[Win64EH] Write .pdata symbol relocations relative to the temporary begin symbol

[mstorsjo]

Summary

Previously the relocations pointed at the public user facing,
possibly external symbol.

When the function itself is weak, that symbol may be overridden at
link time, pointing at another strong implementation of the same
function instead. In that case, there's two conflicting pdata entries
pointing at the same address, and the wrong unwind info might end up
used.

Both GCC/binutils and MSVC produce pdata pointing at internal static
symbols. (GCC/binutils point at the .text section just as LLVM does
after this change, MSVC points at special label type symbols with the
type IMAGE_SYM_CLASS_LABEL and names like '$LN4'.)

This fixes unwinding through an overridden "operator new" with a
statically linked C++ library in MinGW mode. (Building libc++ with
-ffunction-sections and linking with --gc-sections might avoid the
issue too.)

This makes the produced object files a little less user friendly
to debug, but with the other llvm-readobj patches, the unwind info
debugging experience should be pretty much the same.

Alternatively, we could choose to only do this if the function is
marked as weak - producing less consistent output but more
straightforward object files in most cases.

[efriedma]

Makes sense. LGTM

llvm/test/MC/COFF/seh.s

155

This is just to make the CHECK lines easier to read, I assume?

[mstorsjo]

llvm/test/MC/COFF/seh.s

155

Yes; currently they check that the StartAddress and EndAddress point at the same symbol, but without the nop, the EndAddress of func would be resolved by llvm-readobj into smallFunc instead of func + offset - otherwise I'd need to change those CHECK lines.

I guess the alternative would be to add even more logic into the resolving logic in llvm-readobj, see D109649, to not resolve symbol + nonzero offset into a symbol with a zero offset, for the EndAddress case.

[mstorsjo]

llvm/test/MC/COFF/seh.s

155

I amended D109649, with that update I can drop this extra nop.

llvm/lib/MC/MCWin64EH.cpp

static void EmitSymbolRefWithOfs(MCStreamer &streamer,
   streamer.emitValue(MCBinaryExpr::createAdd(BaseRefRel, Ofs, Context), 4);
 }
 
 static void EmitRuntimeFunction(MCStreamer &streamer,
                                 const WinEH::FrameInfo *info) {
   MCContext &context = streamer.getContext();
 
   streamer.emitValueToAlignment(4);
-  EmitSymbolRefWithOfs(streamer, info->Function, info->Begin);
-  EmitSymbolRefWithOfs(streamer, info->Function, info->End);
+  EmitSymbolRefWithOfs(streamer, info->Begin, info->Begin);
+  EmitSymbolRefWithOfs(streamer, info->Begin, info->End);
   streamer.emitValue(MCSymbolRefExpr::create(info->Symbol,
                                              MCSymbolRefExpr::VK_COFF_IMGREL32,
                                              context), 4);
 }
 
 static void EmitUnwindInfo(MCStreamer &streamer, WinEH::FrameInfo *info) {
   // If this UNWIND_INFO already has a symbol, it's already been emitted.
   if (info->Symbol)
streamer.emitValue(
         4);
 }
 
 static void ARM64EmitRuntimeFunction(MCStreamer &streamer,
                                      const WinEH::FrameInfo *info) {
   MCContext &context = streamer.getContext();
 
   streamer.emitValueToAlignment(4);
-  EmitSymbolRefWithOfs(streamer, info->Function, info->Begin);
+  EmitSymbolRefWithOfs(streamer, info->Begin, info->Begin);
   if (info->PackedInfo)
     streamer.emitInt32(info->PackedInfo);
   else
     streamer.emitValue(
         MCSymbolRefExpr::create(info->Symbol, MCSymbolRefExpr::VK_COFF_IMGREL32,
                                 context),
         4);
 }

llvm/test/MC/AArch64/seh.s

 // CHECK-NEXT:   }
 // CHECK-NEXT: ]
 
 // CHECK-NEXT: Relocations [
 // CHECK-NEXT:   Section (4) .xdata {
 // CHECK-NEXT:     0x28 IMAGE_REL_ARM64_ADDR32NB __C_specific_handler
 // CHECK-NEXT:   }
 // CHECK-NEXT:   Section (5) .pdata {
-// CHECK-NEXT:     0x0 IMAGE_REL_ARM64_ADDR32NB func
+// CHECK-NEXT:     0x0 IMAGE_REL_ARM64_ADDR32NB .text
 // CHECK-NEXT:     0x4 IMAGE_REL_ARM64_ADDR32NB .xdata
 // CHECK-NEXT:   }
 // CHECK-NEXT: ]
 
