llvm dev - Aug 2021 - Support for older format versions in RawInstrProfReader

On Mon, Aug 23, 2021 at 10:52 PM Petr Hosek <phosek at google.com> wrote:
> Currently, while IndexedInstrProfReader provides backwards compatibility
> for indexed profile format, RawInstrProfReader only supports the current
> version of raw profile format. Has support for older raw profile format
> versions in RawInstrProfReader ever been considered?
>
> The reason why I'm asking is that we collect code coverage from both
C++
> and Rust, but since Clang and Rust toolchains use different version on
> LLVM, we need to match each profile with a corresponding llvm-profdata
> binary which has been increasing the complexity of our infrastructure quite
> a bit.
>
Is it possible to save one reader binary per raw format version? What is
the challenge of matching up versions with the reader binary?
>
> Using the same llvm-profdata tool would simplify things a lot but would
> require backwards compatibility support in RawInstrProfReader. Would that
> be desirable?
>
Current scheme for raw format allows fast pace change and innovation. It
also allows quite drastical format change. Losing that capability would be
undesirable. Before pursuing this path, the benefit needs to outweigh the
cost.

David
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> On Aug 24, 2021, at 8:24 AM, Xinliang David Li <davidxl at
google.com> wrote:
> 
> On Mon, Aug 23, 2021 at 10:52 PM Petr Hosek <phosek at google.com
<mailto:phosek at google.com>> wrote:
> Currently, while IndexedInstrProfReader provides backwards compatibility
for indexed profile format, RawInstrProfReader only supports the current version
of raw profile format. Has support for older raw profile format versions in
RawInstrProfReader ever been considered?
Justin (cc'd) once explained to me that a key motivating factor for
splitting up the raw and indexed profile formats was to free the runtime from
back-compat concerns.
> 
> The reason why I'm asking is that we collect code coverage from both
C++ and Rust, but since Clang and Rust toolchains use different version on LLVM,
we need to match each profile with a corresponding llvm-profdata binary which
has been increasing the complexity of our infrastructure quite a bit.
> 
> Is it possible to save one reader binary per raw format version? What is
the challenge of matching up versions with the reader binary?
> 
> Using the same llvm-profdata tool would simplify things a lot but would
require backwards compatibility support in RawInstrProfReader. Would that be
desirable?
> 
> Current scheme for raw format allows fast pace change and innovation. It
also allows quite drastical format change. Losing that capability would be
undesirable. Before pursuing this path, the benefit needs to outweigh the cost.
+ 1

vedant
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