Re: [PATCH v4] pgo: add clang's Profile Guided Optimization infrastructure

[Nick Desaulniers <ndesaulniers@google.com>]

On Wed, Jan 13, 2021 at 12:55 PM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> However, I see an issue with actually using the data:
>
> $ sudo -s
> # mount -t debugfs none /sys/kernel/debug
> # cp -a /sys/kernel/debug/pgo/profraw vmlinux.profraw
> # chown nathan:nathan vmlinux.profraw
> # exit
> $ tc-build/build/llvm/stage1/bin/llvm-profdata merge --output=vmlinux.profdata vmlinux.profraw
> warning: vmlinux.profraw: Invalid instrumentation profile data (bad magic)
> error: No profiles could be merged.
>
> Am I holding it wrong? :) Note, this is virtualized, I do not have any
> "real" x86 hardware that I can afford to test on right now.

Same.

I think the magic calculation in this patch may differ from upstream
llvm: https://github.com/llvm/llvm-project/blob/49142991a685bd427d7e877c29c77371dfb7634c/llvm/include/llvm/ProfileData/SampleProf.h#L96-L101

vs this patch:

+ * Note: These internal LLVM definitions must match the compiler version.
+ * See llvm/include/llvm/ProfileData/InstrProfData.inc in LLVM's source code.
+ */
+
+#ifdef CONFIG_64BIT
+ #define LLVM_PRF_MAGIC \
+ ((u64)255 << 56 | \
+ (u64)'l' << 48 | \
+ (u64)'p' << 40 | \
+ (u64)'r' << 32 | \
+ (u64)'o' << 24 | \
+ (u64)'f' << 16 | \
+ (u64)'r' << 8  | \
+ (u64)129)
+#else
+ #define LLVM_PRF_MAGIC \
+ ((u64)255 << 56 | \
+ (u64)'l' << 48 | \
+ (u64)'p' << 40 | \
+ (u64)'r' << 32 | \
+ (u64)'o' << 24 | \
+ (u64)'f' << 16 | \
+ (u64)'R' << 8  | \
+ (u64)129)
+#endif

-- 
Thanks,
~Nick Desaulniers