[llvm] 138b9f1 - [JITLink][ELF] PCRel32GOTLoad relocations are resolved like regular PCRel32 ones

[Stefan Gränitz via llvm-commits]

Author: Stefan Gränitz
Date: 2020-10-24T14:54:38+02:00
New Revision: 138b9f1928454a6096bb928195bacb7ccd46fd00

URL: https://github.com/llvm/llvm-project/commit/138b9f1928454a6096bb928195bacb7ccd46fd00
DIFF: https://github.com/llvm/llvm-project/commit/138b9f1928454a6096bb928195bacb7ccd46fd00.diff

LOG: [JITLink][ELF] PCRel32GOTLoad relocations are resolved like regular PCRel32 ones

The difference is that the former are indirect and go to the GOT while the latter go to the target directly. This info can be used to relax indirect ones that don't need the GOT (because the target is in range). We check for this optimization beforehand. For formal correctness and to avoid confusion, we should only change the relocation kind if we actually apply the relaxation.

Added: 
    

Modified: 
    llvm/lib/ExecutionEngine/JITLink/ELF_x86_64.cpp

Removed: 
    


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diff  --git a/llvm/lib/ExecutionEngine/JITLink/ELF_x86_64.cpp b/llvm/lib/ExecutionEngine/JITLink/ELF_x86_64.cpp
index c3498935bd2d..821a7b3cc44e 100644
--- a/llvm/lib/ExecutionEngine/JITLink/ELF_x86_64.cpp
+++ b/llvm/lib/ExecutionEngine/JITLink/ELF_x86_64.cpp
@@ -125,13 +125,14 @@ static Error optimizeELF_x86_64_GOTAndStubs(LinkGraph &G) {
   for (auto *B : G.blocks())
     for (auto &E : B->edges())
       if (E.getKind() == PCRel32GOTLoad) {
-        assert(E.getOffset() >= 3 && "GOT edge occurs too early in block");
+        // Replace GOT load with LEA only for MOVQ instructions.
+        constexpr uint8_t MOVQRIPRel[] = {0x48, 0x8b};
+        if (strncmp(B->getContent().data() + E.getOffset() - 3,
+                    reinterpret_cast<const char *>(MOVQRIPRel), 2) != 0)
+          continue;
 
-        // Switch the edge kind to PCRel32: Whether we change the edge target
-        // or not this will be the desired kind.
-        E.setKind(PCRel32);
+        assert(E.getOffset() >= 3 && "GOT edge occurs too early in block");
 
-        // Optimize GOT references.
         auto &GOTBlock = E.getTarget().getBlock();
         assert(GOTBlock.getSize() == G.getPointerSize() &&
                "GOT entry block should be pointer sized");
@@ -142,16 +143,13 @@ static Error optimizeELF_x86_64_GOTAndStubs(LinkGraph &G) {
         JITTargetAddress EdgeAddr = B->getAddress() + E.getOffset();
         JITTargetAddress TargetAddr = GOTTarget.getAddress();
 
-        // Check that this is a recognized MOV instruction.
-        // FIXME: Can we assume this?
-        constexpr uint8_t MOVQRIPRel[] = {0x48, 0x8b};
-        if (strncmp(B->getContent().data() + E.getOffset() - 3,
-                    reinterpret_cast<const char *>(MOVQRIPRel), 2) != 0)
-          continue;
-
         int64_t Displacement = TargetAddr - EdgeAddr + 4;
         if (Displacement >= std::numeric_limits<int32_t>::min() &&
             Displacement <= std::numeric_limits<int32_t>::max()) {
+          // 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.
+          E.setKind(PCRel32);
           E.setTarget(GOTTarget);
           auto *BlockData = reinterpret_cast<uint8_t *>(
               const_cast<char *>(B->getContent().data()));
@@ -636,7 +634,8 @@ class ELFJITLinker_x86_64 : public JITLinker<ELFJITLinker_x86_64> {
     char *FixupPtr = BlockWorkingMem + E.getOffset();
     JITTargetAddress FixupAddress = B.getAddress() + E.getOffset();
     switch (E.getKind()) {
-    case ELFX86RelocationKind::PCRel32: {
+    case ELFX86RelocationKind::PCRel32:
+    case ELFX86RelocationKind::PCRel32GOTLoad: {
       int64_t Value = E.getTarget().getAddress() + E.getAddend() - FixupAddress;
       endian::write32le(FixupPtr, Value);
       break;