 // CHECK-NEXT: UnwindInformation [
 // CHECK-NEXT:   RuntimeFunction {
 // CHECK-NEXT:     Function: func
 // CHECK-NEXT:     ExceptionRecord: .xdata

llvm/test/MC/COFF/seh-align1.s

 // CHECK:          RawDataSize: 12
 // CHECK:          RelocationCount: 3
 // CHECK:          Characteristics [
 // CHECK-NEXT:       IMAGE_SCN_ALIGN_4BYTES
 // CHECK-NEXT:       IMAGE_SCN_CNT_INITIALIZED_DATA
 // CHECK-NEXT:       IMAGE_SCN_MEM_READ
 // CHECK-NEXT:     ]
 // CHECK:          Relocations [
-// CHECK-NEXT:       [[BeginDisp:0x[A-F0-9]+]] IMAGE_REL_AMD64_ADDR32NB smallFunc
-// CHECK-NEXT:       [[EndDisp:0x[A-F0-9]+]] IMAGE_REL_AMD64_ADDR32NB smallFunc
+// CHECK-NEXT:       [[BeginDisp:0x[A-F0-9]+]] IMAGE_REL_AMD64_ADDR32NB .text
+// CHECK-NEXT:       [[EndDisp:0x[A-F0-9]+]] IMAGE_REL_AMD64_ADDR32NB .text
 // CHECK-NEXT:       [[UnwindDisp:0x[A-F0-9]+]] IMAGE_REL_AMD64_ADDR32NB .xdata
 // CHECK-NEXT:     ]
 // CHECK:          SectionData (
 // CHECK-NEXT:       0000: 00000000 01000000 00000000
 // CHECK-NEXT:     )
 // CHECK-NEXT:   }
 // CHECK:        UnwindInformation [
 // CHECK-NEXT:     RuntimeFunction {

llvm/test/MC/COFF/seh-align2.s

 // CHECK:          RawDataSize: 12
 // CHECK:          RelocationCount: 3
 // CHECK:          Characteristics [
 // CHECK-NEXT:       IMAGE_SCN_ALIGN_4BYTES
 // CHECK-NEXT:       IMAGE_SCN_CNT_INITIALIZED_DATA
 // CHECK-NEXT:       IMAGE_SCN_MEM_READ
 // CHECK-NEXT:     ]
 // CHECK:          Relocations [
-// CHECK-NEXT:       [[BeginDisp:0x[A-F0-9]+]] IMAGE_REL_AMD64_ADDR32NB func
-// CHECK-NEXT:       [[EndDisp:0x[A-F0-9]+]] IMAGE_REL_AMD64_ADDR32NB func
+// CHECK-NEXT:       [[BeginDisp:0x[A-F0-9]+]] IMAGE_REL_AMD64_ADDR32NB .text
+// CHECK-NEXT:       [[EndDisp:0x[A-F0-9]+]] IMAGE_REL_AMD64_ADDR32NB .text
 // CHECK-NEXT:       [[UnwindDisp:0x[A-F0-9]+]] IMAGE_REL_AMD64_ADDR32NB .xdata
 // CHECK-NEXT:     ]
 // CHECK:          SectionData (
-// CHECK-NEXT:       0000: FCFFFFFF 05000000 00000000
+// CHECK-NEXT:       0000: 00000000 09000000 00000000
 // CHECK-NEXT:     )
 // CHECK-NEXT:   }
 // CHECK-NEXT: ]
 // CHECK:      UnwindInformation [
 // CHECK-NEXT:   RuntimeFunction {
-// CHECK-NEXT:     StartAddress: func {{(\+0x[A-F0-9]+ )?}}([[BeginDisp]])
+// CHECK-NEXT:     StartAddress: .text ([[BeginDisp]])
 // CHECK-NEXT:     EndAddress: func {{(\+0x[A-F0-9]+ )?}}([[EndDisp]])
 // CHECK-NEXT:     UnwindInfoAddress: .xdata {{(\+0x[A-F0-9]+ )?}}([[UnwindDisp]])
 // CHECK-NEXT:     UnwindInfo {
 // CHECK-NEXT:       Version: 1
 // CHECK-NEXT:       Flags [
 // CHECK-NEXT:         ExceptionHandler
 // CHECK-NEXT:       ]
 // CHECK-NEXT:       PrologSize: 0

