[PATCH] DWARFv5: Handle location list for split dwarf.

[nitachra]

>Could you please provide a test snippet, along with the compiler version and command line you use, that generates this error?  Please also put it in the commit message so that anyone inspecting this commit later on can reproduce it.

>Finally, would you mind trying to send the upcoming patches using git-send-email?  It makes it easier to read and comment them.

Hi Simon,

Added the compiler verion, test snippet and the command line used in the commit messaage. However, this error cannot be reproduced right now. First the patch 
https://sourceware.org/pipermail/gdb-patches/2020-January/164841.html needs to go in which has the support for DW_FORM_loclistx.

---
GDB throws the error '<error reading variable: dwarf2_find_location_
expression: Corrupted DWARF expression.>' while printing the variable
value with executable file compiled with -gdwarf-5, -O1 and -gdwarf-split
flags. This patch fixes this error.

Tested by running the testsuite before and after the patch and there is
no increase in the number of test cases that fails. Tested with both
-gdwarf-4 and -gdwarf-5 flags. Also tested -gslit-dwarf along with -gdwarf-4
as well as -gdwarf-5 flags. Used clang version 10.0.0. This is the test case used-

int main()
{
    int arr[] = {1,2,3};
    for(int i = 0; i<3; i++)
        printf("%d",arr[i]);
    return 0;
}

clang -gdwarf-5 -O1 -gsplit-dwarf test.c -o test.out
gdb test.out
gdb> start
gdb> step
gdb> p i
dwarf2_find_location_expression: Corrupted DWARF expression.

gdb/ChangeLog:

   *dwarf2loc.c (enum debug_loc_kind): Added a new kind DEBUG_LOC_OFFSET_PAIR.
    (dwarf2_find_location_expression): Call the function decode_debug_loclists_
    addresses if DWARF version is 5 or more. Add applicable base address if the
    entry is DW_LLE_OFFSET_PAIR from DWO.
    (decode_debug_loclists_addresses): Return DEBUG_LOC_OFFSET_PAIR instead of
    DEBUG_LOC_START_END in case of DW_LLE_offset_pair.
---
 gdb/dwarf2loc.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/gdb/dwarf2loc.c b/gdb/dwarf2loc.c
index 405b239ed4..34795b70f9 100644
--- a/gdb/dwarf2loc.c
+++ b/gdb/dwarf2loc.c
@@ -90,6 +90,11 @@ enum debug_loc_kind
      number that specifies the length, and then a normal location expression
      as in .debug_loc.  */
   DEBUG_LOC_START_LENGTH = 3,
+  
+  /* This is followed by two unsigned LEB128 operands. The values of these 
+     operands are the starting and ending offsets, respectively, relative to 
+     the applicable base address. */
+  DEBUG_LOC_OFFSET_PAIR = 4,
 
   /* An internal value indicating there is insufficient data.  */
   DEBUG_LOC_BUFFER_OVERFLOW = -1,
@@ -231,7 +236,7 @@ decode_debug_loclists_addresses (struct dwarf2_per_cu_data *per_cu,
 	return DEBUG_LOC_BUFFER_OVERFLOW;
       *high = u64;
       *new_ptr = loc_ptr;
-      return DEBUG_LOC_START_END;
+      return DEBUG_LOC_OFFSET_PAIR;
     /* Following cases are not supported yet.  */
     case DW_LLE_startx_endx:
     case DW_LLE_start_end:
@@ -331,7 +336,7 @@ dwarf2_find_location_expression (struct dwarf2_loclist_baton *baton,
       enum debug_loc_kind kind;
       const gdb_byte *new_ptr = NULL; /* init for gcc -Wall */
 
-      if (baton->from_dwo)
+      if (dwarf2_version(baton->per_cu) < 5 && baton->from_dwo)
 	kind = decode_debug_loc_dwo_addresses (baton->per_cu,
 					       loc_ptr, buf_end, &new_ptr,
 					       &low, &high, byte_order);
@@ -357,6 +362,7 @@ dwarf2_find_location_expression (struct dwarf2_loclist_baton *baton,
 	  continue;
 	case DEBUG_LOC_START_END:
 	case DEBUG_LOC_START_LENGTH:
+	case DEBUG_LOC_OFFSET_PAIR:
 	  break;
 	case DEBUG_LOC_BUFFER_OVERFLOW:
 	case DEBUG_LOC_INVALID_ENTRY:
@@ -368,9 +374,11 @@ dwarf2_find_location_expression (struct dwarf2_loclist_baton *baton,
 
       /* Otherwise, a location expression entry.
 	 If the entry is from a DWO, don't add base address: the entry is from
-	 .debug_addr which already has the DWARF "base address".  We still add
-	 base_offset in case we're debugging a PIE executable.  */
-      if (baton->from_dwo)
+	 .debug_addr which already has the DWARF "base address". We still add
+	 base_offset in case we're debugging a PIE executable. However, if the 
+	 entry is DW_LLE_offset_pair from a DWO, add the base address as the 
+	 operands are offsets relative to the applicable base address.  */
+      if (baton->from_dwo && kind != DEBUG_LOC_OFFSET_PAIR)
 	{
 	  low += base_offset;
 	  high += base_offset;
-- 
2.17.1