https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93380
Bug ID: 93380
Summary: Variable Length Array Evaluation will be wrong or
cause GDB to crash due to GCC
Product: gcc
Version: 7.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: robert.dumitru at cyberthorstudios dot com
Target Milestone: ---
We have the following code that runs on a cortex a9 of a Renesas RZ A1 board
and involves the declaration of a variable length array. I will present here
the code, the debug info for the variable and the disassembly code.
The sample code:
int main(void)
{
volatile int n;
int a = 1000;
int id_size = sizeof(cTaskList) / sizeof(V4_T_CTSK);
ID id[id_size];
for(;;){
for(n=0; n<id_size; n++){
(cTaskList[n].task)();
id[n] = nn;
}
}
}
Compiling and building it using the ARM GCC toolchain 7.2.1 we obtain the
following debug information for the variable length array id and also the
following assembly code:
Debug dump for id:
<2><9e2>: Abbrev Number: 38 (DW_TAG_variable)
<9e3> DW_AT_name : id
<9e6> DW_AT_decl_file : 1
<9e7> DW_AT_decl_line : 85
<9e8> DW_AT_type : <0x9f1>
<9ec> DW_AT_location : 3 byte block: 91 68 6 (DW_OP_fbreg: -24;
DW_OP_deref)
The disassembly code:
main():
20021230: push {r11, lr}
20021234: add r11, sp, #4
20021238: sub sp, sp, #24
82 int a = 1000;
2002123c: mov r3, #1000 ; 0x3e8
20021240: str r3, [r11, #-8]
84 int id_size = sizeof(cTaskList) / sizeof(V4_T_CTSK);
20021244: mov r3, #13
20021248: str r3, [r11, #-12]
85 ID id[id_size];/* <-- debug session terminates with this
declaration */
2002124c: ldr r3, [r11, #-12]
20021250: sub r12, r3, #1
20021254: str r12, [r11, #-16]
20021258: mov r3, r12
2002125c: add r3, r3, #1
20021260: mov r0, r3
20021264: mov r1, #0
20021268: mov r2, #0
2002126c: mov r3, #0
20021270: lsl r3, r1, #5
20021274: orr r3, r3, r0, lsr #27
20021278: lsl r2, r0, #5
2002127c: mov r3, r12
20021280: add r3, r3, #1
20021284: mov r0, r3
20021288: mov r1, #0
2002128c: mov r2, #0
20021290: mov r3, #0
20021294: lsl r3, r1, #5
20021298: orr r3, r3, r0, lsr #27
2002129c: lsl r2, r0, #5
200212a0: mov r3, r12
200212a4: add r3, r3, #1
200212a8: lsl r3, r3, #2
200212ac: add r3, r3, #3
200212b0: add r3, r3, #7
200212b4: lsr r3, r3, #3
200212b8: lsl r3, r3, #3
200212bc: sub sp, sp, r3
200212c0: mov r3, sp
200212c4: add r3, r3, #3
200212c8: lsr r3, r3, #2
200212cc: lsl r3, r3, #2
200212d0: str r3, [r11, #-20]
89 for(n=0; n<id_size; n++){
-data-evaluate-expression --thread 1 --frame 0 id
If we try to evaluate id before 200212d0: str r3, [r11, #-20] is executed
we either get the wrong result or gdb will get the wrong size for id and crash
with "../../gdb-7.8.2/gdb/utils.c:1062: internal-error: virtual memory
exhausted.
After this code is executed -200212d0: str r3, [r11, #-20] the memory
contains the correct values.
So maybe the code that writes the correct values to memory should be at the
beginning of the function?
This causes a serious problem if we debug using an IDE based on Eclipse that
has the Variables View open, because gdb will crash immediately after entering
the main function and the user will have no clue why.
Please let me know if you need more details or my explanation is not clear
enough.
Best regards,
Robert
