lldb failed to evaluate address: couldn't materialize: couldn't get the value of variable g_3: failed to read memory DW_OP_piece(4) from file address 0x4028 error: errored out in DoExecute, couldn't PrepareToExecuteJITExpression

[edumoot]

LLDB failed to compute the value of the static global structure variable, g_3, some components of which have been optimized out . We can reproduce this in llvm18.1.2, 17.0.6, and 16.0.3.

clang 4.c -g -O3 -o 4.out 
lldb 4.out
(lldb) br set -l 18
Breakpoint 1: where = 4.out`main [inlined] func_1 at 4.c:18:16, address = 0x0000000000001130
(lldb) r
[…]
* thread #1, name = '4.out', stop reason = breakpoint 1.1
    frame #0: 0x0000555555555130 4.out`main [inlined] func_1 at 4.c:18:16
   15  	    for (g_5 = 0; (g_5 < 2); g_5++)
   16  	    { 
   17  	        int l_20 = 0x95342363;
-> 18  	        g_3.f2 = (l_20 & 0U);
   19  	    }
   20  	    return g_5;
   21  	}
(lldb) p g_3
error: Couldn't materialize: couldn't get the value of variable g_3: failed to read memory DW_OP_piece(4) from file address 0x4028
error: errored out in DoExecute, couldn't PrepareToExecuteJITExpression
(lldb) 

while GDB works well.

gdb 4.out
(gdb) b 18
Breakpoint 1 at 0x1130: file 4.c, line 18.
(gdb) r
[…]
Breakpoint 1, func_1 () at 4.c:18
18	        g_3.f2 = (l_20 & 0U);
(gdb) p g_3
$1 = {f0 = <optimised out>, f1 = <optimised out>, f2 = 3303, f3 = 0, f4 = <optimised out>}
(gdb) 

The source code lists below
cat 4.c

struct S0 {
   const unsigned long  f0;
   unsigned long  f1;
   int f2: 25;
   unsigned int f3: 1;
   unsigned short  f4;
};

static struct S0 g_3 = {4294967295UL, 0x2EAC4530, 3303, 0, 1U};
static int g_5 = 0x6EC7516A;

static const unsigned short func_1(void)
{ 
    struct S0 *l_2 = &g_3;
    for (g_5 = 0; (g_5 < 2); g_5++)
    { 
        int l_20 = 0x95342363;
        g_3.f2 = (l_20 & 0U);
    }
    return g_5;
}

int main(void)
{
    func_1();
    return 0;
}

[llvmbot]

@llvm/issue-subscribers-lldb

Author: Yachao Zhu (edumoot)

LLDB failed to compute the address of the static global structure variable, `g_3`, some components of which have been optimized out . We can reproduce this in llvm18.1.2, 17.0.6, and 16.0.3. ``` clang 4.c -g -O3 -o 4.out lldb 4.out (lldb) br set -l 18 Breakpoint 1: where = 4.out`main [inlined] func_1 at 4.c:18:16, address = 0x0000000000001130 (lldb) r […] * thread #1, name = '4.out', stop reason = breakpoint 1.1 frame #0: 0x0000555555555130 4.out`main [inlined] func_1 at 4.c:18:16 15 for (g_5 = 0; (g_5 < 2); g_5++) 16 { 17 int l_20 = 0x95342363; -> 18 g_3.f2 = (l_20 & 0U); 19 } 20 return g_5; 21 } (lldb) p g_3 error: Couldn't materialize: couldn't get the value of variable g_3: failed to read memory DW_OP_piece(4) from file address 0x4028 error: errored out in DoExecute, couldn't PrepareToExecuteJITExpression (lldb) ```

while GDB works well.

gdb 4.out
(gdb) b 18
Breakpoint 1 at 0x1130: file 4.c, line 18.
(gdb) r
[…]
Breakpoint 1, func_1 () at 4.c:18
18	        g_3.f2 = (l_20 & 0U);
(gdb) p g_3
$1 = {f0 = <optimised out>, f1 = <optimised out>, f2 = 3303, f3 = 0, f4 = <optimised out>}
(gdb) 

