[lldb] Slide eh_frame unwind plan if it doesn't begin at function boundary

This is mainly useful for discontinuous functions because individual
parts of the function will have separate FDE entries, which can begin
many megabytes from the start of the function. However, I'm separating
it out, because it turns out we already have a test case for the
situation where the FDE does not begin exactly at the function boundary.

The test works mostly by accident because the FDE starts only one byte
after the beginning of the function so it doesn't really matter whether
one looks up the unwind row using the function or fde offset.  In this
patch, I beef up the test to catch this problem more reliably.

To make this work I've also needed to change a couple of places which
that an unwind plan always has a row at offset zero.

Author: labath

lldb/source/Plugins/UnwindAssembly/x86/UnwindAssembly-x86.cpp

@@ -73,7 +73,7 @@ bool UnwindAssembly_x86::AugmentUnwindPlanFromCallSite(
 
   int wordsize = 8;
   ProcessSP process_sp(thread.GetProcess());
-  if (process_sp.get() == nullptr)
+  if (!process_sp || !first_row || !last_row)
     return false;
 
   wordsize = process_sp->GetTarget().GetArchitecture().GetAddressByteSize();

lldb/source/Symbol/DWARFCallFrameInfo.cpp

@@ -190,8 +190,12 @@ bool DWARFCallFrameInfo::GetUnwindPlan(const AddressRange &range,
   unwind_plan.SetUnwindPlanForSignalTrap(fde->for_signal_trap ? eLazyBoolYes
                                                               : eLazyBoolNo);
   unwind_plan.SetReturnAddressRegister(fde->return_addr_reg_num);
-  for (UnwindPlan::Row &row : fde->rows)
+  int64_t slide =
+      fde->range.GetBaseAddress().GetFileAddress() - addr.GetFileAddress();
+  for (UnwindPlan::Row &row : fde->rows) {
+    row.SlideOffset(slide);
     unwind_plan.AppendRow(std::move(row));
+  }
 
   return true;
 }

lldb/source/Symbol/UnwindPlan.cpp

@@ -474,7 +474,7 @@ bool UnwindPlan::PlanValidAtAddress(Address addr) const {
   // If the 0th Row of unwind instructions is missing, or if it doesn't provide
   // a register to use to find the Canonical Frame Address, this is not a valid
   // UnwindPlan.
-  const Row *row0 = GetRowForFunctionOffset(0);
+  const Row *row0 = GetRowAtIndex(0);
   if (!row0 ||
       row0->GetCFAValue().GetValueType() == Row::FAValue::unspecified) {
     Log *log = GetLog(LLDBLog::Unwind);

lldb/test/Shell/Unwind/Inputs/eh-frame-small-fde.s

@@ -10,12 +10,20 @@ bar:
 
         .type   foo, @function
 foo:
-        nop # Make the FDE entry start one byte later than the actual function.
+        # Make the FDE entry start 16 bytes later than the actual function. The
+        # size is chosen such that it is larger than the size of the FDE entry.
+        # This allows us to test that we're using the correct offset for
+        # unwinding (we'll stop 21 bytes into the function, but only 5 bytes
+        # into the FDE).
+        .nops 16
         .cfi_startproc
         .cfi_register %rip, %r13
         call    bar
         addl    $1, %eax
-        jmp     *%r13 # Return
+        movq    %r13, %r14
+        .cfi_register %rip, %r14
+        movq    $0, %r13
+        jmp     *%r14 # Return
         .cfi_endproc
         .size   foo, .-foo
 

lldb/test/Shell/Unwind/eh-frame-small-fde.test

@@ -14,9 +14,10 @@ process launch
 
 thread backtrace
 # CHECK: frame #0: {{.*}}`bar
-# CHECK: frame #1: {{.*}}`foo + 6
+# CHECK: frame #1: {{.*}}`foo + 21
 # CHECK: frame #2: {{.*}}`main + 20
 
 target modules show-unwind -n foo
-# CHECK: eh_frame UnwindPlan:
-# CHECK: row[0]: 0: CFA=rsp +8 => rip=r13
+# CHECK:      eh_frame UnwindPlan:
+# CHECK:      row[0]:   16: CFA=rsp +8 => rip=r13
+# CHECK-NEXT: row[1]:   27: CFA=rsp +8 => rip=r14

lldb/unittests/Symbol/UnwindPlanTest.cpp

@@ -62,15 +62,20 @@ TEST(UnwindPlan, PlanValidAtAddress) {
   UnwindPlan::Row row2 = make_simple_row(10, 47);
 
   UnwindPlan plan(eRegisterKindGeneric);
+  // When valid address range is not set, plans are valid as long as they have a
+  // row that sets the CFA.
   EXPECT_FALSE(plan.PlanValidAtAddress(Address(0)));
+  EXPECT_FALSE(plan.PlanValidAtAddress(Address(10)));
 
   plan.InsertRow(row2);
-  EXPECT_FALSE(plan.PlanValidAtAddress(Address(0)));
+  EXPECT_TRUE(plan.PlanValidAtAddress(Address(0)));
+  EXPECT_TRUE(plan.PlanValidAtAddress(Address(10)));
 
   plan.InsertRow(row1);
   EXPECT_TRUE(plan.PlanValidAtAddress(Address(0)));
   EXPECT_TRUE(plan.PlanValidAtAddress(Address(10)));
 
+  // With an address range, they're only valid within that range.
   plan.SetPlanValidAddressRanges({AddressRange(0, 5), AddressRange(15, 5)});
   EXPECT_TRUE(plan.PlanValidAtAddress(Address(0)));
   EXPECT_FALSE(plan.PlanValidAtAddress(Address(5)));