
+# RUN: llvm-mc -triple=x86_64-unknown-linux -relax-relocations=false -position-independent -filetype=obj -o %t/elf_common.o %s
 # RUN: llvm-jitlink -entry=load_common -noexec -check %s %t/elf_common.o
 
         .text
         .file   "elf_common.c"
         .globl  load_common
         .p2align        4, 0x90
         .type   load_common,@function
 

llvm/test/ExecutionEngine/JITLink/X86/ELF_x86-64_small_pic_relocations.s

 # RUN: rm -rf %t && mkdir -p %t
 # RUN: llvm-mc -triple=x86_64-unknown-linux -position-independent -filetype=obj \
 # RUN:         -o %t/elf_sm_pic_reloc.o %s
 # RUN: llvm-jitlink -noexec -slab-allocate 100Kb -slab-address 0xfff00000 \
 # RUN:              -define-abs external_data=0x1 \
-# RUN:              -define-abs extern_in_range32=0xffe00000 \
 # RUN:              -define-abs extern_out_of_range32=0x7fff00000000 \
 # RUN:              -check %s %t/elf_sm_pic_reloc.o
 #
 # Test ELF small/PIC relocations.
 
         .text
         .file   "testcase.c"
 
# jitlink-check: decode_operand(test_call_local, 0) = named_func - next_pc(test_call_local)
         .globl  test_call_local
         .p2align       4, 0x90
         .type   test_call_local,@function
 test_call_local:
         callq   named_func
 
         .size   test_call_local, .-test_call_local
 
-# Check R_X86_64_PLT32 handling with a call to an external. This produces a
-# Branch32ToStub edge, because externals are not defined locally. During
-# resolution, the target turns out to be in-range from the callsite and so the
-# edge is relaxed in post-allocation optimization.
-#
-# jitlink-check: decode_operand(test_call_extern, 0) = \
-# jitlink-check:     extern_in_range32 - next_pc(test_call_extern)
-        .globl  test_call_extern
-        .p2align       4, 0x90
-        .type   test_call_extern,@function
-test_call_extern:
-        callq   extern_in_range32@plt
-
-        .size   test_call_extern, .-test_call_extern
-
 # Check R_X86_64_PLT32 handling with a call to an external via PLT. This
 # produces a Branch32ToStub edge, because externals are not defined locally.
 # As the target is out-of-range from the callsite, the edge keeps using its PLT
 # entry.
 #
 # jitlink-check: decode_operand(test_call_extern_plt, 0) = \
 # jitlink-check:     stub_addr(elf_sm_pic_reloc.o, extern_out_of_range32) - \
 # jitlink-check:        next_pc(test_call_extern_plt)
 # jitlink-check: *{8}(got_addr(elf_sm_pic_reloc.o, extern_out_of_range32)) = \
 # jitlink-check:     extern_out_of_range32
         .globl  test_call_extern_plt
         .p2align       4, 0x90
         .type   test_call_extern_plt,@function
 test_call_extern_plt:
         callq   extern_out_of_range32@plt
 
         .size   test_call_extern_plt, .-test_call_extern_plt
 
 # Test GOTPCREL handling. We want to check both the offset to the GOT entry and its
 # contents.
 # jitlink-check: decode_operand(test_gotpcrel, 4) = \
 # jitlink-check:     got_addr(elf_sm_pic_reloc.o, named_data) - next_pc(test_gotpcrel)
 # jitlink-check: *{8}(got_addr(elf_sm_pic_reloc.o, named_data)) = named_data
 
         .globl test_gotpcrel
         .p2align      4, 0x90
         .type   test_gotpcrel,@function
 test_gotpcrel:
-  movl    named_data@GOTPCREL(%rip), %eax
+  leal    named_data@GOTPCREL(%rip), %eax
 
         .size   test_gotpcrel, .-test_gotpcrel
 
 # Test REX_GOTPCRELX handling. We want to check both the offset to the GOT entry and its
 # contents.
 # jitlink-check: decode_operand(test_rex_gotpcrelx, 4) = \
 # jitlink-check:   got_addr(elf_sm_pic_reloc.o, external_data) - next_pc(test_rex_gotpcrelx)