The source code lists below
cat 4.c

struct S0 {
   const unsigned long  f0;
   unsigned long  f1;
   int f2: 25;
   unsigned int f3: 1;
   unsigned short  f4;
};

static struct S0 g_3 = {4294967295UL, 0x2EAC4530, 3303, 0, 1U};
static int g_5 = 0x6EC7516A;

static const unsigned short func_1(void)
{ 
    struct S0 *l_2 = &g_3;
    for (g_5 = 0; (g_5 < 2); g_5++)
    { 
        int l_20 = 0x95342363;
        g_3.f2 = (l_20 & 0U);
    }
    return g_5;
}

int main(void)
{
    func_1();
    return 0;
}

[edumoot]

This type of error happens when LLDB tries to parse a complex variable, such as a variable of structure, bit fields, or array, where some parts have been optimized out, but the essential parts are still intact.

Let us look through an example of array. After applying '-O3' optimization, g_6[4][0], the key element of the global static array, is kept and updated while other parts are optimized out. In the same environment of LLVM, we get:

clang -g -O3 case.c -o case.out
lldb  case.out
(lldb) br set -line 11
Breakpoint 1: where = case.out`main + 6 at case.c:11:5, address = 0x0000000000001136
(lldb) r
[...]
* thread #1, name = 'case.out', stop reason = breakpoint 1.1
    frame #0: 0x0000555555555136 case.out`main at case.c:11:5
   8   	int main (void)
   9   	{
   10  	    func_1();
-> 11  	    return 0;
   12  	}
(lldb) p g_6
error: Couldn't materialize: couldn't get the value of variable g_6: failed to read memory DW_OP_piece(1) from file address 0x4028
error: errored out in DoExecute, couldn't PrepareToExecuteJITExpression

while GDB shows:

gdb case.out
(gdb) b 11
Breakpoint 1 at 0x1136: file case.c, line 11.
(gdb) r
[...]
Breakpoint 1, main () at case.c:11
11	    return 0;
(gdb) p g_6
$1 = {{<optimised out>}, {<optimised out>}, {<optimised out>}, {<optimised out>}, "\377", {<optimised out>}, {<optimised out>}, {<optimised out>}, {<optimised out>}}

The C source code case.c lists below:

static unsigned char g_6[9][1] = {{254U}, {254U}, {254U}, {254U}, {254U}, {254U}, {254U}, {254U}, {254U}};
static unsigned char func_1(void)
{ 
    g_6[4][0]++;
    return g_6[4][0];
}

int main (void)
{
    func_1();
    return 0;
}

[edumoot]

In LLVM 19.1.0 context, LLDB can also encounter this type of error when certain elements of a union variable have been removed during optimization.

error: Couldn't materialize: couldn't get the value of  xxx:  failed to read memory DW_OP_piece(8) from file address 0x4040 
error: errored out in DoExecute, couldn't PrepareToExecuteJITExpression

[edumoot]

This case suggests that the issue is likely unrelated to the elements of a complex variable being optimized away.

(lldb) ta v
...
(U0) g_240 = <failed to read memory DW_OP_piece(2) from file address 0x4040>
...
(lldb) p g_240
error: Couldn't materialize: couldn't get the value of variable g_240: failed to read memory DW_OP_piece(2) from file address 0x4040
error: errored out in DoExecute, couldn't PrepareToExecuteJITExpression
(lldb) version
lldb version 19.1.0 (https://github.com/llvm/llvm-project.git revision a4bf6cd7cfb1a1421ba92bca9d017b49936c55e4)
  clang revision a4bf6cd7cfb1a1421ba92bca9d017b49936c55e4
  llvm revision a4bf6cd7cfb1a1421ba92bca9d017b49936c55e4

$ gdb
GNU gdb (Ubuntu 15.0.50.20240403-0ubuntu1) 15.0.50.20240403-git
...
(gdb) p g_240
$1 = {f0 = 1, f1 = 1 '\001', f2 = 1 '\001', f3 = 1, f4 = 1}

union U0 {
   int16_t  f0;
   int8_t  f1;
   const uint8_t  f2;
   volatile unsigned f3 : 12;
   int16_t  f4;
};


static union U0 g_240 = {1};