Parse CFI instructions to create SFrame FREs

llvm/lib/MC/MCSFrame.cpp

@@ -21,13 +21,33 @@ using namespace sframe;
 
 namespace {
 
+// High-level structure to track info needed to emit a
+// sframe_frame_row_entry_addrX. On disk these have both a fixed portion of type
+// sframe_frame_row_entry_addrX and trailing data of X * S bytes, where X is the
+// datum size, and S is 1, 2, or 3 depending on which of CFA, SP, and FP are
+// being tracked.
+struct SFrameFRE {
+  // An FRE describes how to find the registers when the PC is at this
+  // Label from function start.
+  const MCSymbol *Label = nullptr;
+  size_t CFAOffset = 0;
+  size_t FPOffset = 0;
+  size_t RAOffset = 0;
+  bool FromFP = false;
+  bool CFARegSet = false;
+
+  SFrameFRE(const MCSymbol *Start) : Label(Start) {}
+};
+
 // High-level structure to track info needed to emit a sframe_func_desc_entry
 // and its associated FREs.
 struct SFrameFDE {
   // Reference to the original dwarf frame to avoid copying.
   const MCDwarfFrameInfo &DFrame;
   // Label where this FDE's FREs start.
   MCSymbol *FREStart;
+  // Unwinding fres
+  SmallVector<SFrameFRE> FREs;
 
   SFrameFDE(const MCDwarfFrameInfo &DF, MCSymbol *FRES)
       : DFrame(DF), FREStart(FRES) {}
@@ -53,7 +73,8 @@ struct SFrameFDE {
                                 MCFixup::getDataKindForSize(4)));
     S.emitInt32(0);
 
-    // sfde_func_start_num_fres
+    // sfde_func_num_fres
+    // TODO: When we actually emit fres, replace 0 with FREs.size()
     S.emitInt32(0);
 
     // sfde_func_info word
@@ -76,10 +97,90 @@ class SFrameEmitterImpl {
   MCObjectStreamer &Streamer;
   SmallVector<SFrameFDE> FDEs;
   ABI SFrameABI;
+  // Target-specific convenience variables to detect when a CFI instruction
+  // references these registers. Unlike in dwarf frame descriptions, they never
+  // escape into the sframe section itself.
+  unsigned SPReg;
+  unsigned FPReg;
+  unsigned RAReg;
   MCSymbol *FDESubSectionStart;
   MCSymbol *FRESubSectionStart;
   MCSymbol *FRESubSectionEnd;
 
+  bool setCFARegister(SFrameFRE &FRE, const MCCFIInstruction &I) {
+    if (I.getRegister() == SPReg) {
+      FRE.CFARegSet = true;
+      FRE.FromFP = false;
+      return true;
+    }
+    if (I.getRegister() == FPReg) {
+      FRE.CFARegSet = true;
+      FRE.FromFP = true;
+      return true;
+    }
+    Streamer.getContext().reportWarning(
+        I.getLoc(), "canonical Frame Address not in stack- or frame-pointer. "
+                    "Omitting SFrame unwind info for this function");
+    return false;
+  }
+
+  bool setCFAOffset(SFrameFRE &FRE, const SMLoc &Loc, size_t Offset) {
+    if (!FRE.CFARegSet) {
+      Streamer.getContext().reportWarning(
+          Loc, "adjusting CFA offset without a base register. "
+               "Omitting SFrame unwind info for this function");
+      return false;
+    }
+    FRE.CFAOffset = Offset;
+    return true;
+  }
+
+  // Add the effects of CFI to the current FDE, creating a new FRE when
+  // necessary.
+  bool handleCFI(SFrameFDE &FDE, SFrameFRE &FRE, const MCCFIInstruction &CFI) {
+    switch (CFI.getOperation()) {
+    case MCCFIInstruction::OpDefCfaRegister:
+      return setCFARegister(FRE, CFI);
+    case MCCFIInstruction::OpDefCfa:
+    case MCCFIInstruction::OpLLVMDefAspaceCfa:
+      if (!setCFARegister(FRE, CFI))
+        return false;
+      return setCFAOffset(FRE, CFI.getLoc(), CFI.getOffset());
+    case MCCFIInstruction::OpOffset:
+      if (CFI.getRegister() == FPReg)
+        FRE.FPOffset = CFI.getOffset();
+      else if (CFI.getRegister() == RAReg)
+        FRE.RAOffset = CFI.getOffset();
+      return true;
+    case MCCFIInstruction::OpRelOffset:
+      if (CFI.getRegister() == FPReg)
+        FRE.FPOffset += CFI.getOffset();
+      else if (CFI.getRegister() == RAReg)
+        FRE.RAOffset += CFI.getOffset();
+      return true;
+    case MCCFIInstruction::OpDefCfaOffset:
+      return setCFAOffset(FRE, CFI.getLoc(), CFI.getOffset());
+    case MCCFIInstruction::OpAdjustCfaOffset:
+      return setCFAOffset(FRE, CFI.getLoc(), FRE.CFAOffset + CFI.getOffset());
+    case MCCFIInstruction::OpRememberState:
+      // TODO: Implement. Will use FDE.
+      return true;
+    case MCCFIInstruction::OpRestore:
+      // TODO: Implement. Will use FDE.
+      return true;
+    case MCCFIInstruction::OpRestoreState:
+      // TODO: Implement. Will use FDE.
+      return true;
+    case MCCFIInstruction::OpEscape:
+      // TODO: Implement. Will use FDE.
+      return true;
+    default:
+      // Instructions that don't affect the CFA, RA, and SP can be safely
+      // ignored.
+      return true;
+    }
+  }
+
 public:
   SFrameEmitterImpl(MCObjectStreamer &Streamer) : Streamer(Streamer) {
     assert(Streamer.getContext()
@@ -88,13 +189,96 @@ class SFrameEmitterImpl {
                .has_value());
     FDEs.reserve(Streamer.getDwarfFrameInfos().size());
     SFrameABI = *Streamer.getContext().getObjectFileInfo()->getSFrameABIArch();
+    switch (SFrameABI) {
+    case ABI::AArch64EndianBig:
+    case ABI::AArch64EndianLittle:
+      SPReg = 31;
+      RAReg = 29;
+      FPReg = 30;
+      break;
+    case ABI::AMD64EndianLittle:
+      SPReg = 7;
+      // RARegister untracked in this abi. Value chosen to match
+      // MCDwarfFrameInfo constructor.
+      RAReg = static_cast<unsigned>(INT_MAX);
+      FPReg = 6;
+      break;
+    }
+
     FDESubSectionStart = Streamer.getContext().createTempSymbol();
     FRESubSectionStart = Streamer.getContext().createTempSymbol();
     FRESubSectionEnd = Streamer.getContext().createTempSymbol();
   }
 
