[MC] [Win64EH] Fix the calculation of the end of epilogs

[mstorsjo]

Summary

Exclude the terminating end opcode from the epilog - it doesn't
correspond to an actual instruction that is included in the epilog
itself (within the .seh_startepilogue/.seh_endepilogue range).

In most (all?) cases, an epilog is followed by a matching terminating
instruction though (a ret or a branch to a tail call), but it's not
strictly within the .seh_startepilogue/.seh_endepilogue range.

This fixes a number of failed asserts in cases where the codegen
has incorrectly reoredered SEH opcodes so they don't match up
exactly with their instructions.

However this still just avoids failing the assertion; the root cause
of generating unexpected epilogs is still present (and fixing that is
a less obvious issue).

[efriedma]

LGTM

llvm/lib/MC/MCWin64EH.cpp

static void ARM64FindSegmentsInFunction(MCStreamer &streamer,
   // Record Start and End of each epilog.
   SmallVector<struct EpilogStartEnd, 4> Epilogs;
   for (auto &I : info->EpilogMap) {
     MCSymbol *Start = I.first;
     auto &Instrs = I.second.Instructions;
     int64_t Offset = GetAbsDifference(streamer, Start, info->Begin);
     assert((Epilogs.size() == 0 || Offset >= Epilogs.back().End) &&
            "Epilogs should be monotonically ordered");
-    Epilogs.push_back({Start, Offset, Offset + (int64_t)Instrs.size() * 4});
+    // Exclue the end opcode from Instrs.size() when calculating the end of the
+    // epilog.
+    Epilogs.push_back({Start, Offset, Offset + (int64_t)(Instrs.size() - 1) * 4});
   }
 
   unsigned E = 0;
   int64_t SegLimit = 0xFFFFC;
   int64_t SegOffset = 0;
 
   if (RawFuncLength > SegLimit) {
 

llvm/test/MC/AArch64/seh-multi-epilog.s

+// This test checks that we emit allow multiple consecutive epilogs without
+// triggering failed asserts.unwind info correctly for epilogs that:
+// RUN: llvm-mc -triple aarch64-pc-win32 -filetype=obj %s -o %t.o
+// RUN: llvm-readobj -u %t.o | FileCheck %s
+
+// CHECK-LABEL:UnwindInformation [
+// CHECK-NEXT:  RuntimeFunction {
+// CHECK-NEXT:    Function: multi_epilog (0x0)
+// CHECK-NEXT:    ExceptionRecord: .xdata (0x0)
+// CHECK-NEXT:    ExceptionData {
+// CHECK-NEXT:      FunctionLength: 20
+// CHECK-NEXT:      Version: 0
+// CHECK-NEXT:      ExceptionData: No
+// CHECK-NEXT:      EpiloguePacked: No
+// CHECK-NEXT:      EpilogueScopes: 2
+// CHECK-NEXT:      ByteCodeLength: 4
+// CHECK-NEXT:      Prologue [
+// CHECK-NEXT:        0x81                ; stp x29, x30, [sp, #-16]!
+// CHECK-NEXT:        0xe4                ; end
+// CHECK-NEXT:      ]
+// CHECK-NEXT:      EpilogueScopes [
+// CHECK-NEXT:        EpilogueScope {
+// CHECK-NEXT:          StartOffset: 2
+// CHECK-NEXT:          EpilogueStartIndex: 0
+// CHECK-NEXT:          Opcodes [
+// CHECK-NEXT:            0x81                ; ldp x29, x30, [sp], #16
+// CHECK-NEXT:            0xe4                ; end
+// CHECK-NEXT:          ]
+// CHECK-NEXT:        }
+// CHECK-NEXT:        EpilogueScope {
+// CHECK-NEXT:          StartOffset: 3
+// CHECK-NEXT:          EpilogueStartIndex: 0
+// CHECK-NEXT:          Opcodes [
+// CHECK-NEXT:            0x81                ; ldp x29, x30, [sp], #16
+// CHECK-NEXT:            0xe4                ; end
+// CHECK-NEXT:          ]
+// CHECK-NEXT:        }
+// CHECK-NEXT:      ]
+// CHECK-NEXT:    }
+// CHECK-NEXT:  }
+// CHECK-NEXT:]
+
+  .text
+  .global multi_epilog
+  .p2align  2
+  .seh_proc multi_epilog
+multi_epilog:
+  stp  x29, lr, [sp, #-16]!
+  .seh_save_fplr_x 16
+  .seh_endprologue
+        nop
+  .seh_startepilogue
+  ldp x29, lr, [sp], #16
+  .seh_save_fplr_x 16
+  .seh_endepilogue
+  .seh_startepilogue
+  ldp x29, lr, [sp], #16
+  .seh_save_fplr_x 16
+  .seh_endepilogue
+  ret
+  .seh_endfunclet
+  .seh_endproc