llvm/test/MC/COFF/seh-align3.s

 // CHECK:          RawDataSize: 12
 // CHECK:          RelocationCount: 3
 // CHECK:          Characteristics [
 // CHECK-NEXT:       IMAGE_SCN_ALIGN_4BYTES
 // CHECK-NEXT:       IMAGE_SCN_CNT_INITIALIZED_DATA
 // CHECK-NEXT:       IMAGE_SCN_MEM_READ
 // CHECK-NEXT:     ]
 // CHECK:          Relocations [
-// CHECK-NEXT:       [[BeginDisp:0x[A-F0-9]+]] IMAGE_REL_AMD64_ADDR32NB func
-// CHECK-NEXT:       [[EndDisp:0x[A-F0-9]+]] IMAGE_REL_AMD64_ADDR32NB func
+// CHECK-NEXT:       [[BeginDisp:0x[A-F0-9]+]] IMAGE_REL_AMD64_ADDR32NB .text
+// CHECK-NEXT:       [[EndDisp:0x[A-F0-9]+]] IMAGE_REL_AMD64_ADDR32NB .text
 // CHECK-NEXT:       [[UnwindDisp:0x[A-F0-9]+]] IMAGE_REL_AMD64_ADDR32NB .xdata
 // CHECK-NEXT:     ]
 // CHECK:          SectionData (
-// CHECK-NEXT:       0000: FCFFFFFF 05000000 00000000
+// CHECK-NEXT:       0000: 00000000 09000000 00000000
 // CHECK-NEXT:     )
 // CHECK-NEXT:   }
 // CHECK-NEXT: ]
 // CHECK:      UnwindInformation [
 // CHECK-NEXT:   RuntimeFunction {
-// CHECK-NEXT:     StartAddress: func {{(\+0x[A-F0-9]+ )?}}([[BeginDisp]])
+// CHECK-NEXT:     StartAddress: .text ([[BeginDisp]])
 // CHECK-NEXT:     EndAddress: func {{(\+0x[A-F0-9]+ )?}}([[EndDisp]])
 // CHECK-NEXT:     UnwindInfoAddress: .xdata {{(\+0x[A-F0-9]+ )?}}([[UnwindDisp]])
 // CHECK-NEXT:     UnwindInfo {
 // CHECK-NEXT:       Version: 1
 // CHECK-NEXT:       Flags [
 // CHECK-NEXT:         ExceptionHandler
 // CHECK-NEXT:         TerminateHandler
 // CHECK-NEXT:       ]

llvm/test/MC/COFF/seh.s

 // CHECK-NEXT:       MEM_READ
 // CHECK-NEXT:     ]
 // CHECK-NEXT:   }
 // CHECK-NEXT: ]
 
 // CHECK-NEXT: Relocations [
 // CHECK-NEXT:   Section (4) .xdata {
 // CHECK-NEXT:     0x14 IMAGE_REL_AMD64_ADDR32NB __C_specific_handler
-// CHECK-NEXT:     0x20 IMAGE_REL_AMD64_ADDR32NB func
-// CHECK-NEXT:     0x24 IMAGE_REL_AMD64_ADDR32NB func
+// CHECK-NEXT:     0x20 IMAGE_REL_AMD64_ADDR32NB .text
+// CHECK-NEXT:     0x24 IMAGE_REL_AMD64_ADDR32NB .text
 // CHECK-NEXT:     0x28 IMAGE_REL_AMD64_ADDR32NB .xdata
 // CHECK-NEXT:   }
 // CHECK-NEXT:   Section (5) .pdata {
-// CHECK-NEXT:     0x0 IMAGE_REL_AMD64_ADDR32NB func
-// CHECK-NEXT:     0x4 IMAGE_REL_AMD64_ADDR32NB func
+// CHECK-NEXT:     0x0 IMAGE_REL_AMD64_ADDR32NB .text
+// CHECK-NEXT:     0x4 IMAGE_REL_AMD64_ADDR32NB .text
 // CHECK-NEXT:     0x8 IMAGE_REL_AMD64_ADDR32NB .xdata
-// CHECK-NEXT:     0xC IMAGE_REL_AMD64_ADDR32NB func
-// CHECK-NEXT:     0x10 IMAGE_REL_AMD64_ADDR32NB func
+// CHECK-NEXT:     0xC IMAGE_REL_AMD64_ADDR32NB .text
+// CHECK-NEXT:     0x10 IMAGE_REL_AMD64_ADDR32NB .text
 // CHECK-NEXT:     0x14 IMAGE_REL_AMD64_ADDR32NB .xdata
-// CHECK-NEXT:     0x18 IMAGE_REL_AMD64_ADDR32NB smallFunc
-// CHECK-NEXT:     0x1C IMAGE_REL_AMD64_ADDR32NB smallFunc
+// CHECK-NEXT:     0x18 IMAGE_REL_AMD64_ADDR32NB .text
+// CHECK-NEXT:     0x1C IMAGE_REL_AMD64_ADDR32NB .text
 // CHECK-NEXT:     0x20 IMAGE_REL_AMD64_ADDR32NB .xdata
 // CHECK-NEXT:   }
 // CHECK-NEXT: ]
 
 
 // CHECK:      UnwindInformation [
 // CHECK-NEXT:   RuntimeFunction {
 // CHECK-NEXT:     StartAddress: [[CodeSect1:[^ ]+]] [[BeginDisp1:(\+0x[A-F0-9]+)?]]
func:
     lea (%rbx), %rsp
     pop %rbx
     addq $24, %rsp
     ret
     .seh_endproc
 
 // Test emission of small functions.
     .globl smallFunc
     .def smallFunc; .scl 2; .type 32; .endef
     .seh_proc smallFunc
 smallFunc:
     ret
     .seh_endproc