Overview of ic decode
ic decode is a tool used to decode instruction cache (icache) traces from executables. It takes a trace file as input and outputs a human-readable disassembly of the instructions that were executed.
ic decode is useful for analyzing the runtime behavior of programs. By looking at the disassembled icache traces, developers can see exactly which instructions were executed and in what order. This helps with debugging, optimization, and understanding program flow.
How ic decode works
ic decode relies on having an icache trace as input. These traces can be generated by hardware perfomance counters or simulator instrumentation. The trace file contains the memory addresses of instructions executed by the CPU.
ic decode takes this list of addresses and disassembles the corresponding instructions using objdump or a similar disassembler. It matches the addresses to locations in the original executable file.
The output is a disassembly listing showing the instructions in chronological order along with their memory offsets. Additional information like cycle counts or processor core ID can also be included if present in the trace file.
Uses for ic decode
Here are some common uses for ic decode:
Debugging
By looking at the disassembly, developers can identify hot spots and bottlenecks at a granular instruction level. This helps pinpoint optimization opportunities.
Program analysis
Disassembly provides insight into control flow and data access patterns. This is helpful for understanding program behavior.
Simulation validation
Icache traces from simulators can be compared to traces from real hardware. The disassembly view makes it easy to verify correct execution.
Performance characterization
The number of times each instruction executes can be aggregated. This helps identify optimization targets.
IC Decode Output Format
The output of ic decode consists of:
- Memory address of instruction
- Disassembled instruction
- Cycle count or other optional columns
Here is an example:
| Address | Instruction | Cycles |
|---|---|---|
| 0x4000b0 | add %rbx, %rax | 10 |
| 0x4000b5 | cmp %rax, %rcx | 2 |
| 0x4000ba | je 0x1234 | 3 |
The disassembled instructions are shown in order along with the execution metrics. This provides an annotated view of the program trace.
Frequently Asked Questions
How do I generate an icache trace file?
Icache traces can be generated using hardware performance counters or instrumentation in processor simulators. The Linux Perf tool has an --icache option to capture icache profiles. Simulators like gem5 can dump icache accesses.
What information is in an icache trace?
A basic icache trace contains the memory addresses of instructions executed. Additional data like cycle counts, core ids, and processor events can also be included. The trace is a chronological list of icache accesses.
What executables can I use with ic decode?
ic decode relies on having a mapping between instruction addresses and locations in an executable. So it needs the original binary/ELF file used to produce the trace. This allows matching addresses to instructions.
Does ic decode require debug symbols?
No, ic decode does not require separate debug symbol information. It disassembles instructions from the binary file without needing separate debugging files.
How is ic decode different from objdump?
objdump can disassemble an entire executable file. ic decode only disassembles the specific instructions present in the trace file. So it only shows the part of the program that was executed.