-  void BuildSFDE(const MCDwarfFrameInfo &DF) {
-    FDEs.emplace_back(DF, Streamer.getContext().createTempSymbol());
+  bool atSameLocation(const MCSymbol *Left, const MCSymbol *Right) {
+    return Left != nullptr && Right != nullptr &&
+           Left->getFragment() == Right->getFragment() &&
+           Left->getOffset() == Right->getOffset();
+  }
+
+  bool equalIgnoringLocation(const SFrameFRE &Left, const SFrameFRE &Right) {
+    return Left.CFAOffset == Right.CFAOffset &&
+           Left.FPOffset == Right.FPOffset && Left.RAOffset == Right.RAOffset &&
+           Left.FromFP == Right.FromFP && Left.CFARegSet == Right.CFARegSet;
+  }
+
+  void buildSFDE(const MCDwarfFrameInfo &DF) {
+    bool Valid = true;
+    SFrameFDE FDE(DF, Streamer.getContext().createTempSymbol());
+    // This would have been set via ".cfi_return_column", but
+    // MCObjectStreamer doesn't emit an MCCFIInstruction for that. It just
+    // sets the DF.RAReg.
+    // FIXME: This also prevents providing a proper location for the error.
+    // LLVM doesn't change the return column itself, so this was
+    // hand-written assembly.
+    if (DF.RAReg != RAReg) {
+      Streamer.getContext().reportWarning(
+          SMLoc(), "non-default RA register in .cfi_return_column " +
+                       Twine(DF.RAReg) +
+                       ". Omitting SFrame unwind info for this function");
+      Valid = false;
+    }
+    MCSymbol *LastLabel = DF.Begin;
+    SFrameFRE BaseFRE(LastLabel);
+    if (!DF.IsSimple) {
+      for (const auto &CFI :
+           Streamer.getContext().getAsmInfo()->getInitialFrameState())
+        if (!handleCFI(FDE, BaseFRE, CFI))
+          Valid = false;
+    }
+    FDE.FREs.push_back(BaseFRE);
+
+    for (const auto &CFI : DF.Instructions) {
+      // Instructions from InitialFrameState may not have a label, but if these
+      // instructions don't, then they are in dead code or otherwise unused.
+      // TODO: This check follows MCDwarf.cpp
+      // FrameEmitterImplementation::emitCFIInstructions, but nothing in the
+      // testsuite triggers it. We should see if it can be removed in both
+      // places, or alternately, add a test to exercise it.
+      auto *L = CFI.getLabel();
+      if (L && !L->isDefined())
+        continue;
+
+      SFrameFRE FRE = FDE.FREs.back();
+      if (!handleCFI(FDE, FRE, CFI))
+        Valid = false;
+
+      // If nothing relevant but the location changed, don't add the FRE.
+      if (equalIgnoringLocation(FRE, FDE.FREs.back()))
+        continue;
+
+      // If the location stayed the same, then update the current
+      // row. Otherwise, add a new one.
+      if (atSameLocation(LastLabel, L))
+        FDE.FREs.back() = FRE;
+      else {
+        FDE.FREs.push_back(FRE);
+        FDE.FREs.back().Label = L;
+        LastLabel = L;
+      }
+    }
+    if (Valid)
+      FDEs.push_back(FDE);
   }
 
