Debugging a Kernel on CPU and GPU: Major Differences
The behavior and commands of Intel Distribution for GDB are very close to the standard GDB. The CPU debugging experience is almost the same; however, there are differences in GPU debugging, coming from the architecture.
In comparison to debugging on a CPU, debugging a kernel on a GPU has a few differences:
Aspect |
Description |
CPU |
GPU |
|---|---|---|---|
Threads and single instruction, multiple data (SIMD) lanes |
When the code is vectorized, threads process vectors of data elements in parallel. SIMD lane is a logical unit of execution for accessing data elements within an instruction. |
The debugger does not support SIMD lanes. Even if your code is vectorized, you cannot switch context to a non-default SIMD lane. |
The debugger supports threads and SIMD lanes. You can switch the context to a particular thread or SIMD lane during debugging. When kernels are compiled in debug mode, most SIMD instructions have 8 or 16 lanes (i.e. execution channels). To refer to a particular SIMD lane of a thread, use a SIMD lane identifier in the format thread ID:lane. To learn more about debugging programs with multiple threads, refer to Chapter 4.10 of Intel® Distribution for GDB* User Guide. |
The following commands behave differently during GPU debugging:
Command |
Description |
Modification |
Example/Comment |
|---|---|---|---|
disassemble |
Disassemble the current function. |
GEN instructions and registers are shown. |
N/A |
|
|
SIMD lanes are supported and SIMD lane switches can occur. |
next [Switching to SIMD lane0] |
thread |
|
SIMD lanes are supported. |
|
thread apply |
Apply a command to the specified SIMD lane of the thread. |
SIMD lanes are supported. |
thread apply 2.3:print element prints the value of the element variable for each active lane of thread 2.3.Useful for inspecting vectorized values. |
info threads |
Display information about threads with ID, including their active SIMD lanes. |
SIMD lanes are supported. |
Use the -stopped option to filter out the unavailable threads. |
info devices |
Display information about the available devices. |
New command. |
N/A |
commands |
Specify a list of commands to execute when your program stops due to a particular breakpoint. |
SIMD lanes are supported. With the /a modifier, breakpoint actions apply to all SIMD lanes that match the condition of the specified breakpoint. |
commands /a print element end prints the element value for all SIMD lanes that match the condition of the specified breakpoint. |
break |
Create a breakpoint at a specified line. |
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