[PATCH] DWARFv5: Handle location list for split dwarf.
nitachra
Nitika.Achra@amd.com
Sun Mar 22 12:56:37 GMT 2020
Hi Simon,
>>On 2020-03-18 10:15 a.m., nitachra wrote:
>>
>> Hi Simon,
>>
>> Thanks for the review.
>>
>>> @@ -332,7 +337,7 @@ 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 (baton->per_cu->version () < 5 && baton->from_dwo)
>>>
>>> Can you please explain the rationale of this change? It's hard to tell whether it is correct or not without an explanation (and I prefer not having to guess your intentions).
>>
>> I made this change because clang and gcc are not generating the DW_LLE_GNU* entries with DWARFv5 and -gsplit-dwarf.
>> Instead they are generating DW_LLE_start* or DW_LLE_offset_pair with DW_LLE_base_addressx.
>Thanks. This is the kind of information that is useful to include in the commit message, that explains why you needed to do such and such change.
Modified the commit message.
>> if (baton->from_dwo && kind != DEBUG_LOC_OFFSET_PAIR)
>> {
>> low += base_offset;
>> high += base_offset;
>> }
>> else
>> {
>> low += base_address;
>> high += base_address;
>> }
>>
>>
>> I made the above change because clang is emitting DW_LLE_offset_pair and DW_LLE_base_addressx for DWARFv5 and -gsplit-dwarf.
>> So we need to add the base address in the case of dwo also, otherwise
>> the low and high address will be incorrect and gdb will give <optimized out> when printing the variable values.
>>
>> These two changes combined will print the correct variable values.
>> Overall the intent for this patch was to support the location list for split dwarf and print the correct variable values.
>Hmm, all I get is <optimized out>. I checked the generated binary a bit closer (test.out and test.dwo), am I supposed to see a .debug_loclists or .debug_loclists.dwo section in there?
>Because I don't see any... Again, this is with this clang version, take from apt.llvm.org:
>clang version 10.0.0-++20200317072632+135744ce689-1~exp1~20200317063228.125
I have included a new test case in the commit message. I was actually using the older clang 10.0.0-rc1 from here https://github.com/llvm/llvm-project/releases. With the new test case
you can reproduce the error with clang-10.0.0 also. You can see the debug_loclist.dwo with this test case.
Regards,
Nitika
---
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 is because DW_LLE_start* or DW_LLE_offset_pair
with DW_LLE_base_addressx are being emitted in DWARFv5 location list
instead of DW_LLE_GNU*. 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-
void bar(int arr[], int l, int m, int r)
{
int i, j, k, n1= m - l + 1, n2= r - m, L[n1], R[n2];
for (i = 0; i < n1; i++)
L[i] = arr[l + i];
for (j = 0; j < n2; j++)
R[j] = arr[m + 1+ j];
}
int main()
{
int arr[] = {12, 11};
bar(arr,0,1,2);
return 0;
clang -gdwarf-5 -O1 -gsplit-dwarf test.c -o test.out
gdb test.out
gdb> start
gdb> step
gdb> step
gdb> step
gdb> step
gdb> p L[0]
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. DW_LLE_start* or DW_LLE_offset_pair
with DW_LLE_base_addressx are being emitted in DWARFv5 instead of DW_LLE_GNU*.
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/dwarf2/loc.c | 18 +++++++++++++-----
1 file changed, 13 insertions(+), 5 deletions(-)
diff --git a/gdb/dwarf2/loc.c b/gdb/dwarf2/loc.c
index 5155cff60d..08c315a381 100644
--- a/gdb/dwarf2/loc.c
+++ b/gdb/dwarf2/loc.c
@@ -92,6 +92,11 @@ enum debug_loc_kind
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,
@@ -232,7 +237,7 @@ 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:
@@ -332,7 +337,7 @@ 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 (baton->per_cu->version () < 5 && baton->from_dwo)
kind = decode_debug_loc_dwo_addresses (baton->per_cu,
loc_ptr, buf_end, &new_ptr,
&low, &high, byte_order);
@@ -358,6 +363,7 @@ 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:
@@ -369,9 +375,11 @@ 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
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