   void emitPreamble() {
@@ -116,7 +300,9 @@ class SFrameEmitterImpl {
     // shf_num_fdes
     Streamer.emitInt32(FDEs.size());
     // shf_num_fres
-    Streamer.emitInt32(0);
+    uint32_t TotalFREs = 0;
+    Streamer.emitInt32(TotalFREs);
+
     // shf_fre_len
     Streamer.emitAbsoluteSymbolDiff(FRESubSectionEnd, FRESubSectionStart,
                                     sizeof(int32_t));
@@ -161,7 +347,7 @@ void MCSFrameEmitter::emit(MCObjectStreamer &Streamer) {
   // Both the header itself and the FDEs include various offsets and counts.
   // Therefore, all of this must be precomputed.
   for (const auto &DFrame : FrameArray)
-    Emitter.BuildSFDE(DFrame);
+    Emitter.buildSFDE(DFrame);
 
   MCSection *Section = Context.getObjectFileInfo()->getSFrameSection();
   // Not strictly necessary, but gas always aligns to 8, so match that.

llvm/test/MC/ELF/cfi-sframe-errors.s

@@ -0,0 +1,31 @@
+// TODO: Add other architectures as they gain sframe support
+// REQUIRES: x86-registered-target
+// RUN: llvm-mc --assemble --filetype=obj -triple x86_64 %s -o %t.o  2>&1 | FileCheck %s
+// RUN: llvm-readelf --sframe %t.o | FileCheck --check-prefix=CHECK-NOFDES %s
+
+
+        .cfi_sections .sframe
+f1:
+        .cfi_startproc simple
+// CHECK: non-default RA register {{.*}}
+        .cfi_return_column 0
+        nop
+// CHECK: {{.*}} adjusting CFA offset without a base register.{{.*}}
+        .cfi_def_cfa_offset 16 // no line number reported here.
+        nop
+// CHECK: [[@LINE+1]]:{{.*}} adjusting CFA offset without a base register.{{.*}}
+        .cfi_adjust_cfa_offset 16
+        nop
+        .cfi_endproc
+
+f2:
+        .cfi_startproc
+        nop
+// CHECK: canonical Frame Address not in stack- or frame-pointer. {{.*}}
+        .cfi_def_cfa 0, 4
+        nop
+
+        .cfi_endproc
+
+// CHECK-NOFDES:    Num FDEs: 0
+// CHECK-NOFDES:    Num FREs: 0

llvm/test/MC/ELF/cfi-sframe.s

@@ -5,8 +5,18 @@
 
 	.cfi_sections .sframe
 f1:
-	.cfi_startproc
+	.cfi_startproc  // FRE 0
+	nop
+	.cfi_def_cfa_offset 16  // FRE 1
+	.cfi_def_cfa_offset 8   // location didn't change. No new FRE, but new offset.
+	nop
+	.cfi_def_cfa_offset 8   // offset didn't change. No new FRE.
+	nop
+	.cfi_def_cfa_offset 16  // FRE 2. new location, new offset.
 	nop
+	.cfi_register 0, 1      // Uninteresting register. No new FRE.
+	nop
+
         .cfi_endproc
 
 f2:
@@ -32,11 +42,11 @@ f2:
 // CHECK:    Function Index [
 // CHECK-NEXT:        FuncDescEntry [0] {
 // CHECK-NEXT:          PC {
-// CHECK-NEXT:            Relocation: {{.*}}32{{.*}}
+// CHECK-NEXT:            Relocation: {{.*}}PC32{{.*}}
 // CHECK-NEXT:            Symbol Name: .text
-// CHECK-NEXT:            Start Address: 0x0
+// CHECK-NEXT:            Start Address: {{.*}}
 // CHECK-NEXT:          }
-// CHECK-NEXT:          Size: 0x1
+// CHECK-NEXT:          Size: 0x5
 // CHECK-NEXT:          Start FRE Offset: 0x0
 // CHECK-NEXT:          Num FREs: 0
 // CHECK-NEXT:          Info {
@@ -51,7 +61,7 @@ f2:
 // CHECK-NEXT:        }
 // CHECK-NEXT:        FuncDescEntry [1] {
 // CHECK-NEXT:          PC {
-// CHECK-NEXT:            Relocation: R_X86_64_PC32
+// CHECK-NEXT:            Relocation: {{.*}}PC32{{.*}}
 // CHECK-NEXT:            Symbol Name: .text
 // CHECK-NEXT:            Start Address: {{.*}}
 // CHECK-NEXT:          }