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@@ -516,4 +516,35 @@ A: LLVM has a -mcpu selector for the BPF back end in order to allow the
By the way, the BPF kernel selftests run with -mcpu=probe for better
test coverage.
+Q: In some cases clang flag "-target bpf" is used but in other cases the
+ default clang target, which matches the underlying architecture, is used.
+ What is the difference and when I should use which?
+
+A: Although LLVM IR generation and optimization try to stay architecture
+ independent, "-target <arch>" still has some impact on generated code:
+
+ - BPF program may recursively include header file(s) with file scope
+ inline assembly codes. The default target can handle this well,
+ while bpf target may fail if bpf backend assembler does not
+ understand these assembly codes, which is true in most cases.
+
+ - When compiled without -g, additional elf sections, e.g.,
+ .eh_frame and .rela.eh_frame, may be present in the object file
+ with default target, but not with bpf target.
+
+ - The default target may turn a C switch statement into a switch table
+ lookup and jump operation. Since the switch table is placed
+ in the global readonly section, the bpf program will fail to load.
+ The bpf target does not support switch table optimization.
+ The clang option "-fno-jump-tables" can be used to disable
+ switch table generation.
+
+ You should use default target when:
+
+ - Your program includes a header file, e.g., ptrace.h, which eventually
+ pulls in some header files containing file scope host assembly codes.
+ - You can add "-fno-jump-tables" to work around the switch table issue.
+
+ Otherwise, you can use bpf target.
+
Happy BPF hacking!