[JITLink][AArch32] Implement R_ARM_PREL31 and stop skipping .ARM.exidx sections (wip)

Author: weliveindetail

llvm/include/llvm/ExecutionEngine/JITLink/aarch32.h

@@ -41,6 +41,8 @@ enum EdgeKind_aarch32 : Edge::Kind {
   /// Absolute 32-bit value relocation
   Data_Pointer32,
 
+  Data_PRel31,
+
   /// Create GOT entry and store offset
   Data_RequestGOTAndTransformToDelta32,
 

llvm/lib/ExecutionEngine/JITLink/ELF_aarch32.cpp

@@ -50,6 +50,8 @@ getJITLinkEdgeKind(uint32_t ELFType, const aarch32::ArmConfig &ArmCfg) {
     return aarch32::Arm_MovtAbs;
   case ELF::R_ARM_NONE:
     return aarch32::None;
+  case ELF::R_ARM_PREL31:
+    return aarch32::Data_PRel31;
   case ELF::R_ARM_TARGET1:
     return (ArmCfg.Target1Rel) ? aarch32::Data_Delta32
                                : aarch32::Data_Pointer32;
@@ -79,6 +81,8 @@ Expected<uint32_t> getELFRelocationType(Edge::Kind Kind) {
     return ELF::R_ARM_REL32;
   case aarch32::Data_Pointer32:
     return ELF::R_ARM_ABS32;
+  case aarch32::Data_PRel31:
+    return ELF::R_ARM_PREL31;
   case aarch32::Data_RequestGOTAndTransformToDelta32:
     return ELF::R_ARM_GOT_PREL;
   case aarch32::Arm_Call:
@@ -147,6 +151,15 @@ class ELFLinkGraphBuilder_aarch32
     // Handling ABI for the Arm® Architecture -> Index table entries
     if (Sect.sh_type == ELF::SHT_ARM_EXIDX)
       return true;
+    // Skip .ARM.extab sections. They are only ever referenced from .ARM.exidx,
+    // i.e. when unwind instructions don't fit into 4 bytes.
+    if (Sect.sh_type == ELF::SHT_PROGBITS && (Sect.sh_flags & ELF::SHF_ALLOC)) {
+      if (Sect.sh_name && Sect.sh_name < Base::SectionStringTab.size()) {
+        StringRef SectionName = Base::SectionStringTab.data() + Sect.sh_name;
+        if (SectionName.starts_with(".ARM.extab"))
+          return true;
+      }
+    }
     return false;
   }
 

llvm/lib/ExecutionEngine/JITLink/aarch32.cpp

@@ -398,6 +398,8 @@ Expected<int64_t> readAddendData(LinkGraph &G, Block &B, Edge::OffsetT Offset,
   case Data_Pointer32:
   case Data_RequestGOTAndTransformToDelta32:
     return SignExtend64<32>(support::endian::read32(FixupPtr, Endian));
+  case Data_PRel31:
+    return SignExtend64<31>(support::endian::read32(FixupPtr, Endian));
   default:
     return make_error<JITLinkError>(
         "In graph " + G.getName() + ", section " + B.getSection().getName() +
@@ -472,9 +474,8 @@ Error applyFixupData(LinkGraph &G, Block &B, const Edge &E) {
   Symbol &TargetSymbol = E.getTarget();
   uint64_t TargetAddress = TargetSymbol.getAddress().getValue();
 
-  // Regular data relocations have size 4, alignment 1 and write the full 32-bit
-  // result to the place; no need for overflow checking. There are three
-  // exceptions: R_ARM_ABS8, R_ARM_ABS16, R_ARM_PREL31
+  // Data relocations have alignment 1, size 4 (except R_ARM_ABS8 and
+  // R_ARM_ABS16) and write the full 32-bit result (except R_ARM_PREL31).
   switch (Kind) {
   case Data_Delta32: {
     int64_t Value = TargetAddress - FixupAddress + Addend;
@@ -496,6 +497,19 @@ Error applyFixupData(LinkGraph &G, Block &B, const Edge &E) {
       endian::write32be(FixupPtr, Value);
     return Error::success();
   }
+  case Data_PRel31: {
+    int64_t Value = TargetAddress - FixupAddress + Addend;
+    if (!isInt<31>(Value))
+      return makeTargetOutOfRangeError(G, B, E);
+    if (LLVM_LIKELY(G.getEndianness() == endianness::little)) {
+      uint32_t MSB = endian::read32le(FixupPtr) & 0x80000000;
+      endian::write32le(FixupPtr, MSB | (Value & ~0x80000000));
+    } else {
+      uint32_t MSB = endian::read32be(FixupPtr) & 0x80000000;
+      endian::write32be(FixupPtr, MSB | (Value & ~0x80000000));
+    }
+    return Error::success();
+  }
   case Data_RequestGOTAndTransformToDelta32:
     llvm_unreachable("Should be transformed");
   default:
@@ -856,6 +870,7 @@ const char *getEdgeKindName(Edge::Kind K) {
   switch (K) {
     KIND_NAME_CASE(Data_Delta32)
     KIND_NAME_CASE(Data_Pointer32)
+    KIND_NAME_CASE(Data_PRel31)
     KIND_NAME_CASE(Data_RequestGOTAndTransformToDelta32)
     KIND_NAME_CASE(Arm_Call)
     KIND_NAME_CASE(Arm_Jump24)

llvm/test/ExecutionEngine/JITLink/AArch32/ELF_relocations_data.s

@@ -57,6 +57,35 @@ got_prel_offset:
 	.size	got_prel_offset, .-got_prel_offset
 	.size	got_prel, .-got_prel
 
+# Dummy personality routine. It will be referenced from .ARM.exidx
+# CHECK-TYPE: {{[0-9a-f]+}} R_ARM_NONE               __aeabi_unwind_cpp_pr0
+	.globl __aeabi_unwind_cpp_pr0
+	.type __aeabi_unwind_cpp_pr0,%function
+	.align 2
+__aeabi_unwind_cpp_pr0:
+	bx lr
+
+# CHECK-TYPE: {{[0-9a-f]+}} R_ARM_PREL31 .text
+#
+# Used in the .ARM.exidx exception tables. The linker must preserve the most-
+# significant bit. It denotes whether the field is an inline entry (1) or a
+# relocation (0).
+#
+# jitlink-check: *{4}(section_addr(out.o, .ARM.exidx)) = (prel31 - (section_addr(out.o, .ARM.exidx))) & 0x7fffffff
+	.globl  prel31
+	.type	prel31,%function
+	.align	2
+prel31:
+	.fnstart
+	.save   {r11, lr}
+	push	{r11, lr}
+	.setfp  r11, sp
+	mov	r11, sp
+	pop	{r11, lr}
+	mov	pc, lr
+	.size	prel31,.-prel31
+	.fnend
+
 # This test is executable with any 4-byte external target:
 #  > echo "unsigned target = 42;" | clang -target armv7-linux-gnueabihf -o target.o -c -xc -
 #  > llvm-jitlink target.o armv7/out.o
@@ -67,5 +96,6 @@ got_prel_offset:
 main:
 	push	{lr}
 	bl	got_prel
+	bl	prel31
 	pop	{pc}
 	.size   main, .-main